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Macromolecular Dimensions and Mechanical
Properties of Monolayer Films of
Sonorean Mesquite Gum
Yolanda L. Lopez-Franco,1 Miguel A. Valdez,*2 Javier Hernandez,1 Ana M. Calderon de la Barca,1 Marguerite Rinaudo,3
Francisco M. Goycoolea1*
1 Laboratory of Biopolymers. Centro de Investigacion en Alimentacion y Desarrollo, A.C. (C.I.A.D., A.C.)P.O. Box 1735 Hermosillo, Sonora, 83000, Mexico
2 Departamento de Investigacion en Polımeros y Materiales, Departamento de Fısica, Universidad de Sonora. Blvd.Transversal y Rosales, 83000, Hermosillo, Sonora, MexicoE-mail: valdez@fisica.uson.mx
3 Centre de Recherches sur les Macromolecules Vegetales, C.N.R.S. affiliated with University Joseph Fourier – B.P. 53,38041, Grenoble, Cedex 9, France
Received: April 22, 2004; Revised: June 25, 2004; Accepted: June 25, 2004; DOI: 10.1002/mabi.200400055
Keywords: gum arabic; interfaces; light scattering; mesquite gum; monolayers
Introduction
Mesquite gum is a natural polysaccharide exudated by the
bark of Prosopis spp. trees. This gum was collected by the
Seri Indians settled in the coast of the Gulf of California in
the state of Sonora, Mexico, who used it to prepare eye
drops.[1] In this region of Mexico, mesquite gum (locally
known as ‘‘chucata’’) is sourced mostly from P. velutina
wild trees and is still used in various domestic applications
(e.g., as a hat hardener) and is commonly chewed in the
rural areas.[2] In the state of San Luis Potosi, mesquite gum
is also collected predominantly from P. laevigata wild trees.
It has also been commercialised in limited amounts to be
used as emulsifier in soft drinks as a substitute of gum
arabic, an extensively used hydrocolloid. In previous
studies, it has been demonstrated that the functionality of
mesquite gum is comparable and even superior in certain
conditions to that of gum arabic.[3–7] As a matter of fact, the
Summary: Mesquite gum sourced from Prosopis velutinatrees and gum arabic (Acacia spp.) were characterized usinglight scattering and Langmuir isotherms. Both gum materialswere fractionated by hydrophobic interaction chromatogra-phy, yielding four fractions for both gums: FI, FIIa, FIIb andFIII in mesquite gum and FI, FII, FIIIa and FIIIb in gumarabic. In mesquite gum, the obtained fractions had differentprotein content (7.18–38.60 wt.-%) and macromoleculardimensions (Mw � 3.89� 105–8.06� 105 g �mol�1, RG�48.83–71.11 nm, RH� 9.61–24.06 nm) and architecturegiven by the structure factor (RG/RH ratio �2.96–5.27). Themechanical properties of Langmuir monolayers at the air-water interface were very different on each gum and theirfractions. For mesquite gum, the most active species at theinterface were those comprised in Fractions IIa and IIb andIII, while Fraction I the p/A isotherm lied below that of thewhole gum. In gum arabic only Fraction III developed greatersurface pressure at the same surface per milligram of materialthan whole gum. This is rationalized in terms of structuraldifferences in both materials. Mesquite gum tertiary structureseems to fit best with an elongated polydisperse macrocoil
in agreement with the ‘‘twisted hairy rope’’ proposal forarabinogalactan proteoglycans.
Compression isotherms of gum arabic and mesquite gum.
Macromol. Biosci. 2004, 4, 865–874 DOI: 10.1002/mabi.200400055 � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Full Paper 865
Ministry of Health of Mexico has granted authorization for
use of mesquite gum in soft drinks, tablets and feeds.
Chemically, mesquite gum is a highly branched complex
proteoglycan. The primary structure of the polysaccharide
component has been described in detail:[8–10] a central
backbone comprised of b(1! 3)-linked D-galactose resi-
dues, to which side oligosaccharide chains of varying size
are attached at O(6); these branches contain predominantly
D-galactose and L-arabinose (known to occur in both
furanose and pyranose forms) and minor proportions of
D-glucoronate, 4-O-methyl-D-glucuronate and L-rhamnose.
A very similar sugar residue composition is shared by
the carbohydrate components of gum arabic from
Acacia senegal and other exudate gums from the acacia
species.[11–16] Besides the carbohydrate components,
mesquite gum bears a proteinaceous component that
accounts for about �4% of its weight.[7] Hence mesquite
gum can be regarded as an arabinogalactan proteoglycan
(AGP) of the Type II,[17] the same type as gum arabic and
related gums. Up to now, the macromolecular structure of
mesquite gum has been rationalized as highly branched
driving it to a globular disordered conformation in solution.
