The elucidation of the action of doxorubicin (DOX) has been considered a challenge for cancer therapy. Using theoretical approaches, we investigated the structure and electronic properties of DOX as a function of pH, which we thought likely to be related to the influence of its tautomers. Regarding the relative stabilities among the tautomers, the results obtained from PM6 were the most similar to those obtained from DFT. The theoretical absorption spectrum for each tautomeric species simply showed a single absorption peak located around 400 nm, in contrast to the experimental absorption spectra in the literature that showed four absorption bands. The experimental evidence was properly explained by considering four tautomeric conformers of DOX. The spectroscopic study of the deprotonated tautomers also suggested the presence of four deprotonated tautomers at more basic pH values. The spectrum at pH 10.08 can be explained by the presence of protonated and deprotonated doxorubicin species.
Density functional theory (DFT) calculations have been performed to develop a systematic structural analysis of Au 13 L 8 3+ , where L = SCH 3 , SeCH 3 , SCH 2 OCH 3 and S(CH 2) 2 NH 2 , in order to examine the influence of different ligands. Binding energy calculations indicate that the gold core is more stabilized by the ligand in the following sequence S(CH 2) 2 NH 2 > SCH 2 OCH 3 > SeCH 3 > SCH 3. Natural bond orbital (NBO) analysis describes the interaction between the gold and the ligands, showing that the strongest electron donation occurs from a lone pair orbital on the sulfur and selenium atoms to the antibonding acceptor σ * (Au−S) and σ * (Au−Se) orbitals, respectively. The NBO analysis allowed to understand the origin of enhanced stability of the [Au 13 (S(CH 2) 2 NH 2) 8 ] 3+. Time-dependent DFT (TDDFT) calculations have been performed to simulate the optical absorption spectra of Au 13 L 8 3+ in gas phase and under the effect of solvents with different polarities. The absorption spectrum of [Au 13 (S(CH 2) 2 NH 2) 8 ] 3+ shows a spectral profile that differs considerably from the others in gas phase and which is strongly affected by the solvent.
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