Inorganica Chimica Acta xxx (2014) xxx–xxx

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Inorganica Chimica Acta

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Theoretical study of the structure, IR and NMRof the bis-peroxo-oxovanadate species containing-histidine peptides

http://dx.doi.org/10.1016/j.ica.2014.03.0360020-1693/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +55 (222)2295500x2830; fax: +55 (222)2295584.E-mail address: francisco.melendez@correo.buap.mx (F.J. Melendez).

Please cite this article in press as: F.J. Melendez et al., Inorg. Chim. Acta (2014), http://dx.doi.org/10.1016/j.ica.2014.03.036

Francisco J. Melendez a,⇑, Abdiel Degollado a, Maria Eugenia Castro b, Norma A. Caballero c,J. Antonio Guevara-García d, Thomas Scior e

a Lab. de Química Teórica, Centro de Investigación, Dpto. de Fisicoquímica, Facultad de Ciencias Químicas, Universidad Autónoma de Puebla, Edif. 105-I, San Claudio y 22 Sur,Ciudad Universitaria, Col. San Manuel, Puebla, Pue. 72570, Mexicob Centro de Química, Instituto de Ciencias, Universidad Autónoma de Puebla, Complejo de Ciencias, ICUAP, Edif. 103H, 22 Sur y San Claudio, Ciudad Universitaria, Puebla,Pue. 72570, Mexicoc Dirección de Profesional, Universidad TecMilenio campus Veracruz, Carretera Boca del Río-Antón Lizardo Km 5.5, Alvarado, Veracruz, Mexicod Laboratory of Research in Bioinorganic and Bioremediation (LIByB), Faculty of Basic Sciences, Technology and Engineering, Universidad Autónoma de Tlaxcala,Campus Apizaco, P.O. Box 140, 90300 Apizaco, Tlaxcala, Mexicoe Facultad de Ciencias Químicas, Universidad Autónoma de Puebla, Av. 14 Sur y San Claudio, Puebla, Pue. 72570, Mexico

a r t i c l e i n f o a b s t r a c t

Article history:Received 22 November 2013Received in revised form 23 March 2014Accepted 25 March 2014Available online xxxx

Keywords:Bis-peroxo-oxovanadate speciesHistidine peptidesIR and NMR spectraDFT calculations

The molecular structures, IR spectroscopy, 1H, 13C and 51V NMR spectroscopy of three bis-peroxo-oxo-vanadate species containing histidine peptides were studies on theoretical grounds. The geometry opti-mizations and IR spectra in gas phase were carried out with the DFT (B3LYP functional) method and thespectra of the peptides under study were obtained with the gauche-including atomic orbital (GIAO)method as implemented in the GAUSSIAN 09 package. The standard 6-31+G⁄ basis set and core-valenceeffective core potential Lanl2dz were employed. New coordination modes were found in the potentialcurves for the rotation through the V�N coordination bond. They bent up and down (Bu, Bd) with theimidazole ring aligned with the V–O(peroxo) bond and also bent with respect to the V@O group. In addi-tion, the parameters describing quadrupolar and chemical shift anisotropy interactions are determinedtheoretically. In particular, this results help to solve apparent inconsistencies found in the 51V NMR solu-tion spectroscopy of the three bis-peroxo-oxovanadate complexes. The gathered evidence furthers theunderstanding of the electronic implications and structure–activity relationships in the case of histidinepeptide-containing bis-peroxo-oxovanadate structures.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

In the last years, the behavior of the vanadium coordinationcomplexes became of great interest due to their active participa-tion in different biological mechanisms, such as biomimetic catal-ysis [1], DNA photocleavage activity [2,3], cellular metabolicactivity [4,5], activation/inhibition of phosphate-dependentenzymes [6–8], calcium metabolism [9,10], gene expression [11],or structure/function of haloperoxidases [12–14], among other.The mono- and bis-peroxo vanadates complexes are of specialinterest due to their insulin mimetic properties [15–18] and theactivation/inhibition mechanisms of cancer [19,20].

In the 90’s, Conte et al. focused on the amino acid histidinebecause they were intrigued by its linkage to the coordinationsphere of vanadium in biological systems [21–23]. Other research

groups studied the interaction between mono/bis peroxo-vana-dates and peptides in solution by 51V NMR [24,25]. In particular,it has been demonstrated that histidine-containing vanadium com-plexes yield more stable products than those using other peptides[24]. Up to now, the X-ray structure of mono/bis peroxo-vanadateslinked to histidine peptides has not yet been reported. However,the coordination forms of vanadium linked to imidazole ring ofthe histidine residue have been characterized by either spectro-scopic techniques [22–25] or theoretical studies [26–35].