This would account for its high solubility in water and
Newtonian rheology even at very high polymer concentra-
tion (ca. 50%).[18] However, in this study we revise this
proposal and favor a more elongated structure.
Although the overall composition of carbohydrate
domains of mesquite gum seems to be closely related
to that of gum arabic, both materials can distinctively be
distinguished by different sets of antibodies which spe-
cifically recognize non-reducing chain termini of the
peripheral chains,[19] as well as by crossed immunoelec-
trophoresis.[20] Besides, studies by electrophoresis with
Yariv reagent embedded in the second dimension, have
shown that AGPs from gum arabic differ from those of
mesquite gum[21] and other AGPs of wild tomato.[22]
Fractionation of gum arabic using hydrophobic affinity
chromatography demonstrated that three main fractions can
be isolated.[23] Analysis of the fractions showed that each
contained similar proportions of the various sugars and
differed principally in their molecular masses and protein
contents. The bulk of the gum (84.4 wt.-% of the total) was
shown to be comprised by the so-called arabinogalactan
(AG) fraction with weight-average molecular weight (Mw)
of 2.78� 105 from light scattering measurements and
low in protein (0.35 wt.-%). The second major fraction
(10.4 wt.-% of the total), or AGP, had a higher molecular
weight of 1.45� 106 and contained a greater proportion of
protein (11.18 wt.-%). The third minor fraction (1 wt.-% of
the total), or glycoprotein fraction (GP), had a molecular
weight of 2.5� 105 and the higher protein content (50%
wt.-%). The proteinaceus component of the first two
fractions had similar amino acid distributions, with
hydroxyproline and serine being the most abundant. Work
by Williams et al.,[24] using 1H and 13C NMR spectroscopy
and methylation analysis showed that there were no major
differences in the sugar constituents of the above three
fractions. In a recent study,[25] mesquite gum from
P. laevigata produced both in vitro from stem segments of
mesquite tree and in situ by wild trees was fractionated by
hydrophobic affinity chromatography. Five fractions were
identified for both materials. In the case of gum from wild
trees, Fraction I represented 85 wt.-% of the total gum, had a
molecular mass of 9.3� 105 as determined by gel
permeation chromatography (GPC) (calibrated with dex-
tran standards) and was low in protein (2.3 wt.-%).
Fractions IIa and IIb which represented �11 wt.-% of the
total gum, were found with molecular masses of 6.9� 105
and 5.9� 105, respectively and low protein content (1.4 and
4.3 wt.-%, respectively). Two minor fractions, IIIa and IIIb,
accounted for 3.7 and 1.8 wt.-% of the total gum, had
molecular masses of 1.1� 105 and 3.5� 104, respectively,
and accounted for the greater protein content (35.7 and
50.0 wt.-%, respectively).
For gum arabic, the tertiary structure of the fraction with
species of largest molecular mass (Mw � 1.0� 106), has
been described in terms of a ‘wattle blossom’ macro-
molecular assembly,[23,26,27] by virtue of which few (�5)
discrete polysaccharide domains of Mw � 2� 105 are held
together by a short peptide backbone chain. The key
experimental evidence to support this hypothesis showed
that hydrolysis of the fraction with largest molecular mass
with a protease produced a five-fold reduction in molecular
mass and these fractions were no longer prone to
proteolysis. Alternatively, a ‘‘twisted hairy rope’’ structure
has also been proposed, whereby a linear polypeptide
backbone bears polysaccharide domains in a repeating
pattern.[28] In this proposal, the gum arabic glycoproteins
are represented by a statistical model with a fundamental
7 kDa subunit, where polysaccharide side chains attach to a
polypeptide backbone (Hyp/Pro-rich) of >400 residues
(�150 nm long; 5 nm diameter; axial ratio �30:1) in a
highly regular and ordered fashion (every 10 to 12 residues
repetitive peptide units), forming a twisted hairy rope.
Evidence to this proposal has come mostly from transmis-
sion electron microscopy (TEM) and it is consistent with
the fact that large macromolecules such as AGPs, migrate
through a primary cell wall with a nominal molar mass
cutoff of 68 kDa. Neither of these type of large assemblies
appear to exist in mesquite gum. Instead, the experimental
evidence available to date, argues in favor that mesquite
gum comprises predominantly individual loose proteogly-
can domains. This hypothesis is consistent with differences
in rheology, hydrodynamic molecular dimensions, and area
per molecule adsorbed to a oil-water interface when both
materials have been subjected to similar test condi-
tions.[18,20,21] It is accepted that the complex heterogeneous
and amphiphilic structure of gum arabic is central to its
unique functionality and utilitarian value, namely as an
emulsifier and stabilizer of citrus flavor based soft-
866 Y. L. Lopez-Franco, M. A. Valdez, J. Hernandez, A. M. Calderon de la Barca, M. Rinaudo, F. M. Goycoolea
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
drinks.[29,30] In a recent study, the mechanical properties of
monolayers spread at an air-water interface for whole and
fractionated gum arabic from A. senegal and A. seyal
fractions were investigated. In both gums it was observed
that the maximal surface pressure of whole gum was higher
than the corresponding parameter for each of their fractions,
implying that there was a synergistic effect between the
various proteoglycan molecular species comprising the
whole gum.[31] Such studies have not so far been undertaken
in mesquite gum.
