A DFT/TDDFT study of complexation ability of the calcium cation

complexes with Lateral chain ligand of Taxol

Abstract: Theoritical investigation of the [Taxol-Ca] and [LC1-

Ca] complexes in gas phase employing DFT and TD-DFT theory

methods have been study. The electronic structure of the complexe

have been taken into the count. Charge distribution one donor an

acceptor atoms are evaluated by Mulliken analysis . The charge

distribution indicate that the ligands transfer their negative

charge to Calcium ion Ca2+ during formation of the complexes

(LMCT). Electronic excitation energies of Ca2+ complexe are

calculated. It is found that the Lateral chain ligand has the

same total density as Taxol structure.

In this work we investigate the performance of the DFT method,

augmented with an empirical dispersion function (DFT-D), paired

with the PCM implicit solvation model, for the computation of

noncovalent interaction energies of biologically-relevant,

solvated model complexes. It is found that this method describes

intermolecular interactions within and (-like) environments with

roughly the same accuracy as in the gas phase. Another important

finding is that, when environmental effects are taken into

account, the empirical dispersion term associated with the DFT-D

method need be modified very little (or not at all), in order to

obtain the optimum, most well balanced, performance.

1

Keywords: Taxol, Ca2+ ion,conformation, binding energy, DFT/TDDFT,

UV-Visible.

INTRODUCTION

Cancer’s deseases (CD) is caused by an uncontrolled division of

abnormal cells in a part of the body. It is the consequences of a

combination of complex factors related to heredity, lifestyle and

environment1-3. It is caused by an accelerated cellular growth

accumulating genetic alteration as they progress to a more

malignant phenotype 4 and are the second major cause of the human

death in industrialized countries5.

These death rates were significantly reduced in the past decades

due in part to the successful use of chemotherapeutic agent3. non-

proteinogenic β –amino α-hydroxy acids are found in many natural

products and drugs, for example N-benzoyl-3-phenylisoserine as a

side-chain of taxol (generic name Paclitaxel) (figure1) which is

one of the most popular chemotherapeutic agents used nowadays for

treatment of breast, ovarian, and lung cancers6,7.

Soon after its isolation, taxol was shown to have a unique mode

of action it blocks cancer cell division by promoting the

polymerization and stabilization of the protein tubulin to

microtubules and inhibits the de-polymerization of microtubules

back to tubulin. Although an atomistic structure of tubulin has

been proposed8.

Four main groups (Na, K, Mg, and Ca) and ten transition (V, Cr,

Mn, Fe, Co, Ni, Cu, Zn, Mo,

and Cd) metals are currently known or thought to be required for

normal biological functions in humans9 , the medical consequences

of insufficient quantities where known.

2

Besides, the most common ions essential for biological processes

are K+,Na+,Mg2+ and Ca2+. Ca2+, in contrast to Mg2+, is found

principally in extracellular locations. Its main function is to

stabilize structural components in biological organisms- e.g.

extracellular proteins, cell membranes, cell walls and

extracellular de posits such as bone. In addition, it assumes

regulatory functions within cells10-12

The Ca2+ cation also has anticariogenic effects demonstrated in a

set of clinical trials13,14. Previous data suggest that carcinogenic

stimuli cause local increase in the Ca2+concentration leading to

activation of proto oncogenes and to inactivation of tumor-

suppressor genes, which lead to the manifestation of a malignant

phenotype. Tumor cell proliferation maybe stimulated by

persistent increase of Ca2+, in contrary the transitory fulminic

increase of Ca2+ induce the activation of mitochondrial apoptotic

pathway15

In the other hand, intracellular calcium concentration may play a

role in the development of chemoresistance. Altered

Ca2+ homeostasis of cell is correlated with cisplatin or Taxol

resistance in Non-small cell lung cancer  (NSCLC) cell lines

(A549 and EPLC) or small cell lung cancer (SCLC) cell line

(H1339)16,17

W. Boehmerle et al. suggestion is understand the etiology of the

potent dose and therapy limiting side effects is still poorly

understood and difficult to explain with the know Taxol

interaction partners. Their conclusion of analysis is the

affinity of taxol for (NCS-1) protein and stabilized both probe

and Ca2+ depending characteristics18. However, their finding is

3

that taxol also affects intracellular Ca2+ signaling care should

be given when interpreting the results obtained with taxol as a

microtubule modifying drug. Their observations (that taxol has

new binding partner, NCS-1, and that binding to NCS-1 lead to

initiation of cytosolic Ca2+ oscillation) suggest that taxol’s

effects when used as a research to will be more complex than

originally expected.