Precisely, such theoretical studies have had importance in solv-ing problems of biological interest, involving mono/bis peroxo-vanadates species. For example, QM/MM theoretical calculationswere performed to elucidate the coordination forms of vanadiumwith vanadium-dependent enzymes using vanadates [26–28]. In2009, Geethalakshmi et al. [28] published 51V NMR chemical shiftsat B3LYP/AE1 + theory level for the linkage of peroxo-vanadateswith vanadium-dependent enzymes. Moreover, B3LYP theoreticalstudies of the optimization of the peroxo-vanadates linked to

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peptides, using an effective core potential with Lanl2dz basis setfor 51V, were conducted in order to determine the coordinationsphere of vanadium in these compounds [29–31]. On theoreticalgrounds, Conte et al. [32–34] studied the ligand system NH4VO3/H2O2/histidine as a mimetic model of haloperoxidases active siteusing ab initio calculations. Then, Yu et al. [35] studied the reactionbetween of diperoxovanadates with organic ligands [OV(O2)2L]n�

(n = 1 or 3, L = oxalate or H2O) in solution. Their studies were com-plemented with B3LYP theoretical calculations including effectivecore potential with Lanl2dz for 51V and PCM solvent model toexplain the experimental observations. The authors concluded thatsolvation effects stabilize one particular isomer over all others andthat has significant implications on the reactivity and stability ofvanadium-containing systems with regard to biological functions.

In the present work, a theoretical study of the molecular struc-tures and the spectroscopic characterization by IR and 1H, 13C and51V NMR of three histidine peptide-containing bis-peroxovanadatecomplexes, [VO(O2)2(H2O)]� was carried out. The complexes corre-spond to the reaction products of [VO(O2)2(H2O)]� with Gly-L-His(GH), Gly-Gly-L-His (GGH), and Gly-L-His-Gly (GHG). Their com-plexes structures were fully optimized by DFT calculations withthe B3LYP functional. The LANL2DZ pseudopotential for the vana-dium atom and the 6-31+G(d) basis set for the remaining atomswere used. The characterization of the vibrational normal modeswas achieved applying the same theory level, the signal correspond-ing to the characteristic V@O stretching was adequately assigned,confirming the experimental data. In addition, the calculations ofthe vibration frequencies indicated that the optimized structuresof the complexes correspond to minimum energy structures. The1H, 13C and 51V NMR spectra characterization was achieved thanksto the GIAO method with the same basis set. In particular, the calcu-lated 51V chemical shifts are in good keeping with the experimentaldata from literature [25]. Finally, the solvent effects (with D2O andDMSOd6) were evaluated using the popular PCM model.

2. Computational methods

Calculations on the electronic ground state of vanadium (V)complexes were carried out using B3LYP density functional theory[36,37] and the 6-31+G(d) basis set was applied to all ligands [38].A relativistic effective core potential (ECP) [39] on V atom was usedto replace the inner core electrons keeping the outer core electrons(13 explicit electrons for neutral V). The geometries were fullyoptimized without symmetry constraints.

Vibrational frequencies were calculated to obtain the IR spec-trum and to identify all true local minimums (only real frequen-cies), while frequencies were scaled by a 0.9600 factor [40]. NMRchemical shielding calculations were performed for the complexesunder the gauge- independent atomic orbital (GIAO) [41] methodfrom the B3LYP optimized geometry. Calculations were conductedin gas phase and with the PCM model [42,43] to describe the sol-vent effect, using D2O (e = 88.0) and DMSOd6 (e = 46.7). In theapproximation, the cavity is created by a series of overlappingspheres. At each self-consistent field (SCF) step, we calculatedthe reaction field in the solvent due to the electrostatic field ofthe solute wave function. The aforementioned reaction field wasincluded in the Fock operator (Kohn–Sham Hamiltonian) in orderto calculate the orbitals of the density functional wave functionof the solute. All calculations were performed under Gaussian 09[44].

3. Results and discussion

The results are divided into two parts: structural parameters ingas phase, including the solvent D2O and DMSOd6 effect and a

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spectroscopy study of IR and NMR of 1H, 13C and 51V in both phasesfor the three complexes. The latter were synthesized by thepeptides with V2O5 and NH4OH reaction [25].

3.1. Molecular structures

The optimized bond lengths and angles in the three GH, GGHand GHG complexes which were obtained by means of DFT meth-ods are given in Table 1, while the labeling of atoms included the[VO(O2)2(H2O)]� group coordinated to imidazole ring is plotteden Fig. 1.

In the active site of some haloroperoxidases, for example, inchloroperoxidase from the fungus Curvularia inaequaliz, HIS496coordinates to vanadate (V) by its e–N [45,46]. The start geome-tries of the three complexes, GH, GGH and GHG, were obtainedfrom an X-ray study of bis-peroxo oxovanadate ion [16] as wellas theoretical studies concerning the ion in coordination with his-tidine [28–30]. The optimized structural parameters in the gasphase and those, including the solvent D2O and DMSOd6 effectsfor the three complexes calculated at the B3LYP/6-31+G(d) leveland using the Lanl2DZ basis set of Wadt and Hay [39,47,48]employing an effective core potential (ECP) are summarized inTable 1.

In general, for the GH, GGH and GHG complexes, the values ofbond lengths, bond angles and dihedral angles did not show signif-icant changes among these with the B3LYP method and they are inaccordance with those previously reported for other vanadiumcomplexes similar [29,30] to those calculated in this work. There-fore, it is confirmed that the methodology used in this study is ade-quate for different molecules of this kind [49]. In all optimizedgeometries no important differences were found, the geometricparameters for VO(O2)2 moiety changes very little between thegas phase values and those with the solvent D2O and DMSOd6

effect in the three complexes. However, several particular differ-ences are seen (Table 1).