Like gum arabic, mesquite gum also forms and stabilizes
oil-in-water emulsions and has the ability to encapsulate
orange citrus oil during spray drying.[20,32,33] This study
aims to glean further insight to the structure and functional
properties of mesquite gum sourced from the predominant
mesquite species of the plains of the Sonorean Desert,
P. velutina. The gum was fractionated using standard
hydrophobic affinity chromatography in order to keep in
line with previous studies in gum arabic and mesquite gum.
Multi-angle laser light scattering and dynamic light
scattering were used to characterize the molecular dimen-
sions and the architecture of the obtained gum fractions.
The behavior of the individual fractions in monolayers
adsorbed at the air-water interface was also studied using a
Langmuir balance in order to gain understanding of the
relationship between the macromolecular structure of
the various fractions and the phenomena involved in the
adsorption process. The gained knowledge is compared
with the properties of gum arabic characterized under
similar experimental protocol.
Experimental Part
Mesquite gum sample was a batch of hand sorted tears (Grade‘‘A’’) selected from a gum stock gathered in the spring of 1998from various local suppliers in Hermosillo, Sonora, Mexico.The gum arabic (Acacia spp.) sample and the rest of thechemical substances were all analytical grade and obtainedfrom Sigma-Aldrich Chemicals (St Louis, MO). Milli-Q waterwas obtained by a filtration system (Millipore Inc. Mexico) andused throughout. Mesquite gum sample was pre-treated bydissolving it in water at �4.5 vol.-%, filtered through acellulose filter in a pilot plant system (Zeta Plus1, Cuno Inc.USA) and freeze-dried until utilized.
Hydrophobic Interaction Chromatography
A column of dimensions 2.5� 24 cm was packed with phenyl-Sepharose CL–4B gel (Pharmacia, Uppsala, Sweden) using4.2 mol � dm�3 NaCl as eluent solution. A 100 cm3 volume ofa 10 wt.-% solution of mesquite gum in 4.2 mol � dm�3
NaCl was applied to the column and eluted successively by4.2 mol � dm�3 NaCl, 2.0 mol � dm�3 NaCl and finally water.The flow rate was set to 40 cm3 � h�1. Absorbance wasmonitored at 280 nm by using a low-pressure chromatographicsystem (Bio-Rad, CA, USA). The obtained fractions weredialyzed extensively against water and freeze-dried.
Protein
The protein content in the various samples was determinedusing a combustion N analyzer LECO Mod. FP 528 (LecoMexico). Conversion factors of 6.53 and 6.60 were used inthe calculations of mesquite gum[34] and gum arabic,[35]
respectively.
Total Sugars
The content of total sugars was determined by the phenol-sulfuric method.[36]
Optical Rotation
Optical rotation measurements were recorded for 1 wt.-%aqueous gum solutions in a cell of path length 1 dmthermostated at 20� 0.1 8C. Angular rotation measurementswere recorded using a Perkin Elmer 341 polarimeter.
Light Scattering Measurements
The static and dynamic light scattering measurements wereperformed using an ALV-5000 digital correlator system(Langen-GmbH, Germany) fitted with a temperature controlset at 25� 0.1 8C. The scattered light, vertically polarized withan l0¼ 632 nm Argon laser (30 mW), was measured atdifferent angles in the range of 408–1508. The reduced elasticscattering I(q)/KC, with K¼ 4p2n0
2(dn/dc)2(I90/R90)/l04NA, was
measured in scattering angle steps of 108, where n0 is therefractive index of the standard (toluene), I90and R90are theintensity and the Rayleigh ratio of the standard at y¼ 908,respectively, C is the sample concentration in g � cm�3, I(q) isthe intensity scattered by the sample after subtraction ofthe scattering contribution of solvent, and NA is Avogadro’snumber. The increment of the refractive index (dn/dc) wastaken as 0.150 ml � g�1, a value previously found for varioussamples of gum arabic in several previous studies.[37]
This same dn/dc value was also used for mesquite gum andfor the fractions of both materials. The scattering angle q isgiven by q¼ (4pn/l0)sin(y/2) and n is the refractive index ofthe medium (n¼ 1.33). The data were analyzed using the wellestablished Zimm equation.[38]
Kc
Ry¼ 1
MwPðyÞþ 2A2C ðC ! 0; y ! 0Þ ð1Þ
where Ry is the Rayleigh ratio for the angle y, A2 is the secondvirial coefficient of the molecule, Mw is the weight-averagemolecular mass and P(y) is the form factor which depends onthe shape of molecules and is approached for small q by1 � 1
3R2
Gq2 and RG is the radius of gyration of the molecule.Several form factors were tested for different geometriesaccording to the information given in the software of theinstrument (ALV 5000/E/WIN Software).