Taxol is a compound that selectively stabilizes microtubules

and has been used extensively in studies of microtubule

function19. M. P .Mattson et al were employed Taxol to test the

hypothesis that the state of microtubule polymerization

influences neuronal Ca2+ homeostasis and vulnerability to

excitotoxicity, there data obtained with taxol suggested

that stabilization of microtubules suppresses Ca2+ influx

and protects neurons against excitotoxicity. However,

the possibility remained that taxol was protecting neurons

against excitotoxicity by a mechanism unrelated to

its microtubule stabilizing activity20. The mechanisms by which

microtubules and other cytoskeletal elements affect ion channel

function are not known21. However, Taxol administration is limited

by serious side effects including cardiac arrhythmia, which

cannot be explained by its microtubule-stabilizing effect.

Recently, neuronal calcium sensor 1, a calcium binding protein

that modulates the inositol-1,4,5-trisphosphate receptor

(InsP(3)R), was described as a binding partner of Taxol and as a

substrate of calpain. They have examined calcium signaling

processes in cardiomyocytes after treatment with Taxol to

investigate the basis of Taxol-induced cardiac arrhythmia. Taxol

treated cells had increased expression of NCS-1, an effect also

detectable after Taxol administration in vivo22. Several4

chemotherapeutic drugs are believed to cause an enhanced calcium

signal leading to hyperactivation of neurons23.

Figure 1: Chemical Structure of paclitaxel (Taxol) and biological

activity region of Lateral chain and DBAC.

Considering the topological structure (figure 1), Paclitaxel

(PTX) is a complex natural product with a rigid baccatin core

composed by the DBAC region, which is a natural substance

isolated from the leaves of Taxus species24 and four flexible side

chains emanating from C2, C4, C10 and C13. PTX is highly

substituted poly-oxygenated cyclic diterpenoid characterized by

the taxane ring system and it differs from other known taxanes

either in substitution pattern, the nature of the ester side-

chain or in the presence of the oxetane ring (D-ring) system.

Potent antimitotic activity has seen to be restricted to taxol

which possess an N-benzoyl-3 phenylisoserine side-chain at C13

atom. The lateral chain is essential for the antitumor activity of

taxoid family which is characterized by terpene core25,26.

Hydrophobic interaction plays a role, owing to the presence of

lipophilic ester groups on the chain and on the terpenoid core27.5

Likewise the LC molecule has no symmetry and exhibits a

significant internal rotational degree of freedom due to

rotations around the single bonds in the central part of the

carbon skeleton (figure 3). Therefore, study on conformational

landscape of lateral chain may provide insights and may support

practical efforts for medicine.

D.S. Galvão and co-workers have performed a theoretical study28 on

the electronic structure of 10-deacetylbaccatin-III by semi-

empirical methods PM3 and ZINDO. They have identified the

relative importance (in terms of electronic features) of specific

DBAC’s molecular regions and concluded that regions that are

considered important for the biological activity of taxol can be

traced by the electronic features already present in DBAC

regions.

Compared the topological structure to Paclitaxel (PTX), Lateral

Chain (LC) has a simple structure in term of molecular size, it

has 46 heavy atoms while Paclitaxel has 113 atoms. The LC makes a

good candidate for preliminary investigation.

Our main was to find a model system in which the Taxol molecule

interacts with some metallic cations while Ca2+, Fe2+ and Cu2+

present in biological fluids. The major focus of this work is on

the calcium anticancer drug taxol chelation, which is presented

as a case of study theoretically the scope of the problem. For

the calculations on isolated molecules (gas phase), we will

extend the approach of these previous studies to LC residue

attached to carbon C11 of the moiety and benzoate connected to

nitrogen N (-C3’) substituent of the C13 side chain (figure 2), in

order to come as to the structure of Taxol.

6

As a ligand, considering a lateral chain of taxol, it contains

three strong chelating sites in competition (figure 2).

Interaction of the LC with metal ion, both via their phenyls

groups, NH…OCar and NH…OEt, have been subject of details studies,

with OCar and OEt are the carboxylic and Ethoxy oxygen atoms,

respectively (see figure 2).

We determine whether or not LC can bind to Ca2+ ion to form

chelated complex, both via their phenyl groups or amide groups.

Figure 2: Structure of lateral chain, arrows indicates the

possible complexes sites via degree of freedom of the dihedral

angle φ, studied in this work.

Mulliken charge transfer complexes are being regarded as an

important materials for application in mechanism of drug action29 .

Thus, we have computed a variety of properties, like charge

transfer (CT) interaction between electron donor / acceptor and

the vibrational spectra of LC ligand. we attempted to

accommodate on a different way a wider forms complexes of the

[[LC-Ca]+ and [Taxol-Ca]+ with different stabilities. The effect

of the OH- counterion for complexes cation spectrum has been also

analyzed. To the best of our knowledge, there is no report of the

stability of Taxol complexes with Ca2+.

