Electronic Structure of Some a 3 Adenosine-receptor Antagonist ——a Structure Activity Relationship

DFT quantum chemical computations have been carried out at the B3LYP/6-31G(d) level. Full geometry optimization has been performed and equilibrium geometries for a new series of phenyl thiazoles have been located. Ground state electronic properties, charge density distributions, dipole moments and its components have been calculated and reported. Effect of substituents on the geometry and on the polarization of the studied series of compounds are analyzed and discussed. Some structural features have been pinpointed to underline the affinity and selectivity of the studied compounds as adenosine A 3-receptor antagonists. Results of the present work indicate that activity towards A 3 receptor sites is directly correlated with both of the polarity and the co-planarity of the thiazole.


Introduction
Adenosine, a metabolite of adenine nucleotides, is a physiological regulator of several cellular activities and cellular metabolism.It acts as an autacoid and activates Gprotein-coupled membrane receptors.These receptors are present on almost every cell.However, receptor subtype distribution and densities vary greatly.In our previous work [1] a QSAR model has been developed for 1,3dimethylxanthines as adenosine receptor antagonists.The model is capable of predicting the affinity towards both A 1 and A 2 receptors.Furthermore, several diverse classes of heterocyclic compounds have been developed [2][3][4] and reported as selective antagonists for A 3 receptors.Recently, thiazole and thiadiazole analogues have been described as the possible core skeletons of A 3 receptor antagonists with moderate affinity and selectivity [5][6].
The current study aims to present the ground state electronic properties of some new thiazole derivatives with expected biological activity, namely, A 3 Adenosinereceptor antagonist.A structure-activity correlations (SAR) will be attempted.Compounds studied in the present work were prepared [7] through microwave-assisted cross-coupling reactions in water, and microwave-assisted Suzuki cross-coupling reactions.These methods have been reviewed [8,9].

Materials and Methods
Theoretical computations carried out throughout this work were performed using the Gaussian 2003 program package [10].Full geometry optimizations were performed at the DFT (Density Functional Theory) level of theory [11].The B3LYB method [12,13] has been adopted.

Electronic Structure of 4-Phenylthiazole
The choice of the appropriate basis set is of prime impor-tance.The size of the molecules studied in the present work is medium to large, hence, a cost-effective study of basis sets, is in order.Such a study revealed that the 6-31G(d) basis set provides the minimum acceptable level of accuracy.This point can be appreciated by inspection of Table (1), where the optimized geometric parameters, atomic charges and dipole moments of 4-phenylthiazole are presented, at the 6-31G and the 6-31G(d) levels of theory.Inclusion of the d-polarization functions has but limited effect on the geometric parameters of 4phenythiazole.Such effect is confined to the sulphur atom region where significant deformation is observed.Upon d-orbital inclusion, the C-S bond length shows >3% shortening whereas, the C-S-C bond angles show widening by ~1 degree.This angle widening is due to the much better extension in space of the d-orbitals.This extension ensures a better description of the molecular structure.
The effect on the charge density distribution and on the dipole moment is more pronounced.Thus, the net charge on "S" is reduced from a value of 0.4 e to a value of 0.25 e upon inclusion of "d" functions.The "d" polarization function enabled a much better description of the "S" electron-affinity.The effect in this case is not localized; it is transmitted to all atoms of the 5-membered ring.In case of the 6-31G(d) basis set, the N-atom is able to accumulate 25% more negative charge, whereas, C5 became more positive.This indicates the direction of migration of the electron charge density.The 6membered ring, on the other hand, shows but little dependence on the d-functions.The overall redistribution of the charge density is reflected in the increase in the magnitude of the dipole moment to a value of 1.044 D.
The direction of the dipole moment shows also dramatic dependence on the "d" functions.Figure (1) shows the dipole moment vectors upon using the 6-31G and the 6-31G(d).The change in direction is most probably due to the fact that polarization functions are able to provide much better description of the lone pair electrons on both "S" and "N".
The above discussion indicates clearly that the 6-31G (d) basis set is much more capable of describing the molecular electronic properties of the studied molecules.Consequently, this basis set is adopted throughout the present study.

Substituted 4-Phenylthiazole
Substituted 4-phenylthiazoles studied in the present work fall into two main groups: in the first, substituent (X) is in the five-membered ring and the second represent variation of the substituent (Y) in the six-membered ring.Our discussion of the main molecular features will be along these two lines.