The hydrodynamic radius, RH, was obtained for dilutedsamples from dynamic light scattering measurements at anincidence angle of 908 through the Stokes-Einstein relation:
D0 ¼ kBT
6pZRH
ð2Þ
Macromolecular Dimensions and Mechanical Properties of Monolayer Films of Sonorean Mesquite Gum 867
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
where kB is the Boltzmann constant, T is the absolute temper-ature and Z the viscosity of the solvent. Samples were dissolvedin 20-mmol � dm�3 NaCl solutions at 3, 5, 7 and 10 mg �ml�1.Solutions were filtered twice with 0.8mm filters (Millipore) andonce with 0.2 mm filter.
p/A Isotherms
Compression isotherms (p/A diagrams) were established using aNima Langmuir-Blodgett balance (Model 611) at 25� 0.1 8C.Instrument surface tension precision was 0.1 mN �m�1. Thecompression rate was kept constant at 50 cm2 �min�1 and thesubphase was 1 mol � dm�3 KCl solution.
Avolume of 100 ml aqueous solution (10 wt.-%) of a samplewas spread onto the water surface using a Hamilton microsyringe with its needle very near the surface. Purity of thesubphase was checked by compression of the surface withoutsample. The surface was regarded as clean until the pressurewas lower than 0.5 mN �m�1. The elapsed time after spreadingthe sample and before compressing was 40 min. The pH valuesof whole mesquite gum and whole gum arabic solutions were�4.5 and �4.3, respectively.
Results and Discussion
Hydrophobic Interaction ChromatographicFractionation
Figure 1 shows the elution chromatographs corresponding
to the separation of mesquite gum and gum arabic in order
of increasing hydrophobicity. The observed UV profiles
(l¼ 280 nm) are proportional to the proteic fraction
associated with the eluting species. Gum recovery, protein
contents and specific optical rotation for these fractions are
given in Table 1. Notice that both materials yield three
major fractions following step-wise elution with 4.2 mol �dm�3 NaCl, 2.0 mol � dm�3 NaCl and water. The peak
described by the least hydrophobic component in mesquite
gum (Fraction I) shows an almost identical elution pattern
to the equivalent fraction of gum arabic. This peak
represents the major component of mesquite gum material,
comprising 94.77 wt.-% of the recovered gum and has a
7.18 wt.-% protein content. This fraction can be regarded as
the AG fraction. The second peak in mesquite gum eluted
with 2.0 mol � dm�3 NaCl and appeared split in further two
sub-fractions. These were labeled as IIa and IIb and studied
separately and can be regarded as the constituents of an
AGP fraction. Fraction IIa accounted for 2.49 wt.-% of the
Table 1. Physicochemical analysis of fractions of mesquite gum (P. velutina) and gum (Acacia spp.), isolated by hydrophobic affinitychromatography.
Material Fraction Eluent Gum recovery Total sugars Protein [a]D58920
% % %
Mesquite Gum Whole 93.13 5.90a) þ55.10(P. velutina) I 4.2 mol � dm�3 NaCl 94.77 91.50 7.18a) þ57.15
IIa 2 mol � dm�3 NaCl 2.49 76.80 23.20b) þ46.64IIb 2 mol � dm�3 NaCl 1.17 n.d.c) n.d.c) n.d.c)
III H2O 1.56 61.40 38.60b) þ27.61Gum Arabic Whole 99.20d) 2.24d) �28.77(Acacia spp.) I 4.2 mol � dm�3 NaCl 89.08 97.00d) 0.35d) �26.29
II 2 mol � dm�3 NaCl 9.40 91.20d) 11.18d) �32.52IIIa H2O 1.42 45.20d) 47.30d) n.d.c)
IIIb H2O 0.10 n.d.c) 50.00d) n.d.c)
a) Protein conversion factor of 6.53 for mesquite gum[34] and 6.60 for gum arabic.[35]
b) Calculated by difference from the total sugar content.c) n.d. not determined because sample amount was not enough.d) Randall et al.[23]
Figure 1. Elution UV profile (l¼ 280 nm) of mesquite gum andgum arabic following hydrophobic interaction chromatography onPhenyl-Sepharose CL-4B. Fractions were eluted (step-wise) using4.2 mol � dm�3 NaCl, 2.0 mol � dm�3 NaCl and MilliQ water.