7

Computational details

Calculations reported here were performed with Gaussian 03

software package with a spin-restricted formalism29. Full geometry

optimization for different LC conformations and [LC-Ca]+ , [LC-

CaOH] and [Taxol-Ca]+, [Taxol-CaOH] were carried out by using

Density Functional Theory (DFT) with the Becke thee-parameter

hybrid functional (B3LYP) method30-32. 6-31G(d,p) is the standard

basis sets and a popular polarized basis set which adds p

functions to hydrogen atoms in addition to the d functions on

heavy atoms.

In this study one of the points we would like to evaluate, is

whether the geometry of LC with metal cation can form an active

complex. A preliminary geometry optimization of the deprotonated

LC as ligand has revealed that the most stable structure is also

same structure of protonated LC. In the optimized structure of

Ca(LC)2, four oxygen atoms from carboxylate groups and two

nitrogen atom make an “octahedral-like” environment around

calcium ion (figure 5) the most stable (LC)2Ca complex correspond

to hexacoordinated structure in which Ca2+ ion interact with two

nitrogen and four oxygen atoms of LC dimer .

Molecules are studied in gas phase adopting the zwiterionic form

to the carbon 3΄ set to NH for lateral chain. in the first step,

the geometries were fully optimized at the B3LYP/6-31G(d,p)

level for all atoms.

Although there is qualitative agreement between these methods,

the B3LYP hybrid density functional method appears to give a more

accurate representation of the energetic of LC.

8

The analyses of vibrational frequencies indicated that optimized

structures of complexes were at stationary points corresponding

to local minima without imaginary frequencies. The electronic

charge distribution of different complexes were evaluated by

Mullikan theory34 enable us to characterize the donor and acceptor

molecule.

We initially used the rotational step of 10 degrees to the C3’-C2’-

C1’-O1’ dihedral angle using B3LYP/6-31G(d,p). The complete

rotation of 180° shown existence of two best conformation (in

terms of energy) and were selected in both structures of 3’-N

benzoyl form hydrogen bound with …Ocar and with …OEt atoms

respectively .

Although, W. Boehmerle et al show experimental evidence of the

binding of taxol for (NCS-1) protein and stabilized both probe

and Calcium ion18.The LC dimer chelats with calcium ion (complex

(LC)2...Ca2+) has been carried out, in order to explore binding to

the chemotherapeutic drug as possible interaction of LC activity

of taxol. The electronic spectrum calculations of individual

molecules and ion complexation and their effects on spectral

properties were made by using time dependent density functional

(TDDFT) theory method with B3LYP/6-31G(d,p) basis set in gas

phase34 . In absorption or fluorescence spectra are observed when

the LC1 and Taxol are complexed by a Ca2+ .

Density functional DFT studies have been performed for three

possible structures of calcium complexes with LC assuming hexa-

coordination with two LC ligands. Calculation were carried out

using the B3LYP method with the 6-31G(d,p) basis sets.

It should be mentioned that Shen Li et al.35 performed

conformational studies of benzene-water complex at vdW-DF, they

suggest that either the H atoms or the O atom of water can9

interact with benzene, depending on the location of the water.

The water molecule will interact with the rim of the benzene ring

via its O atom.

Ground state electronic structure S0 calculation of complexes has

been performed in gas phase, the optimum structures were verified

by the absence of imaginary frequencies. The 20 lowest singlets

excited states of complexes were calculated within TDDFT

formalism. We used in place of the δ functions Gaussian having a

FWHM with 0,3 eV .For the better interpretation of Photoluminescence properties,

triplet excited states T1 of complexes have been optimized in gas

phase. Long-wavelength emission bands have been calculated with

ΔSCF approch [[56] Scalmani, G.; Frisch, M. J. J. Chem. Phys. 2010, 132, 114110. [57]

Andreiadis, E. S.; Imbert, D.; Pécaut, J.; Calborean, A.; Ciofini, I.; Adamo, C.;

Demadrille, R.; Mazzanti, M. Inorg. Chem. 2011, in press.

Results and Discussion

Conformational study of neutral and zwitterionic LCoptimization in gas phase

The first part of the computation is devoted to optimize the

geometry of LC at varying dihedral angle θ in order to find

stable structures LC1 and LC2 by B3LYP/6-31G(d,p) (figure 2),

and we determine ground state geometry for the LC ligand. The

result showed that rotation around C13-C10 bond is

restricted by the tendency to form intramolecular hydrogen bond

interaction between N-H…OEt and N-H…Ocar for LC1 and LC2

respectively.