From the data it becomes evident that the r(O4–H16) and r(O6–H17) bond lengths calculated from the three complexes vary inorder of GH > GHG > GGH and GGH > GHG > GH, respectively inthe both phase. Furthermore, there is a variation of 0.031 and0.141 Å between the gas phases and the solvent effect respectively,for the three complexes. Also it can be seen from Table 1 that r(V1–N10) only vary 0.034 Å between both phases.

In these complexes studied here, it was found that ligands bindto metal atoms by using the e–N, the bong lengths in the imidazolering lie in the range of 1.322–1.384 Å, as found for the imidazole inthe solid state [35,50]. On the other hand, in GH complex the val-ues are 1.581, 1.242 and 1.273 Å for r(C16–C17), r(C17–R18) andr(C17–O19) in gas phase, respectively, and in the range of 1.257–1.565 Å for another phase; both sets of data are similar to GGHand GHG complexes. In addition, the r(C17–R18) bond length cal-culated vary in order of GH > GGH > GHG for the gas phase and sol-vent effect, respectively. Therefore, this effect can be due to the sizeof the substituent.

However, the r(C21–O22) and r(N20–H) bond lengths calcu-lated are shorter in GGH than in GH and GHG complexes, respec-tively and both phases. Similarly, in GH complex the values are1.988, 1.957 and 0.997 Å for r(O3–H9), r(O5–H8) and r(O22–H),respectively, shortest that in GHG and GGH complexes and in therange of 0.996–2.032 Å. In contrast, all the bond lengths are moreshort with solvent effect, however, the r(N20–C21) and r(N13–H)bond lengths calculated have higher values 1.023–1.324 Å in gasphase.

Concerning the valence angles and planarity, the anglesbetween atoms O3–V1–O5 and O4–V1–O6 calculated for GGHcomplex was slightly smaller than for GH and GHG complexes,1�, which is in agreement with reported by Yu et.al. [35]. In both

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Table 1Optimized geometrical parameters of the equilibrium structures of the complexes GH, GGH and GHG at B3LYP/6-31+G(d) level plus the Lanl2DZ basis set. The numberingconvention is shown in Fig. 1. Bond lengths in Å and angles in degrees.

Parameters Gas phase Solvent effect

D2O DMSOd6

GH GGH GHG GH GGH GHG GH GGH GHG

Bond lengthsV1@O2a 1.599 1.592 1.599 1.602 1.603 1.603 1.602 1.603 1.603V1–O3a 1.895 1.894 1.893 1.901 1.898 1.897 1.901 1.898 1.897V1–O4a 1.857 1.867 1.858 1.861 1.863 1.863 1.860 1.863 1.863O3–O4a 1.444 1.426 1.444 1.446 1.445 1.445 1.446 1.445 1.445O5–O6a 1.445 1.424 1.445 1.446 1.447 1.446 1.446 1.447 1.446V1–N10a 2.167 2.141 2.165 2.127 2.121 2.122 2.128 2.122 2.123O4–H16a 2.497 2.432 2.444 2.469 2.397 2.408 2.475 2.397 2.408O6–H17a 2.358 2.486 2.432 2.450 2.699 2.551 2.445 2.968 2.548N10–C11 1.374 1.378 1.373 1.383 1.384 1.383 1.383 1.384 1.383C11–C12 1.374 1.374 1.375 1.374 1.375 1.375 1.374 1.375 1.375C12–N13 1.386 1.382 1.386 1.381 1.381 1.381 1.381 1.381 1.381N10–C14 1.322 1.352 1.323 1.331 1.331 1.332 1.331 1.331 1.332C12–C15 1.495 1.498 1.495 1.499 1.499 1.500 1.500 1.500 1.500C15–C16 1.553 1.554 1.555 1.557 1.559 1.557 1.557 1.560 1.557C16–C17 1.581 1.578 1.563 1.568 1.565 1.564 1.568 1.565 1.564C17–R18 1.242 1.256 1.295 1.262 1.268 1.310 1.262 1.267 1.310C17–O19 1.273 1.260 1.289 1.257 1.257 1.274 1.257 1.257 1.274C16–N20 1.475 1.466 1.473 1.471 1.459 1.469 1.471 1.459 1.470N20–C21 1.284 1.324 1.283 1.299 1.342 1.297 1.298 1.341 1.297C21–O22 1.314 1.246 1.315 1.298 1.241 1.300 1.298 1.241 1.300CH2–R23 1.464 1.456 1.464 1.463 1.452 1.463 1.463 1.452 1.463O3–H9 1.988 2.032 1.993 1.913 1.932 1.933 1.914 1.935 1.933O5–H8 1.957 2.002 1.948 1.924 1.902 1.898 1.924 1.900 1.898N13–H 1.023 1.033 1.025 1.032 1.035 1.046 1.032 1.035 1.046N20–H 1.076 1.033 1.085 1.027 1.019 1.032 1.028 1.019 1.032O22–H 0.997 1.844 0.996 1.031 2.135 1.029 1.031 2.131 1.029