868 Y. L. Lopez-Franco, M. A. Valdez, J. Hernandez, A. M. Calderon de la Barca, M. Rinaudo, F. M. Goycoolea
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
recovered gum and had a 23.2 wt.-% protein content, while
the recovered amount of Fraction IIb was not enough to
allow its analytical characterization. A third fraction was
eluted as a single peak and represented 1.56 wt.-% of the
recovered weight of the gum and had the highest protein
content, 38.6 wt.-%, and can be compared with the GP
fraction proposed for gum arabic. In the case of gum arabic,
the first major peak represented 89.08 wt.-% of the material
(Fraction I or AG fraction). The second peak, Fraction II
(AGP fraction), eluted as single monomodal profile,
represented 9.40 wt.-%, while the third peak, Fraction III
(GP fraction) appeared split in two fractions and both
accounted for 1.52 wt.-%. The results found for fractionated
gum arabic compare very well with those previously
documented data using an identical experimental protocol
for hydrophobic chromatographic separation.[23,29,37]
While the results on mesquite gum almost coincide with
those of Orozco-Villafuerte et al.,[25] differing essentially
in that Fraction III is reported as eluting as two components.
This difference may stem on subtle structural differences in
the glycoprotein components associated with the distinctive
species botanical origin.
Further physicochemical information of mesquite gum
and gum arabic and their various comprising fractions was
provided by the data given in Table 1 of the specific optical
rotation, [a]D20. It is interesting to notice that as the hydro-
phobic nature of the constituent species increases in
mesquite gum, the material becomes less dextrorotatory.
It can be argued that this is a consequence of the varying
composition in the protein fraction or in the nature of the
sugar fraction. It is also shown that [a]D20 is nearly inde-
pendent on the protein yield.
Macromolecular Dimensions Characterization
Until recently there was almost no biophysical data about
the size, conformation and molecular weight of mesquite
gum. We have used multi-angle laser light scattering to
deepen the present understanding of the macromolecular
properties of mesquite gum and its fractions and compare
them with those of gum arabic. Figure 2 shows representa-
tive Zimm plots obtained for both whole materials obtained
by static light scattering used to derive RG and Mw data.
Gum arabic has been investigated for several decades
due to its widespread use worldwide. According to Randall
et al.,[23] the AGP fraction of gum arabic has an RH¼22.8� 0.5 nm corresponding to a molecular mass of
1.45� 106. The AG fraction had a RH¼ 9.2� 0.5 nm
corresponding to a molecular mass of 2.79� 105. Similar
results were found by Osman et al.[39] They identified the
individual AG, AGP and GP components and found a RH for
the AG fraction between 8 and 12 nm with a small propor-
tion of the AGP fraction with an RH¼ 30 nm.
Islam et al.[40] found that the molecular weight of the
whole gum arabic was about 560 000 and their results
were consistent with the ‘‘wattle blossom’’ proposal to
describe the tertiary structure of the AGP fraction by virtue
of which a number of polysaccharide domains are linked to
a common polypeptide chain, a model proposed originally
by Fincher et al.[17] for AGPs. Later, Connolly et al.[26]
calculated the molecular mass of the blocks of polysacchar-
ide obtained after treatment with pronase of the order of
2� 105. Williams and Langdon[41] undertook GPC studies
monitoring the column eluent simultaneously by UV,
refractive index, on-line photon correlation spectroscopy
and on-line multi-angle laser light scattering (MALLS)
detection, measured the RH and RG of gum arabic. They
found that the ratio RG/RH was a function of the shape of the
molecule. For spheres this ratio has a value of 0.788 while
for coils and rods it has a value >2.0. A ratio RG/RH� 1 for
all the eluting species was consistent with the proposal of
the highly branched compact structure. One of the few
works that has addressed the value of RH of whole mesquite
Figure 2. Zimm plot of gum arabic (a) and mesquite gum (b) in2 mmol � dm�3 NaCl at 25 8C.
Macromolecular Dimensions and Mechanical Properties of Monolayer Films of Sonorean Mesquite Gum 869
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
gum is by Goycoolea et al.[18] Using dynamic light scat-
tering to investigate the dependence of RH on the ionic
strength, the values of RH were found to vary from 8.2 to
10.6 nm. However, no data of RG were available to date on
mesquite gum and its fractions.
The results for Mw, RH and RG on mesquite gum and gum
arabic and their fractions are given in Table 2. As observed
for the case of gum arabic, the results for Mw for the
Fractions I and II are similar to the ones found by Randall
et al.[23] A difference of 20% is found in the case of our Mw
for the whole gum arabic and the value found by Islam
et al.[40] When we compared our own results with the ones
found by Williams and Langdon,[41] it was found that the
values of the ratio RG/RH� 1� 0.5, for whole gum and
Fraction III of gum arabic were in reasonably good
agreement. However, note that for the Fraction II of gum
arabic, RG/RH> 2, a result that is in conflict with the idea
that the AGP fraction has a predominantly branched
compact structure assumed by the ‘wattle blossom’
assembly. Indeed, a ‘‘twisted hairy rope’’ model has been
proposed to account for the tertiary structure of gum arabic
AGP fraction as a competing model to the ‘wattle blossom’
one.[28] Our values agree reasonably well with the
previously reported Mw values for mesquite gum fractions
separated using a GPC system calibrated with protein and
dextran standards.[25] However, the use of standards may
not give very accurate results in elongated macromolecules.