10

Figure 3: Structures optimized for LC1 and LC2 calculated at

the B3LYP/6-31G (d,p), Bond lengths are in Å.

We observe weak intramolecular hydrogen bond interaction

between N-H… OEt of LC1 and between N-H…OCar of LC2 compound. The

energy level for both forms is the same (ΔE=0,0015kcal/mol).

In order to form complex with calcium, we assumed that the

zwiterionic form of LC is the deprotonation of the –NH in the

amine, their different ways are:

In zwitterionic form as β-amino acids which are known to

exist as zwitterions in the solid state and in solution and in

the no-ionized, neutral form in the vacuum phase34 the

zwitterionic form exists frequently in physiological conditions

but depends on the temperature and pH in cells. Existence of

11

intramolecular proton transfer (N-H O=C) . Intermolecular

proton transfer between (C=O) of the first LC and the (H-N) of

the second one.

Therefore, molecules are studied in gas phase adopting the

zwiterionic form of LC with NH deprotonated according to the

following reaction, LC-(N-H) → LC-(N:)- + H+.

LC(N:)- is believed to complex with Ca2+ ion . It appears that the

lowest energy lying and zwitterionic conformers were found to be

close in energy about 4.6 eV (106,07kcal/mol) in gas phase and

4.72 eV (108,84 kcal/mol) in PCM at DFT level. These energies

are more less to that obtained with cyclopentadienyl

deprotonation [ reference sihem).

Table 1, summarize the result for some relevant bond lengths,

valence and dihedrals angle in order to compare the predicted

B3LYP geometries of protonated , deprotonated LC and the highest

stable [LC1-Ca]+ complex. It has been observed that there is such

difference regarding bond length, valence and dihedrals angle

between various atoms of LC1,

Zwiterionic LC1 form and ([LC1/Ca]+) complex . The bond lengths

value between various atoms of, LC and LC deprotonated varied

about 0.01 Å at 0.07 Å and between LC deprotonated and [LC1/Ca]+

varied around 0.01 Å at 0.04 Å. In the other hand, valence angle

between LC and LC deprotonated varied around 0° to 10° and

between LC deprotonated and ([LC1/Ca]+ varied about 0° to 4°.

Table 1: Optimization parameters of LC1, zwiterionic LC1 form and

the highest stable [LC1/Ca]+ complex at B3LYP calculation in

ground state.

Parameter LC1 Zwiterio [LC1/Ca]+

12

nic

LC1

complex

Bond lengths

O7-N25

O7-C8

C8-C10

C10- C13

C13- N25

N25-C26

C26- O27

Valence

angle

O7- C8-C10

C8-C10-C13

C10-C13- N25

C13- N25-C26

N25-C26- O27

Torsional

angle

O7-C8-C10-C13

C8-C10-C13-N25

C10-C13-N25 -C26

C13-C10-C8-O7

Dipole

moment (D)

2.88

1.34

1.52

1.56

1.45

1.30

1.22

112.

73

112.

44

109.

78

122.

26

122.

30

-

61.7

7

73.3

5

126.

84

1.68

5.72

2.93

1.33

1.52

1.62

1.42

1.38

1.23

113.06

112.10

109.93

116.94

121.63

-77.96

58.46

81.10

- 25.70

3.35

3.05

1.42

1.53

1.57

1.46

1.36

1.22

113.44

113.80

110.08

114.76

129.0

-78.67

65.76

134.58

-3.58

15.34

13

aBond lengths R in Å, angles θ and dihedral

angles φ in degrees.

The most important point to be mentionned here is that the bond

length O7-N25 of oxygen O7 of LC1 increase in zwiterionic form and

in the complex about 0.05 and 0.17Å respectivelly.

Geometries and binding energies of LC-Ca complexes.

* [LC-Ca] + complex

Molecules are studied in vaccum level adopting form of LC, which

the total charge is equal one. It is believed to complex with ion

metals as Ca2+. Depending of the initial geometry, the lateralchain ligand (Fig.5) contains several coordination sites, i.e.

ethoxy atom and carboxy atom as well as nitrogen atom

deprotonated of amide groups of zwiternionic LC. Therefore, it

also can coordinate the metal calcium by phenyl groups on/down.

The interact ion between the metal ion and the ligand is

predominantly electrostatic. Several local minima of the

potential energy were found for LC and Ca2+ ion complexes are

displayed with their relative energies in figure 5. For complexes

5 and 6, zwiternionic form of LC deprotonated of NH amine is

complexed with Ca2+.

14

Figure 5: Lowest energy ground state structure calculated for

Lateral chain complexes with Ca2+ obtained at B3LYP/6-31G (d,p).

Relative energies in kcal/mol. [Hydrogen atoms are omitted for

clarity in (1) to (4) ].