Substituted-2-Methylthiazoles
Substitution by a methyl group is not expected to cause major geometric deformation.Careful inspection (table supplementary material) of the main geometric parameters of 4-phenyl-2-methylthiazole reveals that there is no noticeable change in N 1 -C 2 or C 2 -S 3 bonds in the immediate vicinity of the substitution center.This is true for all bond lengths.However, there seems to be considerable deformation in bond angles.The N 1 C 2 S 3 bond angle is reduced by ~2° upon 2-methyl substitution, an effect which causes an increase in ring strain.However this effect is not transmitted to the 6-membered ring.
2-methyl substitution causes polarization of the σframework in a direction opposite to that of the polarization of the π framework.This increase in σ polarization is reflected in two main effects.First, the increase in the negative charge density accumulated on the N atom and the reduction of the dipole moment by ~16%.This polarization and the subsequent reduction in polarity would certainly have the effect of reducing the activity towards the A 3 -receptors [1].
Substitution in C 9 of the six-membered ring by a halogen atom has a considerable effect on the charge redistribution.While the effect on the C-C bond length is almost negligible, the dipole moment increases dramatically from a value of 0.87 D to 2.98 and 2.88 D for the chloroand the bromo-derivatives, respectively.The geometric and electronic features of the chloro-and the bromo-derivatives are given in tables supplementary material.
The high electronegativity of the halogen atom induces polarization in both the σ-and the π-frameworks, of the phenylthiazole moiety.Both the chloro-and the bromo-derivatives show their dipole moment vectors pointing in the same direction, towards the 5-membered ring.This indicates a net charge transfer from the 6-to the 5-membered ring.It is very important to realize that, the planarity of 4-phenylthiazole is due to the fact that the π-system extends all over the entire σ-framework.This planarity is a very important geometric feature for adenosine receptor antagonist [14].Substitution by a chloro-, a bromo-or a methyl group lead to an increase in the tightness of binding.This has the direct consequence of keeping the two rings co-planar.Substitution by an aryl group in the phenyl ring, would certainly add to the σ/π polarization.Careful inspection of the geometric and electronic features of aryl-4-phenyl-2-methylthiazoles studied in this work (tables supplementary material) reveals that, extending the π-system has the direct consequence of enhancing the polarity of the system.It is also evident that the conjugation is much tighter on the aryl-phenyl region.This shift of π-conjugation, away from the 5-membered ring, has two main effects.First, reduction in magnitude of the dipole moment by almost 20%, and lifting of co-planarity.The thiazole ring is out of the plane of the rest of the molecule.It should be noticed that, although the dipole moment has been reduced considerably, yet its direction is not significantly affected.This is most probably due to the fact that the directional character of the lone-pair electrons on "S" and on "N" plays a dominant role in this respect.Pchlorophenyl substitution has a pronounced effect on the polarity of the molecule.Thus, the thiazole ring is forced back towards the molecular plane and the dipole moment has increased to 3.15 D. The effect of the chlorine atom substitution is to considerably polarizes the σ-framework in a direction opposite to that of the π-system.Replacement of 4-chloro by a 4-methyl group causes a reduction of the magnitude of the dipole moment to 0.462 D and forcing the thiazole ring out of the molecular plane.
Thus, an electron-withdrawing substituent in the 4phenyl moiety is essential for the A 3 -adenosine antagonist receptors reactivity.This is due to the enhanced polarity and co-planarity of the molecule.This is true for all substituted-2-methylthiazole derivatives studied in the present work.Table (2) summarizes the dipole moment values, its components and the dihedral angles for the studied 2-methylthiazoles.
It is also important to examine the variation of the electron-donating strength and the electron affinity values of the studied molecules.Table (3) presents the ionization energies (I.P) and electron affinities (E.A) of the studied molecules.Substitution in the 6-membered ring has but little effect on the donating strength.However, this substitution has considerable effect on the electronaffinity.The halogen atom substitution has pronounced effect in lowering the LUMO i.e. increasing the electronaffinity of the molecule.This point is of prime importance in determining the activity as A 3 receptor antagonists.

2-N-methylthiazole-2-amine and Its Derivatives
Table (4) summarizes the dipole moment data and dihedral angles of the studied compounds.The π-electron density redistribution in this series of compounds seems to be dominated by a localized conjugation in the thiazole-amine moiety.This would impose a subsequent cross-conjugation in the phenylthiazole region.This is reflected in the magnitude of the dihedral angles reported in Table (4).In general, the aryl group is out of the plane of the thiazole amine moiety.Substitution in the 4position of the phenyl ring does not change much the non -coplanarity of the studied molecules.Substitution with a Cl or with a Br atom enhances the polarity considerably; both the magnitude and direction of the dipole moments are affected.Thiazole amines are of better electron donating and of less electron-accepting strength then the corresponding methylthiazoles.Table (5) presents the ionization energies and electron affinities of the studied thiazole amines.
The geometric and electronic features of aryl-4-phenyl -2-N-methylthiazoles-2-amine studied in this work are analyzed (presented in (tables, supplementary material)).This series of compounds do not show significant geometry changes upon substitution.However, thiazole amines are characterized by a pronounced increase ( ~20%) of the negative electron density accumulated on the thiazole nitrogen atom.The variation of the charge density upon substitution is remarkable and reflects itself in the alteration of the direction of the dipole moments (cf. Figure 3).