The value of RH found in this study for whole mesquite
gum is very close to the one previously found.[18] Yet, the
ratio RG/RH> 2 for whole and fractionated species, indicate
the possibility of an elongated structure for all the fractions
of mesquite gum. This result suggests the existence of
important differences in the structure between the corres-
ponding fractions of mesquite gum and gum arabic.
Figure 3a and 3b show the unweighted size (RH) dis-
tribution curves of both gum materials and their constituent
fractions. The various macromolecular species that com-
prise gum arabic show broad dispersion in size spanning
three orders of magnitude (2 to 1 000 nm). There is close
similarity in the monomodal distribution curves of whole
gum and Fraction I, as this fraction accounts for most of
the weight of the gum. Close inspection of the curves show
that Fraction I species have overall somewhat lower RH
values than do the species comprising the bulk material. In
turn, it can be appreciated that Fraction II has the narrowest
dispersion and it is also monomodal. Further inspection of
the data, reveals that molecular species of Fraction IIIa are
split under a bimodal size distribution. Fraction IIIb appears
to be of similar size than the larger species of Fraction IIIa.
In the case of mesquite gum, the registered size distribution
curves of the bulk material and its fractions (Figure 3b)
described very similar patterns to those of gum arabic.
Indeed, Fraction III also showed a clear bimodal size
distribution.
Dynamic light scattering studies with native mesquite
gum and gum arabic and their fractions indicate that all the
samples show some degree of polydispersity, so that our
Table 2. Molecular size for whole and fractionated mesquitegum in comparison with whole and fractionated gum Arabic.
Material Fraction Mw RG RH RG/RH
g �mol�1 nm nm
Mesquite gum Whole 3.86� 105 50.47 9.48 5.32(P. velutina) I 3.89� 105 49.82 9.61 5.27
IIa 4.84� 105 55.30 13.17 4.20IIb 3.82� 105 48.83 15.68 3.08III 8.06� 105 71.11 24.06 2.96
Gum Arabic Whole 4.39� 105 35.02 22.36 1.57(Acacia spp.) I 2.92� 105 52.26 14.96 3.49
II 1.19� 106 47.64 18.52 2.57IIIa 8.88� 105 40.93 43.58 0.94IIIb 1.34� 106 69.93 66.20 1.06
Figure 3. Unweighted probability distribution curves of thehydrodynamic radius of gum arabic and its fractions (a) andmesquite gum and its fractions (b).
870 Y. L. Lopez-Franco, M. A. Valdez, J. Hernandez, A. M. Calderon de la Barca, M. Rinaudo, F. M. Goycoolea
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
results of hydrodynamic radius must be interpreted
cautiously and as average values. Due to this polydispersity
and/or aggregation, perhaps as a consequence of the
polyelectrolyte character of the samples and concentration,
there are more than two relaxation modes in the correlation
curves and hence it makes very difficult to differentiate
them in pure slow and a fast relaxation modes, under a
treatment similar to the one conducted by Buhler and
Rinaudo on chitosan, a semiflexible biopolymer, using a
sample of low polydispersity (Mw=Mn ¼ 1.3).[42]
Static laser light scattering was used to characterize the
average molecular weight and the architecture (radius of
gyration) of the obtained gum fractions. By computing
the factor form for different molecular structures with
the available different configurations of the instrument, we
observed that whole mesquite gum and its fractions were
best fit by a model for a polydisperse coil structure with a
ratio Mw=Mn ¼ 2. In Figure 4 we show the Kratky function,
P(U)*U2 vs U for the three fractions of mesquite gum
(Figure 4d, 4e and 4f) and gum arabic (Figure 4a, 4b and 4c),
where U¼ q�Rg. Inspection of the curves of mesquite
gum Fractions I (Figure 4d), IIb (Figure 4e) and III
(Figure 4f) shows that in all cases the experimental data
deviate significantly from a globular (hard sphere) structure
and follow closely the polydisperse coil model. The result
for Fraction I is not very clear because of the large
fluctuation occurred in the experiment. While in the case of
gum arabic, Fractions I (Figure 4a), II (Figure 4b) and IIIa
(Figure 4c) all could be fitted by the three different models,
including the hard sphere one.
p/A Isotherms
Whole mesquite gum and gum arabic isotherms were
obtained by spreading the samples and compressing the film
balanceand then measuring thechange in thesurfacepressure
at the air-water interface. The corresponding compression
isotherms of the three fractions of both arabic and mesquite
gums are shown in Figure 5a and Figure 5b, respectively. It is
important to stress the fact that gum samples in the present
study were not dissolved in a mixture of 2-propanol/water as
in the method used by Fauconnier et al.[31]
Inspection of Figure 5a shows that the bulk material and
three fractions of gum arabic present essentially the same
behavior at large surface area per milligram of gum. Further,
as the surface areas are reduced during compression there is a
gradual increase in surface pressure and some kind of
condensed phase is invariably attained in all cases. Closer
inspection shows that the Fraction IIIa isotherm lies at
somewhat greater values than whole gum and the rest of the
Figure 4. Form factor for fractions I (a), II (b) and IIIa (c) from gum arabic and fractions I (d), IIb (e) and III (f) frommesquite gum obtained from static and dynamic light scattering.