The structural geometry analysis suggests that LC is able of

strongly binding with three Ca2+ ion in different sites of

complexation. The most stable ground state complexes are

displayed in fig. 5 with their relative energies. The energy

difference between the different complexes is not negligible

excepted for [Ph1-Ca]2+ Down compared at [Ph1-Ca]2+up . The

binding energies ∆E of a complex is calculated as:

∆E=Ecomplex−(ELC+Eion)

15

Where Ecomplex and ELC are the complex and the free LC energies,

respectively.

*[(LC)2 -Ca] complex.

In –NCCCOO- group, both O and N atoms are recommended as

coordination atoms in all kinds of complexes, and the

characteristic structure determines the strong chelating ability

of the LC with metal ions. In order to assign the positions of

Ca2+ ion in the complexes, three possible structures of calcium

complexes with LC assuming hexacoordination with two LC ligands.

Therefore, the Ca2+ ion chelates with, two nitrogen and four

oxygen of (LC)2-Ca complexes (figure 6).

Figure 6: Optimized structures of [(LC)2-Ca] complexes obtained at

B3LYP/6-31G(d,p) level [Hydrogen atoms are omited for clarity].

In Table 2, we present the Ca2+ binding energies obtained in

vacuum level using DFT level. The most stable

structure of each type of monomer complex follow an order:

5> 6 > 1 > 4 > 2 ≈ 3, as calculated using B3LYP/ 6-31+G (d)

method (Table 2). The BE of the studied complexes between

different structures is the largest for complex 5 with -502, 20

kcal/mol owing less flexible ligand. For complexes 1,2,3,4 have

16

high flexibility leading smaller binding energies, and type of

coordination is different. In the other hand the calculations of

the dimer complex have shown that the binding sequences follow

the order: 7 > 9 > 8. The stability of Ca-(LC)2 complex is

evidently far away from the other studied.

Table 2: Binding energies for calcium ion of monomer Lateral

Chain molecule calculated at B3LYP/6-31G(d,p) level for structure

optimized.

In the Complex 7,

all oxygen atom i.e.

two oxygen atoms of,

carbonyl and ethoxy

groups,

17

complexe

s

Ground stateComplexation energy

(kcal/mol)

Monomer

[Ph1-Ca]2+up

(1) -175,08

[Ph1-

Ca]2+down (2)

-161,12

[Ph2- Ca]2+-

up (3)-161,18

[Ph2-

Ca]2+down (4)

-128,25

[LC1-Ca]+

(5)-502,20

[LC2-Ca]+

(6) Dimer

[(LC1)2-Ca]

(7)

[(LC2)2-Ca]

(8)

[(LC1-LC2)-

Ca] (9)

-394,11

-689,27

-673,10

-681,13

respectively and two nitrogen atoms are involved in coordination

with Ca2+ cation (figure 5). In complex 6, the calcium is

hexacoorditaned, whereas in complex 8 and 9, the coordination of

calcium is four and five, respectively.

In contrast, in the different structures of monomer [LC-Ca]+

complexes only the oxygen atoms from carbonyl groups of LC

ligands (complexes 2 and 4) is engaged in the coordination

process. In the other hand two oxygen atoms from carbonyl groups

of LC ligands (complexes 1 and 3) are engaged in the coordination

process. Nitrogen and one oxygen atoms in complexes 5 and 6 are

coordinated to Ca2+ cation (figure 4).

Atomic charge and orbital molecular analysis

Table 3 summarized the Mulliken charge in the complexes and in

the free ligands, the inter-atomic distances between the Calcium

and the oxygen and the nitrogen coordinated atoms, were

calculated at B3LYP/6-31+(d,p) level of theory.

Table 3: The interatomic distances (Å) and Mulliken partial

charge (a.u) for O and N atoms of LC coordinating calcium cation

in complexes structures calculated by B3LYP/6-31G(d,p)

complexes Cation

charge

Coordinatin

g

atom

Coordination

atom

partial charge

Distance

coordinating

atom→cationComp.

Lig.[Ph1-Ca]2+up +1,334 O6 -0.593 2.43

18

(1) O8 -0.503

-0.598

-0.538

2.32

[Ph1- Ca]2+down

(2)

+1,372 O27 -0.692

-0.525

2.19

[Ph2- Ca]2+-up

(3)

+1.243 O7

O12

-0.599

-0.478

-0.599

-0.5375

2.40

2.41

[Ph2- Ca]2+down

(4)

+1,645 O27 -0.472

-0.526

2.69

[LC1-Ca]+

(5)

+1.268 N25

O9

-0.702

-0.572

-0.569

-0.503

2.41

2.39

[LC2-Ca]+

(6)