Bond anglesO3–V1–O5a 90.8 89.7 90.7 89.8 89.8 89.7 89.8 89.8 89.7O4–V1–O6a 133.0 134.0 133.2 136.1 136.0 136.0 136.0 136.0 136.0O3–V1–O4 45.3 44.5 45.3 45.2 45.2 45.2 45.2 45.2 45.2O2–V1–O3 109.1 109.1 109.5 106.8 107.7 107.4 106.9 107.7 107.5O2–V1–O6 112.4 112.2 112.2 111.2 111.3 111.4 111.3 111.3 111.4O5–H8–O7 149.5 148.3 150.3 148.1 150.1 150.2 148.1 150.4 150.1H8–O7–H9 98.6 99.3 98.5 98.9 98.9 98.9 98.9 99.0 98.9V1–N10–C11 126.5 126.3 125.4 127.5 125.9 126.4 127.5 125.9 126.4C12–N13–C14 107.7 108.1 105.5 108.7 105.3 108.5 108.6 108.8 108.5R18–C17–O19 129.6 128.7 130.3 127.4 126.4 128.8 127.5 126.4 128.9N20–C21–O22 121.5 126.2 121.7 121.1 124.3 121.3 121.1 124.3 121.3Dihedral anglesO2–V1–O3–O4 102.9 103.0 102.6 103.8 102.8 103.1 103.8 102.8 103.1O5–H8–O7–H9 4.5 6.9 5.7 4.1 5.4 4.3 4.0 5.4 4.3O2–V1–N10–C14 �85.4 �94.7 �97.1 �84.9 �110.7 �98.4 �84.5 �110.6 �98.3N10–V1–O4–H24aV1–N10 �22.1 �13.8 �13.0 �20.2 0.98 �9.3 �20.7 0.88 �9.4N10–V1–O6–H25aV1–N10 14.2 23.1 23.7 16.6 32.2 26.2 16.1 32.1 26.1C11–C12–C15–C16 123.1 128.8 117.7 133.7 133.8 133.0 133.5 133.7 133.0H–N20–C21–O22 171.3 171.0 171.5 175.5 172.8 175.4 175.5 172.7 175.3O22–C21–CH2–R23 9.8 1.7 11.0 0.0 2.0 0.6 0.0 2.2 0.64

a Ref. [35]. Histidine molecule: 1.606; 1.884; 1.865; 1.448 Å; 91.2�; 133.9�; �21.9�; 21.6�.

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phases, these values are in the range of 89–91� and 133–136�,respectively. All three complexes showed an O3–V1–O4 angle ofca. 45�. The tetrahedral character is also observed in the [VO(O2)2]-� moiety, with valence angles O2–V1–O3and O2–V1–O6 in the107–112� range. The trends in the distortion of the water moleculeupon substitution of histidine-like ligands are correct, with anglessmaller than 109� for all complexes isolated and solvent effect. It isalso noted that with the histidine-like ligands substituents, theV1–N10–C11 angle vary in order of GH > GGH > GHG, while theC12–N13–C14 angle vary in order of GGH > GH > GHG with differ-ences of 2�. In the R18–C17–O19 and N20–C21–O22 angles theorder of the values is GHG > GH > GGH to both phases.

The dihedral angles experience a large variation for differentligands, except the O2–V1–O3–O4 angle whose values are in therange of 102–103�. Thus, on going from GH to GHG and then toGGH, the dihedral angle of O5–H8–O7–H9 in water molecule

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respect to [VO(O2)2(H2O)]� moiety increases from 4.5� to 5.7�and then to 6.9� in gas phase and decreasing values with solventeffect. On the other hand, in gas phase the values calculated ofthe dihedral angle O2–V1–N10–C14 are �85.4, �94.7 and 97.1�to GH, GGH and GHG complexes, respectively. However, takingaccount the solvent D2O and DMSOd6 effect, the order the valuesis GGH > GHG > GH. This can also be seen for the values calculatedof torsional angles N14–V1–O4–H24 and N14–V1–O4–H25 that ongoing from GHG to GGH and then to GH, increases from �13.0� to�13.8� and then to �22.1� and decreases from 23.7� to 23.1� andthen to 14.2�, respectively. Comparing these torsional angles withother authors [35], it is observed that the substituent on the imid-azole is turned slightly out-of-plane. In contrast, the average values0.93� by GGH and �9.35� by GHG are lower with solvent effect.Finally, to the remainder of the molecular chain, the dihedral angleof O22–C21–CH2–R23 in GGH, GH and GHG complexes increases

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O6

O4

O5

O3V1

O2

N10

O7

H9

H8C11 C13

N13C14

H24

C15

H

H25C16

HH N20

C21O22 CH2

R18

O

R23

GHGGHGHG

ONCH2COOH

NH2HNCOCH2NH3NH2

Complex

C17

R18

O19

H

R23H

Fig. 1. Molecular structure of the three GH, GGH and GHG complexes coordinated to[VO(O2)2(H2O)]� moiety and, labeling of their main atoms.

