Towards understanding mode of action of L-dopa and carbidopa: DFT/TD-DFT analyses of their electronic and vibration spectra

1378-1386<span style="font-size:11.0pt;font-family: " times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";mso-bidi-font-family:="" mangal;mso-ansi-language:en-gb;mso-fareast-language:en-us;mso-bidi-language:="" hi"="" lang="EN-GB">The electronic absorption and vibrational spectra of L-dopa (LD) and carbidopa (CD) have been measured experimentally in different solvents and also computed theoretically. FTIR spectra of LD and CD have been computed theoretical at the B3LYP/6-311++G** level of theory. <span style="font-size: 11.0pt;font-family:" times="" new="" roman","serif";mso-fareast-font-family:calibri;="" mso-bidi-font-family:mangal;mso-ansi-language:en-gb;mso-fareast-language:en-us;="" mso-bidi-language:hi"="" lang="EN-GB">A scaling factor of 0.95 results in good correspondence between calculated and experimental spectra. Vibrational modes have been assigned and similarities and differences between the FTIR spectra of LD and CD are discussed. Two IR marker bands have been identified for CD. Based on TD-DFT/TPSSh calculations, the leading transitions contributing to the electronic absorption of LD and CD are proposed. These findings have been discussed in the context of the experimentally observed spectra reported for LD and CD in different solvents. NTO analyses clearly indicate that most of the leading transitions in LD and partially in CD involve a sizable charge transfer from the aromatic catechol moiety to the aliphatic amino acid side chain. The short wavelength transition, however, shows an opposite trend.</span