+1.159 N25

O7

-0.711

-0.572

-0.595

-0.478

2.30

2.40

[(LC1)2-Ca]

(7)

+1.119 O10

N26

O28

O10'

N26'

O28'

-0.535

-0.526

-0.644

-0.572

-0.640

-0.503

-0.538

-0.478

-0.645

-0.572

-0.639

-0.526

2.47

2.47

2.39

2.47

2.47

2.39

[(LC2)2-Ca] +1.121 O28 -0.649 -

0.526

2.37

19

(8) N26

O28'

N26'

O8

O8'

-0.644

-0.572

-0.649

-0.526

-0.644

-0.572

-0.530

-0.478

-0.530

-0.478

2.46

2.37

2.46

2.72

2.72

[(LC1-LC2)-Ca]

(9)

+1.081 O28

N26

O10

O28'

N26'

O10

-0.654

-0.526

-0.639

-0.572

-0.549

-0.503

-0.653

-0.503

-0.651

-0.572

-0.538

-0.473

2.39

2.47

2.42

2.39

2.42

2.61

Table 3 shows that the charge of LC in the complexes decreased

compared to the free ligand. While the charge of Calcium

increased. For instance, before complexation the charge on

calcium is +2, while the charge of the calcium in monomer complex

4 is +1,645 a.u. and in dimer complex7 is +1.119 a.u.

This indicates that ligand transfer their negative charges

occurs to calcium ions during complexe formation with increase

the electron density of calcium in the order : (6) > (3) > (5) >

20

(1) > (2) for monomer LC complex. In the other hand for dimer

complex the electron density of calcium increases in the order :

1=2 > 3. LC of taxol has favourable donor-centers, namely oxygen

atoms from carbonyl (C=O), ethoxy (C-O) and nitrogen (N-C)

groups. They create good binding sites for hexadentate cation

for complex 7 and pentadentate cation for complex 9 (see table

2).

This results shows that ligand in dimer complex has better donor

atoms than in monomer complex. The maximum Mulliken charge (<2)

for all the complexes has been found on the

Ca – center atom and atoms coordinated.

Furthermore, the Mulliken’s electronic charge on the O atom of

the complex in its ground state is found to be more negative than

that obtained in the isolated LC molecule. For example, the

Mulliken’s electronic charges on the N atom of free ligand and in

complex 5 is (-0.572 a.u.) and (-0.702 a.u.) respectively by

DFT (B3LYP) calculations. In the other hand , the Mulliken’s

electronic charge on the calcium ion with +2 before

complexation , becomes less positive +1.268 a.u) for complex 5

about 0.732 e has been transferred. This indicates that

appreciable amount of electronic charge has been transferred from

LC ligand to Ca2+ in complex.

Photophysical properties

* Absorption spectra

Molecular orbital calculations using G09 and Gausview package on

selectedcomplexes were carried out to gain more insight into the

photophysical properties of the complexes. the features of the

highest occupied (HOMO-1 and HOMO-7) and the lowest unoccupied

(LUMO and LUMO+1) frontier orbitals mainly involved in the

21

transition are dipected in figure 7, and the description on

energy gaps of each transition are listed in the same figure .

Electronic excitation energies of LC, Taxol , [LC-Ca]+, and

[Taxol-Ca]+, [LC-CaOH], and [Taxol-CaOH] species have been

studied at the TD-DFT/B3LYP with 6-31+G(d,p) calculation in gas

phase. For all complexes, we have considered the most stable one

(with high binding energy) as like as complex 5 and complex

[Taxol-Ca]+ (see table 2). Orbital surfaces and energies for LC,

Taxol , [LC-Ca]+, and [Taxol-Ca]+ ,[LC-CaOH] and [Taxol-CaOH] are

given in figure 7.

In LC1 and Taxol complexes, the highest occupied MOs are

localized on the Elecronic excitation energies and the absorption

coefficient of the most intense bands are given in table 4. A

band with large oscillator coefficient for [LC-Ca]+, and [Taxol-

Ca]+ complexes appears at 414 and 1453 nm respectivelly, while

these bands decrease with used OH- counter-ion in [LC-CaOH] and

[Taxol-CaOH] neutral complexes about 302 and 438 nm respectively.

Table 4: Vertical excitation energies (eV), oscillator strengths

(f) and configurations of excitations for Taxol and LC1 with and

without counter-ion atoms (OH-) for complexes.

Complexes Wavelength

(nm)

f Configurati

on

Transiti

on

properti

esLC1

Taxol

[LC1-Ca]+

270

427*

414

0.58423

0.08304

0.0344

H-1 L

H

L

H-7 L

--

--

22

(LM

CT)[Taxol-Ca]+

[LC1-CaOH]

[Taxol-

CaOH]

1453

302

438

0.0142

0.0224

0.0234

H L

H

L+1

H-1

L+1

(LM

CT)

(LL

CT)

(L

LCT) *Experimental Wavelength36 (273nm).

