RESUMEN
The accurate determination of the nonpolar surface area of glycans is vital when utilizing liquid chromatograph/mass spectrometry (LC-MS) for structural characterization. A new approach for defining and computing nonpolar surface areas based on continuum solvation models (CS-NPSA) is presented. It is based on the classification of individual surface elements representing the solvent accessible surface used for the description of the polarized charge density elements in the CS models. Each element can be classified as polar or nonpolar according to a threshold value. The summation of the nonpolar elements then results in the NPSA resulting in a very fine resolution of this surface. The further advantage of the CS-NPSA approach is the straightforward connection to standard quantum chemical methods and program packages. The method has been analyzed in terms of the contributions of different atoms to the NPSA. The analysis showed that not only atoms normally classified as nonpolar contributed to the NPSA, but at least partially also atoms next to polar atoms or N atoms. By virtue of the construction of the solvent accessible surface, atoms in the inner regions of a molecule can be automatically identified as not contributing to the NPSA. The method has been applied to a variety of examples such as the phenylbutanehydrazide series, model dextrans consisting of glucose units and biantennary glycans. Linear correlation of the CS-NPSA values with retention times obtained from liquid chromatographic separations measurements in the mentioned cases give excellent results and promise for more extended applications on a larger variety of compounds.
RESUMEN
Theoretical descriptions of excited state proton transfer (ESPT) have had various degrees of success. This work presents a theoretical description of the photodissociation of the 7-hydroxyflavylium cation (7-HF), the fundamental chromophoric moiety of anthocyanin natural plant pigments. ESPT of 7-HF is promoted by a significant shift of charge away from the OH group in the first singlet excited state, leading smoothly to the excited conjugate base and a protonated water cluster. Several factors contribute to the consistency of the results of the present study: (1) the theoretical approach (TD-DFT with the B3-LYP functional and def2-TZVP basis set utilizing Grimme's D3 dispersion correction); (2) the modeling of the solvent effect combining hydrogen bonding of the photoacid to a cluster of discrete water molecules in a water-like continuum solvent (COSMO); (3) the large S1 -S2 energy gap of flavylium cations; and (4) the electrostatics of the ESPT in which a proton is transferred from a cationic photoacid to water without Coulombic interaction between the proton and the conjugate base.