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from 1.7� to 9.8� and then to 11.0�, diverging from almost 0� as thesize of the substituent when considering the solvent effect. In theFig. 2 shown the optimized structures for the three GH, GGH andGHG complexes in gas phase.

Table 2 shows the relative values of energy for each the com-plexes with respect to most stable to both phases. As can be seen,the three complexes are found to be the most stable in the solventD2O effect, followed by the solvent DMSOd6 effect and gas phase,with energy differences in the GH and GHG complexes of 1.12and 96.68 kcal/mol, respectively, whereas in the GGH complexwe obtained the greatest energy difference: 1.52 and 130.32 kcal/mol, respectively.

As expected, the GHG complex is found to be the most stable,followed by the GGH complex with energy difference in gas phaseof 4.64 kcal/mol and the GH complex with an estimated energy byexcess. In contrast, the GGH complex is found to be more stablethan the GHG complex in the solvent D2O and DMSOd6 effects,with energy differences of 29.00 and 28.61 kcal/mol, respectively.From these results, it is clear that the reactivity of the histidine-likeligands substituents (GH, GGH and GHG) depends on a combinedeffect of the intrinsic binding strength between ligand and[VO(O2)2(H2O)]� ion in the three complexes [35].

We obtained the following values of dipole moment for eachthe complexes, the values in gas phase are 17.45, 20.63, and15.95 Debye for the GH, GGH and GHG complexes, respectively.Whereas that in the solvent D2O effect, the values are 25.07,45.61 and 26.52 Debye, respectively, and 24.90, 45.48 and 26.40Debye, respectively, in the solvent DMSOd6 effect. The dataobtained are consistent with those previously reported for othervanadium complexes similar to those calculated in this study,therefore, is true to say that the level of theory used in this workfor the structural study of vanadium complexes is appropriate.

Fig. 2. Optimized molecular geometries of the three GH, GGH and GHG complexescoordinated to [VO(O2)2(H2O)]� moiety at B3LYP/6-31+G(d) level and using theLanl2DZ basis set in gas phase.

3.2. Infrared spectra

Fig. 3 shows the calculated frequencies in the 300–4000 cm�1

range at the B3LYP level and the 6-31+G(d) and Lanl2DZ basisset in gas phase and the solvent D2O and DMSOd6 effects of GHcomplex coordinated to [VO(O2)2(H2O)]– moiety. No imaginaryfrequencies in the three phases, indicating that the structurespredicted are a minimum on the potential energy surface. The

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experimental data reported for this compound shows two bandsthan those obtained from the peptides before forming the com-plexes. The experimental bands lie between 950 and 970 cm�1

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Table 2Relative energies values (kcal/mol) of the GH, GGH and GHG complexes at B3LYP/6-31+G(d) level plus the Lanl2DZ basis set in both phase.

Phase GH GGH GHG

Gas 96.68 130.32 96.68D2O 0.00a 0.00b 0.00c

DMSOd6 1.12 1.52 1.12

a �1280.7129 a.u.b �1488.6991 a.u.c �1488.7065 a.u.

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related to the stretching vibration of V@O group. In addition, at870 cm�1 another stretching vibration of the O–O group occurs,which is typical for peroxooxovanadate compounds [51,52].Theoretically, the IR spectra of the GH complex coordinated to a

Fig. 3. Infra-red (IR) spectra of the GH complex coordinated to the [VO(O2)2(H2O)]� mowere carried out in gas phase as well as for solvent D2O and DMSOd6 effects.

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[VO(O2)2(H2O)]– moiety look very similar in all three phases (seeFig. 3). In both phases can be seen very well to the vibrationsdue to m(V@O) and m(O–O) stretching, for example in the gas phaseshows a sharp peak at 1044 cm�1 related to the signal m(V@O) andother sharp peak shortly at 968–977 cm�1 assigned to the m(O–O)symmetric stretching mode.

Also this region shows a sharp peak at 959 cm�1 correspondingto mas(O–O) asymmetric stretching mode. Concerning the glycine(Gly) residue, the scissors deformations coupling with thed(HOH) bending mode of H2O was computed at 1751–1758 cm�1

with strong intensity. The stretching mode of Gly, denoted asm(GlyN–C), was computed at 1789 cm�1 and is more intense thanthe last. As can be seen in Fig. 3, several vibrations appear at higherfrequencies characterized by three sharp peaks at 3308, 3454and 3568–3595 cm�1 of relative intensities, corresponding to them(GlyN–C = O� � �H) stretching mode in the amide group, the

iety at the B3LYP/6-31+G(d) level including the Lanl2DZ basis set. The calculations

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Fig. 4. Infra-red (IR) spectra of the GH, GGH and GHG complexes. They are coordinated to [VO(O2)2(H2O)]�moiety at the B3LYP/6-31+G(d) level with the Lanl2DZ basis set fora typical gas phase situation.

Table 3Vibrational frequencies (cm�1) and relative infrared intensities (%) of the three complexes, GH, GGH and GHG, calculated at B3LYP/6-31+G(d) level plus the Lanl2DZ basis set ingas phase.