Density is focus in same ligant of LC1 or Taxol appeared in

following figure.

Figure x : Total density of LC1 (in the left) and Taxol

(in the right) structures calculated by 6-31G(d,p)

23

-a-

-b-

24

-c-

-d-

Figure 7: Electronic density of LC1 and Taxol complexes

calculated by TD-DFT

25

Spectre d'absorption/émission de LC1-Ca : l'absorption présente

une bande qui est du à une transition intense HOMO-7/LUMO à

laquelle correspond un λmax de l'ordre de 414nm. Le spectre

d'émissions quant à lui présente aussi une bande qui résulte de

la transition HOMO-1/LUMO de longueur d'onde λmax = 937nm. Cette

émission se situe dans le proche IR.

Density is focus in same ligant of LC1 or Taxol appeared in

following figure.

26

Figure x : Total density of LC1 (in the left) and Taxol

(in the right) structures calculated by 6-31G(d,p)

Conclusion

in this work , we report DFT and TD-DFT. LC1 of taxol acts as

hexadentate ligand on forming complexes with calcium ion. TDDFT

method can be utilized successfully to evaluate the CT between

Taxol and Ca2+ in [Taxol-Ca]+ and [Taxol-Ca-OH] complex in vacuum

level. Frontier molecular orbital calculations reveal that there

is receivable opportunity to electron transfer from LC1/Taxol to

Ca2+ ion in [LC1-Ca]+ and [Taxol-Ca]+ CT that correspond to LMCT

process ( donnor-acceptor complex).

The result from the present investigations indicate that [Taxol-

Ca]+ CT complex could be a potential candidate to blocking cancer

cell division by promoting the polymerization of the protein

tubulin to microtubules.

References

1-The Biological Basis of Cancer; McKinnel, R. G., Ed.; Cambridge

University Press: Cambridge, 1998.

2- Lodish, H.; Baltimore, D.; Berk, A.; Zipursky, S. L.;

Sudaira, P. M.;Darnell, J.; Molecular Cell Biology; Scientific

American Books: New York, 1995.27

3-Cancer Biology; Ruddon, R. W., Ed.; Oxford University Press:

New York, 1987.

4- Sugimura, T. Multistep carcinogenesis: A 1992

perspective. Science 1992, 258, 603-607.

5- EIU Marketing in Europe, Trade Reviews, 337, December

1990.

6- Dubois, J. Expert Opin. Ther. Pat. 2006, 16, 1481–1496.

7- Marupudi, N. I.; Han, J. E.; Li, K. W.; Renard, V. M.;

Tyler, B. M.; Brem, H. Expert Opin. Drug Saf. 2007, 6, 609–621.

8- Nogales, E.; Wolf, S. G.; Downing, K. H. Nature 1998, 391

199-202. Lo¨we, J.; Amos, L. A. Nature 1998, 391, 203-206.

9- STEPHEN J. LIPPARD, ″METALS IN MEDICINE″, Department of

Chemistry Massachusetts Institute of Technology

10-WILLIAMS, R. J. P. (1970). The biochemistry of

sodium, potassium, magnesium a nd calcium. Q. Rev.

Chem. Soc., 24, 331-65.

11- WILLIAMS, R. J. P. (1976). Calcium chemistry and

its relation to biological function. In Calcium in

biological systems, Syrup. Soc. exp. Biol., 30th, 1-

17. London, Cambr idge University Press.

12- WILLIAMS, R. J. P. (1977). Calcium chemistry and its

relation to protein binding. In Calcium binding proteins

and calcium Junction, edited by R. H. Wasserman et al.,

3-12. Amsterdam, Elsevier North-Holland.)

13- D. Kandelman, G. Gagnon, A 24-month clinical-study of

the incidence and progression of dental-caries in relation

to consumption of chewing gum containing xylitol in school

preventive programs, J. Dent. Res. 69 (1990) 1771–1775

14- K.K. Makinen, C.A. Bennett, P.P. Hujoel, P.J.

Isokangas, K.P. Isotupa, H.R. Pape,Xylitol chewing gums and28

caries rates: a 40-month cohort study, J. Dent. Res.74

(1995) 1904–1913.

15- Jaffe LF. A calcium-based theory of carcinogenesis. Adv

Cancer Res.2005 ; 94:231–63

16- Padar S, van Breemen C, Thomas DW, Uchizono JA, Livesey

JC, Rahimian R. Differential regulation of calcium

homeostasis in adenocarcinoma cell line A549 and its Taxol-

resistant subclone. Br J Pharmacol. 2004; 142:305–16.

