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Internet Electronic Journal of Molecular Design - IEJMD, ISSN 1538-6414, CODEN IEJMAT
ABSTRACT - Internet Electron. J. Mol. Des. September 2003, Volume 2, Number 9, 589-598

Density Functional Studies of Molecular Polarizabilities. Part 11. The Dipole Polarizability of the Most Stable Conformation of 3'-Azido-3'-deoxythymidine AZT
Javier Hernandez, Humberto Soscun, and Alan Hinchliffe
Internet Electron. J. Mol. Des. 2003, 2, 589-598

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Abstract:
We report an ab initio Hartree-Fock HF and Density Functional Theory DFT study of the dipole polarizability α of 3'-azido-3'-deoxythymidine (AZT) compound. Geometry was fully optimized at the HF/6-31+G(d,p) and BLYP/6-31+G(d,p) levels of theory using X-ray data as starting structure. These calculations have allowed us to find the most stable conformational structure of AZT. This structure shows a torsion angle between thymine and furanose rings larger than 90° and a nonlinear azido group located trans to the C2'-C3' bond of furanose ring. This conformation, verified by vibrational analysis, involves a hydrogen interaction between the oxygen atom in the position 2 of the thymine ring and the hydrogen atom in the position 3' of the furanose ring (O-H bond lengths are 2.4618 and 2.4536 Å at HF and BLYP, respectively). The calculated geometry of the furanose, azide and thymine fragments agree with X-ray AZT structure and with other DFT calculations, whereas the orientation of the thymine ring in the optimized structure shows strong discrepancies with the orientation in the crystal. The static dipole polarizability a was evaluated using the 6-311++G(3d,3p) basis sets with HF, BLYP and B3LYP methods. The polarizability results were compared with values from the empiric atomic methods of Miller, Thole, Swart and Jensen, giving average polarizability for AZT that range between 166 to 214 au values, whereas the corresponding values at HF/6-311++G(3d,3p), B3LYP/6-311++G(3d,3p) and BLYP/6-311++G(3d,3p) approaches are 157.27, 189.77 au and 182.34 au, respectively. These results indicated that the best value for AZT α lies in the middle of the range between the smaller and the largest empiric polarizability value, that correspond to the Miller and Thole methods.

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