Macromolecular Dimensions and Mechanical Properties of Monolayer Films of Sonorean Mesquite Gum 871
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
fractions even when both isotherms follow very similar
profiles. This is probably due to the fact that this fraction
contributes mostly to the hydrophobic properties of the gum
arabic, as it bears the highest protein content (Table 1).
Differences in amino acid composition profiles among the
various fractions, may also underlie their capacity to adsorb
at the air-water interface. It is expected that non-polar amino
acids will contribute significantly to the monolayer isotherm.
Another way of analyzing the p/A curves is by comparing
the surface area per milligram of gum at a fixed similar
surface pressure (5 mN �m�1). We observe that curves of
Fractions I and II of gum arabic achieve this pressure at
smaller areas than do whole gum and Fraction III species.
This could be directly related to the protein content, size
and/or shape (i.e., related to RG/RH) of the involved
molecules adsorbed at the interface. Synergy between the
AG, AGP and GP fractions of gum arabic has been sug-
gested to account for the greater surface pressure at the end
of compression (maximal surface pressure) attained by
whole gum arabic, which was higher than the corresponding
parameter for each fraction.[31] Our results seem to confirm
this hypothesis, as the contribution of Fraction III in weight
terms to the weight of whole gum is very small indeed so as
to claim an additive effect. Notice that the GP fraction
(Fraction IIIa and IIIb), has the greater RG values and lower
RG/RH ratio, indicative of a large yet spherical species. The
consequence of these features on the adsorption behavior
might be to allow closer packing at the air-water interface,
thus effectively attaining a lower surface tension (i.e.,
greater surface pressure) at smaller areas. Fraction II, or
AGP fraction, has a protein content of 11.18 wt.-% and a
RG/RH ratio of 2.57 (Table 2), indicative of a different
conformation than the spheroid one of Fraction IIIa and
IIIb, which is presumably closer to that of an elongated rod.
Inspection of the p/A isotherms of mesquite gum
(Figure 5b) reveals important differences in monolayer
properties among the various analyzed fractions. Notice
that isotherm of Fraction I lies consistently below the one of
the whole gum and starts to rise at lower surface area
(�0.003 m2 �mg�1). By contrast, isotherms of Fraction IIa,
IIb and III start to show elevation of the surface pressure at
the onset of the compression cycle. Besides, Fraction IIa
attains the highest surface pressure (�37 mN �m�1) at the
end of the compression cycle compared with the other
fractions, including those of gum arabic. In order to account
for the observed mechanical properties of the monolayers of
mesquite gum and its fractions differences in protein
content, macromolecular dimensions and conformation of
the constituent species need be considered. Fraction I, with
7.18 wt.-% protein content produces the lowest surface
pressure, roughly in line with Fraction I of gum arabic.
Although the protein content of this fraction appears to be
greater by an order of magnitude than Fraction I of gum
arabic, the presence of protein by itself is not sufficient
so as to modify the interfacial adsorption. Therefore, the
importance of the conformation and topology of the
molecule and the hydrophilic/hydrophobic sites adsorbed
at the interface is highlighted. It is interesting to notice that
Fraction I of mesquite gum had a large deviation from the
globular conformation as assessed from the RG/RH ratio
�5.27, pointing towards an elongated structure. This
coincides with the isotherms of the fractions of gum arabic,
where also Fraction I with the highest value of RG/RH give
the lowest surface pressure value at smaller areas. By
contrast, the species curves with lower RG/RH ratios start
rising at larger areas (Fractions IIb and III).
Comparing the behavior for the isotherms of both whole
gums, we observe that their isotherms reach the surface
pressure of 5 mN �m�1 at the approximate the same area
(�0.002 m2 �mg�1). In spite of the fact that two isotherms
of the fractions of mesquite gum reached the highest
pressure, gum arabic reached a higher surface pressure at
the smallest compressed area compared with the value
reached by the whole mesquite gum. This can be explained
with the fact that mesquite gum contains a somewhat larger
Figure 5. Compression isotherms of gum arabic and its fractions(a) and mesquite gum and its fractions (b).