4-Phenyl-2-N-phenylthiazole-2-amine and Its Derivatives
Table (6) presents the dipole moments, its components and dihedral angles of the studied compounds.Inspection of geometric data of these compounds (tables and figures, supplementary material) indicates clearly that:  All the studied molecules show accumulation of the negative charge density on the thiazole N-atom.This negative charge facilitates hydrogen bonding which is a very important structural feature related directly to the ability to bind to the A 3 receptor sites. The NH-phenyl derivatives are non-coplanar.Indicating cross-conjugation between the phenyl and the thiazole moieties.It should be noted that the chlorine and bromine atom substituents have exactly the same effect as noted before.First, it reduced the dihedral angles forcing the phenyl ring towards coplanarity.Second, it enhances the polarity of the molecule as indicated by the magnitude of the dipole moments (cf.Substitution in general, has but little effect on the geometric features of phenylthiazoles, the major geometric effect, however, is traced in the co-planarity of the molecules studied.
Thus, while 4-phenylthiazole itself is coplanar, substitution in the 5-membered ring lefts this co-planarity.It is very important to notice the effect of substitution by Clor Br-, in the phenyl ring.This substitution has the geometric effect of restoring the co-planarity back.It seems that Cl-or Br-substituents would have a marked effect on the activity of phenylthiazole as A 3 -receptor antagonists.
Substitution has a remarkable effect on the dipole moments, magnitudes and directions.Analysis of dipole moments show fluctuations of their values by substitution.It has been indicated that activity towards A 3receptor sites is directly proportional to the polarity of the thiazole.In the present work, the enhanced polarity of some phenylthiazoles has been analyzed and attributed to σ-/π-polarization.In some cases, especially for N-CH 3 and N-Ph substitutents, cross-conjugation has but a very little effect on the polarity of molecules.This elaborates upon our previous conclusion that, the contributions from the lone-pair electrons dominate the dipole moments.
Analysis of the effect of substitution, on the net electronic charge on the thiazole "N" atom, shows that the NH-CH 3 and NH-Ph derivatives accumulate more than 0.53e on the thiazole "N" atom.On the other hand, substitution in the phenyl ring has but little effect on the amount of charge on the thiazole N atom.This charge is of prime importance in H-bond formation with A 3 -receptor sites.This hydrophilic interaction seems to underlie the selectivity for the human A 3 -receptor.In vitro, the thiazole moiety is surrounded by many hydrophobic amino acids.
The tendency of the studied phenylthiazoles to act as electron-donors or electron-acceptors has also been in-vestigated.The energies of the HOMO's of the studied series of thiazoles are around 5 eV.This low ionization potential indicates a high tendency toward electron donation.Furthermore, aryl-2-methylthiazoles are characterized by low lying LUMO's, indicating considerable electron-affinity.This is of prime importance in enhancing the activity towards A 3 -receptor sites.
The present analysis of the structure and electronic properties of phenylthiazoles focuses on four main structural features.The first feature is the co-planarity of molecules.The second is the polarity, as indicated by the dipole moments and their directions.The third feature is the net negative charge on the thiazole "N" atom.This charge enhances H-bond formation; a property of prime importance for binding to A 3 -receptor sites.Finally, the fourth feature is the electron donating and accepting tendency of the studied thiazoles.A substituent that forces co-planarity increases the dipole moment and the charge accumulated on the thiazole "N" atom, and lowers the LUMO, would certainly enhance the activity as potent and selective adenosine A 3 -receptor antagonists.

Figure 1 .
Figure 1.Perspective view of dipole moment orientation of 4-phenylthiazole using 6-31G and 6-31G (d).The origin of coordinate system used is located at the center of mass at point "1" for each molecule; the orientation of dipole moment is along the bold line from 1 to 2.
Figure (2) presents the geometries of the studied thiazoles and the corresponding dipole moment vectors.

Figure 2 .
Figure 2. Perspective view of dipole moment orientation of substituted-2-methylthiazole.The origin of coordinate system used is located at the center of mass at point "1" for each molecule; the orientation of dipole moment is along the bold line from 1 to 2.

Figure 3 .
Figure 3. Perspective view of dipole moment orientation of substituted-2-N-methylthiazole-2-amine.The origin of coordinate system used is located at the center of mass at point "1" for each molecule; the orientation of dipole moment is along the bold line from 1 to 2.