No. GH GGH GHG IR Frequencies Assignment

Frequencies Intensities Frequencies Intensities Frequencies Intensities

1 959 30 971 8 959 22 mas(O–O) + mas(O–O)2 977 11 987 100 976 12 ms(O–O) + ms(O–O) + d(Imi)3 1044 50 1056 44 1044 41 m(V@O)4 1751 81 1712 10 1720 52 das(COO) + m(C–H) + d(N–H) + d(H2O)5 1789 74 1771 5 1794 63 m(GlyN–C) + d(GlyO–H) + d(GlyCH2)6 3308 100 2858 26 3322 86 m(GlyN–C@O� � �H)7 3454 91 3281 51 3432 100 m(ImiN–H)8 3568 77 3608 62 3561 61 ms(O–H)water

9 3595 16 3645 1 3597 18 mas(O–H)water

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Table 4Theoretical and experimental 1H and 13C NMR chemical shifts (in ppm) of the three GH, GGH and GHG complexes calculated at B3LYP/6-31+G(d) level plus the Lanl2DZ basis set.The values are referenced against the chemical shift of TMS calculated at the same theory level.

Complex Atom Theoretical Experimentala

Chemical shifts, d (ppm)

GH Gas phase D2O DMSO-d6 Solid state D2O DMSO-d6

H1 3.88 3.63 3.58 – 3.62 4.79H3 4.48 4.33 4.50 – 4.38 4.60H5 2.65,3.03 3.03,3.10 2.89,3.00 – 2.89,3.11 3.15,3.47H7 8.30 7.08 7.70 – 7.06 7.37H8 8.26 8.04 8.70 – 8.00 8.68C1 39.38 35.91 40.15 41.6 35.8 46.7C2 157.40 161.16 161.31 167.3 161.8 177.4C3 61.13 49.63 52.53 54.8 49.9 56.0C4 170.08 171.34 172.24 177.0 172.2 179.8C5 32.43 22.45 28.55 28.1 22.8 27.9C6 125.28 124.73 132.34 128.8 123.8 129.8C7 123.12 119.44 116.65 117.8 120.3 117.2C8 131.43 130.04 134.83 137.0 132.4 133.2

GGH H1 3.60 3.85 3.56 – 3.77 –H3 3.71 3.98 3.68 – 3.86 –H5 3.74 3.86 3.57 – 4.41 –H7 3.08 2.98 2.70 – 3.02,3.16 –H9 7.63 7.29 7.00 – 7.13 –H10 8.10 7.77 7.50 – 8.10 –C1 45.42 41.40 34.66 42.6 40.8 –C2 168.68 157.35 148.48 170.0 167.9 –C3 43.94 42.45 35.68 42.6 42.7 –C4 151.40 160.44 171.60 170.0 170.5 –C5 58.06 56.82 48.09 56.1 54.4 –C6 163.45 170.56 175.74 177.4 177.0 –C7 30.87 29.72 22.97 27.7 27.5 –C8 129.03 126.32 123.48 128.7 125.0 –C9 121.28 120.10 111.39 120.0 117.0 –C10 129.02 135.32 139.57 136.6 137.1 –

GHG H1 3.51 3.73 3.82 – 3.68 –H3 3.83 4.18 4.47 – 4.53 –H5 4.20 4.03 3.59 – 3.74 –H7 2.87/2.90 2.89,3.02 2.88/3.01 – 3.09,3.20 –H9 7.88 7.38 7.18 – 7.18 –H10 7.19 8.02 7.09 – 8.09 –C1 43.38 41.42 33.64 42.6 40.7 –C2 155.68 167.67 158.81 171.0 167.2 –C3 53.26 56.15 51.42 53.0 53.0 –C4 158.79 163.82 155.00 171.0 171.8 –C5 49.39 50.87 42.13 42.6 43.5 –C6 170.09 173.30 164.47 176.0 176.6 –C7 32.21 31.27 22.55 28.0 27.1 –C8 127.25 126.65 117.75 127.4 125.6 –C9 123.53 119.95 111.23 118.0 117.8 –C10 131.43 136.17 120.48 137.1 137.6 –

a Ref. [25].

Table 5Theoretical and experimental 51V NMR chemical shifts (in ppm) of the three GH, GGHand GHG complexes calculated at B3LYP/6-31+G(d) level plus the Lanl2DZ basis set.The values are referenced to the chemical shift of VOCl3 calculated at the same theorylevel.

Complex Theoretical Experimentala

Chemical shifts, d (ppm)[51VO(O2)2�H2O]�

Gas phase D2O DMSO-d6 D2O

GH �686.71 �784.02 �782.92 �752.0GGH �706.15 �784.93 �783.84 �754.0GHG �674.51 �786.18 �785.02 �748.0

a Ref. [25].