872 Y. L. Lopez-Franco, M. A. Valdez, J. Hernandez, A. M. Calderon de la Barca, M. Rinaudo, F. M. Goycoolea
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
proportion of the Fraction I, which does not produce a
noticeable surface pressure as can be observed in Figure 5b.
The analysis of the monolayer mechanical properties was
complemented by deriving the parameters for the limiting
area, namely A0 and e. A0 denotes the limiting area which is
determined by extrapolation at the intersection of the abs-
cissa axis with the tangent of the isotherm atp¼ 2 mN �m�1
and e ¼ �A dpdA
is the film elasticity measured from the
isotherm curve at the final part (pressure around
20 mN �m�1) of the isotherm.
From the results shown for both gums in Table 3, we
observe that A0 follows the same behavior for both fractions
of the gums. The value for A0 grows in the order Fraction
I< Fraction II< Fraction III. Whole and Fraction I of both
gums share very close A0 values among themselves. The
result agrees with the one found by Fauconnier et al.[31] for
two different samples of gum arabic. However, in our case,
the value of A0 for the whole samples is larger than the one
for the corresponding A0. This result agrees well with the
fact that the RH of Fraction I is the smallest for both gums
fractions. A0 indicates monolayer expansion and in our case
we observe that Fraction III is the most expanded for both
gums. In the case of gum arabic, Fraction III together
with Fraction II (AGP fraction) give the most important
contribution to the interfacial properties of whole gum
arabic isotherm.[31]
We also notice that fractions of both mesquite and arabic
samples show the same behavior for monolayer expansion.
This is larger for Fraction III, then follows Fraction II and
this value is shorter for Fractions I. This is according
with the behavior of the hydrodynamic radius as also
noticed by Fauconnier et al.[31] Comparing A0 for each
fraction of both gums, we notice that A0 is almost the same
for the whole gums and is larger for Fractions II and III of
mesquite gum than the corresponding fractions of gum
arabic, indicating strong differences of the molecules at the
air-water interface.
In the case of the film elasticity e; this parameter
measures the film elasticity and the resistance to the change
of area.[43] We observe in Table 3 that most of the fractions
and the whole gum of gum arabic produce more resistant
films than the corresponding ones of mesquite gum. Only
the film of Fraction IIa of mesquite gum results more
resistant than any other measured film of both gums. As
mentioned by Fauconnier et al.[31] this behavior explains
the excellent emulsifying properties of gum arabic. We
compared the behavior of both whole gums at the air-water
interface by performing one complete cycle by compres-
sing and expanding to the original area. Figure 6 shows
the results of this experiment for both gums. Clearly, the
hysteresis area is very small probably because of the
flexibility of molecules at the interface.[44] On the other
hand, the curves do not return to the original pressure, this
would mean that compressing and expanding molecules at
the interface produces some changes on the molecular
structure that could improve their hydrophobicity. These
Table 3. Limiting area and film elasticity extracted fromisotherm curves of gum arabic (Acacia spp.) and its fractionsand mesquite gum (P. velutina) and its fractions.
Material Fraction A0 e
m2 �mg�1 mN �m�1
Mesquite Gum Whole 3.50� 10�3 19.20(P. velutina) I 2.37� 10�3 20.00
IIa 5.10� 10�3 34.50IIb 5.97� 10�3 12.01III 5.64� 10�3 5.26
Gum Arabic Whole 3.55� 10�3 33.40(Acacia spp.) I 2.88� 10�3 24.80
II 4.30� 10�3 25.28IIIa 4.95� 10�3 25.44IIIb n.d.a) n.d.a)
a) n.d. not determined because sample amount was insufficient.
Figure 6. Cycle of compression and expansion of the spreadmonolayer of gum arabic (a) and mesquite gum (b).
Macromolecular Dimensions and Mechanical Properties of Monolayer Films of Sonorean Mesquite Gum 873
Macromol. Biosci. 2004, 4, 865–874 www.mbs-journal.de � 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
phenomena indicate that both samples behave as real mono-
layers at the air-water interface and that most of molecules
at the interface stay bounded at the interface after com-
pressing and expanding the area of the film without loosing
any appreciable amount of molecules at the interface.
Conclusion
The characterization of mesquite gum showed differences
in its chemical surface properties in comparison with gum
arabic. According to the structure factor and light scattering
evidence, the macromolecular structure of mesquite gum
resembles a polydisperse macrocoil in agreement with the
‘‘twisted hairy rope’’ proposal advanced for gum arabic
AGP in previous studies. Such structural differences be-
tween the constituent macromolecular species comprising
both materials underlie also different adsorption and mec-
hanical properties when they are adsorbed as a monolayer at
the air-water interface. They also confirm that mesquite
gum is an effective natural emulsifier.
Acknowledgements: CONACYT is gratefully acknowledgedfor grant No. ER074 ‘‘Materiales Biomoleculares’’ and forstudentship support to one of us (Y. L. Lopez-Franco).
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