Abstract
In the present study, prior MD investigation or Monte-Carlo search, our step-by-step search for the real lowest energy conformers and the properties of taxol (Also called paclitaxel) is presented theoretically. The paper completes the old paper of Ballone and Marchi (JPC A 1999) which carried out LDA calculations on taxol. A total of 216 initial trial structures were generated by all combinations of internal single-bond rotamers and optimized at the B3LYP/3-21G* level and further optimized at the B3LYP/6-311G* level. A total of 12 unique conformers are found, and their relative energies, dipole moments, rotational constants, zero point vibrational energy, and harmonic frequencies are determined. Their relative electronic energies were determined at the M06L/6-311G(2df,p) level. Combined with statistical mechanics principles, conformational distributions at various temperatures are computed. Characteristic Hbonding types are classified and demonstrated in the taxol structure. It is found that various hydrogen bonds (red shift H-bonding, blueshifted H-bonding and dihydrogen bond) coexist in the title compound. The NBO analysis was performed showing the change of the charge and stereoelectronic effect of different radicals. The UV spectra of the lowest-lying conformer of taxol in methanol are investigated with the TD CAM-B3LYP/6-311+G(2df,p) calculations. The S0-S1, S0-S2, and S0-S3 excitations of taxol are mixed ππ*/πσ*/nπ* transitions at 4.32, 5.02, and 5.13 eV, respectively. The theoretical IR spectrum and UV absorption spectrum of the taxol agree with the available experimental data of taxol very well. Calculations in the solution using SCRF/CPCM method at the M06L/6- 311G(2df,p) level showing the hydrophobic nature of taxol.
Keywords: Anticancer drug taxol, Absorption spectra, Conformational distribution, Hydrogen bond, Topological analysis, Vibrational frequencies
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