F.J. Melendez et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx 7

m(N–H) stretching mode of the imidazole group and ms(O–H)symmetric stretching, as well as mas(O–H) asymmetric stretchingmodes in the water moiety, respectively. Hence, it is expectedthat the IR spectra – which were calculated with DFT method fromthe GGH and GHG complexes coordinated to [VO(O2)2(H2O)]�

moiety – exhibit similar characteristics of the infrared bandfrequencies in the gas phases when including the solventD2O and DMSOd6 effects. Furthermore, Fig. 4 displays the IRspectra calculated in gas phase for the three complexes of GH,GGH and GHG. All calculated frequencies are positive andcorrespond to minima. Table 3 constitutes a collection of the valuesof theoretical harmonic frequencies and intensities with morerepresentativeness thanks to the B3LYP method for the threecomplexes in gas phase. In Table 3, the 2nd, 4th, and 6th columnslist their relative intensities (%), which were normalized to theintensity of the strongest line. In addition, the table includes therelative contributions of the different modes regarding computedfrequency.

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3.3. 1H, 13C and 51V NMR spectroscopy

The theoretical spectra of the three complexes were calculatedby using GIAO with input data taken from optimized structures

), http://dx.doi.org/10.1016/j.ica.2014.03.036

Fig. 5. Schematic display of the variation between experimental and theoretical 51VNMR chemical shifts (in ppm) of the three complexes which are coordinated to[VO(O2)2(H2O)]� moiety (GH, GGH and GHG).

8 F.J. Melendez et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx

which were calculated at B3LYP level in the gas phase and also inthe solvent D2O and DMSO-d6. In order to express the chemicalshift, d (ppm), the 1H and 13C NMR spectra of the tetramethylsilane(TMS) molecule was determined applying the same method andbasis set for use as a reference case. The isotropic shielding con-stants values ri were transformed to chemical shifts in gas phaserelative to TMS using di (ppm) = rTMS – ri, where rTMS = 32.07and 190.85 ppm for H and C atoms, respectively. Furthermore,the data obtained from rTMS for H and C atoms are similar concern-ing the solvent effects. The energy levels studied in NMR spectros-copy treat the spin eigen-states of chemically bonder nuclei inpresence of an external magnetic field. Despite the complex natureof the interactions of the magnetic moments of the NMR-activenuclei with the magnetic field, the main features of NMR spectramay be satisfactorily accounted for by the solution of the energyequation that involve an effective spin-Hamiltonian, i.e., the elec-trons do not appear at all, and the nuclei are represented only bytheir intrinsic spins as well as their associated magnetic moments[53]. The 1H and 13C NMR data were recorded at 75.43 MHz with aJeol GSX-300 MHz in D2O and DMSO-d6 solution using TMS as thereference structure.

For compound GH it was possible to obtain the 1H and 13C NMRspectra in D2O and DMSO-d6 solution and for the solid state, whichwere compared with those obtained in the gas phase and the sol-vent D2O and DMSO-d6 effects at B3LYP theory level while using6-31+G(d) and plus Lanl2DZ basis set. However, for compoundGGH it was only possible to obtain the 13C NMR experimental spec-tra in the solid state and D2O solvent, which were compared atsame theory level. Identical experimental results were obtainedfor the GHG complex as shown in the same table. The experimental[25] and theoretical chemical shifts of 1H and 13C, in ppm, arerecollected and listed in Table 4.

In general, as it is shown in Table 4, the theoretical d (ppm) datalie in good keeping with experimental values for all three com-plexes (GH, GGH and GHG). Their d values for H were calculatedand appear in the 2.6–8.7 ppm range in both phases. Hence, itcan be assumed that the values calculated in the 2.0 – 4.0 ppmrange are well predicted. They lie in the range of chemical shift val-ues for typical protons experimentally measured at 2.6 ppm, whichare typically reported for ethyl moieties [54]. Also, the values wereconsistent with those reported in the literature for imidazole com-pounds at 8.5 ppm [55].

For the GH complex, the d for 13C in position of 1 to 8 (C1 to C8)were found with a strong shift, the higher frequencies of the signalsof the carbon atoms of the amino acid chain shows a coordination ofthe carbonyl oxygen atoms to the vanadium. The d values were cal-culated in the 60.0 to 170.0 ppm range in gas phase, while in thesolvent D2O and DMSO-d6 effect, the computational data reproducethe experimental values in the range of chemical shift values in thecase of 13C – all of which – assures us a good description of solvationtheoretical model (PCM) used to described the strong interactionwith the solvent. The 13C NMR signals of the carbon atoms attachedin the position of 1 to 10 for the GGH and GHG complexes alsoshowed theoretical chemical shifts in good agreement with theexperimental measurements. The values were calculated in the30.0–170.0 ppm range in gas phase for both complexes. In particu-lar, the average values which were obtained with respect to exper-imental data in solid state were 5.2 and 3.1 ppm, respectively.However, the error percentages slightly decrease upon using thePCM model approach with the solvent D2O effect. Thusly, the gasphase values of the 13C NMR spectrum of GH can be compared withthat obtained in simulations of the solvent D2O and DMSO-d6

effects. They indicate a possible interaction through the imidazolering and histidine carboxylate since they are slightly shifted tolower fields. Moreover, since the chemical shift values calculatedfor the GGH complex were found in the same range as the GH

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complex, similar effects on its NMR spectrum can be expectedand were indeed found. On the other hand, Guevara et al. wereunable to obtain good NMR spectra in DMSO-d6 solvent for theGHG complex because of the strong interaction of this solvent withthe complexes [25], therefore of the theoretical 13C NMR spectrumfor GHG complex, we can predict the chemical shifts for carbonsusing the PCM model in good approximation and reported our val-ues calculated in the 33.0–165.0 ppm range, indicating that there isan additional interaction through the carbonyl groups.