17-  Schrodl K, Oelmez H, Edelmann M, Huber RM, Bergner A.

Altered Ca2+-homeostasis of cisplatin – treated and low

level resistant non-small-cell and small-cell lung cancer

cells. Cell Oncol. 2009; 31:301–15.

18- Review PNSA. 2006 N°48.

19- Horwitz, S.B., Mechanism of action of taxol, Trends

Pharmacol.Sci.,13(1992) 134-136.

20- K. Furukawa, M.P.Mattson, Taxol stabilizes [Ca2+] and

protects hippocampal neurons against

excitotoxicity ,Brain Research 689 (1995) 141-146.

21- Katshutoshi Furukawa, Mark P. Mattson, Brain Research

689 (1995) 141-146.

22- J.H. JH Benbow,T.T.Mann, Barbara E. Ehrlich, J.Biol Chem

287(45):37907-16(2012).

23- Ramos-Franco, J., Caenepeel, S., Fill, M., and Mignery,

G. (1998) Biophys J 75, 2783-2793.

24- A. Stierle, D. Stierle, G. Strobel, G. Bignami, P.

Grothaus, Bioactive metabolites of endophytic fungi of

pacific yew, Taxus Brevifolia, paclitaxel, taxanes, and other

bioactive compounds, in:G.I. Georg, T.T. Chen, I. Ojima,

D.M. Vyas (Eds.), Taxane Anticancer Agents, A.C.S. Symposium

29

Series 583, American Chemical Society,Washington, DC, 1995,

pp. 81–97.

25- C.S. Swindell, N.E. Krauss, Biologically active taxol

analogues with deleted A-ring side chain substituents and

variable C-2 configurations, J. Med. Chem. 34 (1991) 1176–

1184.

26- L.R. Jayasinghe, Structure-activity studies of

antitumor taxanes synthesis of novel C13 side chain

homologated taxol and taxotere analogs, J. Med. Chem. 37

(1994) 2981–2984.

27- M. Suffness, Overview of paclitaxel research: progress

on many fronts, in: G.I. Georg, T.T. Chen, I. Ojima, D.M.

Vyas (Eds.),Taxane Anticancer Agents, A.C.S. Symposium

Series 583, American Chemical Society, Washington, DC, 1995,

pp. 1–17.]

28- S.F. Braga, D.S. Galvão A semiempirical study on the

electronic structure of 10-deacetylbaccatin-III. J. Mol.

Graph. and Model. 21 (2002) 57–70).

29- A. Korolkovas, Essentials of Medicinal Chemistry, 2nd

ed., Wiley, New York, 1988 (chapter 3).[10] R. Mondal

(Karan), S.C. Lahiri, J. Ind. Chem. Soc. 76 (1999) 347.

30- Frisch, M. J. Trucks, G. W.; Schlegel, H. B.; Scuseria,

G. E.; Robb,M. A.; Cheeseman, J. R.;Montgomery, J. A., Jr.;

Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.;

Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi,

M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.;

Hada,M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,

J.;Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,

H.; Klene, M.; Li,X.; Knox, J. E.; Hratchian, H. P.; Cross,

J. B.; Adamo, C.; Jaramillo, J.;Gomperts, R.; Stratmann, R.30

E.; Yazyev, O.; Austin, A. J.; Cammi, R.;Pomelli, C.;

Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G.

A.;Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;

Dapprich, S.; Daniels,A. D.; Strain, M. C.; Farkas, O.;

Malick, D. K.; Rabuck, A. D.;Raghavachari, K.; Foresman, J.

B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;Clifford, S.;

Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.;

Piskorz,P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith,

T.; Al-Laham, M. A.;Peng, C. Y.; Nanayakkara, A.;

Challacombe, M.; Gill, P. M. W.; Johnson,B.; Chen, W.; Wong,

M. W.; Gonzalez, C.; Pople, J. A.

31- Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (19)

32- Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37,

785–789. (20)

33- Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.;

Frisch, M. J. J. Phys. Chem. 1994, 98, 11623–11627.

34- R.S.Mulliken, J.A,Chem.Soc.74 (1952) 811.

35- E. D. Glendening, A. E. Reed, J. E.Carpenter, F.

Weinhold, 3.1. ed.

36- Shen Li,Valentino R. Cooper,T. Thonhauser, Aaron Puzder,

and David C. Langreth. A Density Functional Theory Study of the Benzene-

Water Complex. J. Phys. Chem. A 2008, 112, 9031–9036.

37- M.C.Wani, H.L.Taylor, M.E.Wall, P.Coggon, A.T.McPhail,

J.Am.Chem. Soc.93, Z1971.2325-2327.

31