Intriguingly, the experimental signal on the 51V NMR spectra ofcoordination complexes of imidazole or N-methylimidazole bis-peroxovanadate was found in �750 ppm, possibly attributed tothe nitrogen coordination of the vanadium [61]. In addition, twosignals appeared at �750 and �740 ppm for the histidine coordi-nation complexes, which corresponds to the imidazole coordina-tion or a chelate made of the imidazole and carbonyl groups[61,62]. An HETCOR experiment could assign the resonances. 51VNMR spectra were recorded at 78.85 MHz, while chemical shiftvalues were referenced to VOCl3 by using spectral width 105 Hzand with number transients (64–300).

Moreover, 51V NMR spectroscopy has become an excellent ana-lytical tool for several vanadium compounds [56,57]. The geome-tries of the vanadium atom in many complexes do not changedramatically upon oxidation [58]. Previous studies revealed thatthe unusual shielding of the 51V resonances for vanadium catecho-late complex can be attributed to a number of structural incre-ments from regular substituted effects, and that the largestcontribution arises from the non-oxo nature of the these com-plexes [59]. The basic property, such as the electric field gradient(EFG), chemical shift anisotropy (CSA) and 51V chemical shift (withan accuracy of about 100 ppm), can be computed using appropriatethe conceptual framework of DFT [60].

In the present study also we reported the theoretical 51V NMRresults of the corresponding GH, GGH and GHG vanadatecomplexes. The results obtained at the GIAO-B3LYP/6-31+G(d)/Lanl2DZ theory level and experimental values for three complexes

), http://dx.doi.org/10.1016/j.ica.2014.03.036

F.J. Melendez et al. / Inorganica Chimica Acta xxx (2014) xxx–xxx 9

summarized in Table 5. The results for the experimental and theo-retical values summarized in this table showed that the bestapproach is when the solvent effect is considered. These vanadatecomplexes are characterized by d(51V) chemical shift values near�748, �752 and �754 ppm for GH, GGH and GHG complexes,respectively. While similar values are computed at the GIAO-B3LYP level for the three complexes with differences of Dd = 32,31 and 38 ppm and Dd = 30, 30 and 37 ppm including the solventD2O and DMSO-d6 effect, respectively. The resonances of the threecomplexes were also obtained in gas phase, the differencescomputed slightly shielded by about 62 ppm (d = �687, 706 and674 ppm for GH, GGH and GHG complexes, respectively). Apossible reason for this balance may be to consider the solvationeffect in the calculation as well as taking into account a singlewater molecule, which can be quite sizeable for anions such as[VO(O2)2�H2O]� or [VO(O2)�H2O]� [63]. We believe that a deviationof 40 ppm is still within the aforementioned error tolerance. Itshould be noted that the correct sequence of resonances is repro-duced correctly if is considered the solvent effect, but stronglyunderestimated when is only reproduced in gas phase (see Fig. 5).

4. Conclusions

In this work, fully optimized geometries for the three bis-per-oxo-oxovanadate species containing histidine peptides complexeswere obtained by DFT calculations showing parameters very simi-lar to the X-ray data reported for similar compounds. Employingthe present computational approach, we presented a goodreproduction of the energies, molecular geometries andexperimental 1H, 13C and 51V spectra. Solvent effect of D2Oand DMSO-d6 were included with the PCM model for betterspectroscopic characterization.

The theoretical IR and 1H, 13C and 51V spectra adequately repro-duced the experimental data. The assignment of normal vibrationalmodes as the mas(O–O) and ms(O–O) stretching due to VO(O2)2

moiety were calculated and adequately assigned theoretically.Similarly, the m(GlyN–C@O� � �H) mode had not been assignedexperimentally showed a band in the 2800–3300 cm�1 rangefor the three complexes. The bands from the ms(O–H)water andmas(O–H)water vibrational modes of the three GH, GGH and GHGcomplexes was reproduced and assigned in the 3500–3600 cm�1

range by the Density Functional Theory at B3LYP/6-31+G(d)/Lanl2DZ level employed. These bands appear in this IR regionbecause the water molecule is not coordinated to vanadium, point-ing to the interesting possibility that the changes in the first or sec-ond coordination sphere of vanadium produced notable change inthe calculated 51V chemical shift values of up to ca. 62 ppm. Thus,the solvent effect may be an important factor governing the chem-ical shifts and will be the base for a further structural assignmentof VO-histidine-containing peptides complexes based on elec-tronic, vibrational and 51V NMR spectroscopy, and could enhancethe understanding of the structure characteristics on vanadiumcomplexes of biogenic ligands.

Acknowledgement

One of us A.D.E. wishes to thank CONACYT (Mexico) for finan-cial support through a doctoral fellowship 172001.

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