气相中疏水氨基酸的单电子氧化还原性质

Characteristics of One Electron Redox Behavior of Hydrophobic Amino Acids in Gas Phase

采用密度泛函理论在B3LYP/DZP++水平上研究气相中疏水氨基酸的单电子氧化还原性质.计算表明:发生单电子氧化反应时,侧链较小的甘氨酸、丙氨酸、脯氨酸、缬氨酸、亮氨酸、异亮氨酸丢失电子的主要部位是氨基、α-碳和羧基,对应着相对较大的绝热电离能(8.52-9.15 eV);而半胱氨酸、甲硫氨酸、苯丙氨酸、酪氨酸、色氨酸因侧链丢失较多负电荷,其电离能有所降低.气相中疏水氨基酸从外界捕获的电子主要驻留在羧基或氨基的氢原子外侧以及分子的骨架上,形成具有偶极边界结构和价键结构的混合状态阴离子,绝热电子亲和势在-0.08至-0.63 eV之间.由于氨基酸的电离能较大且电子亲和势为负值,所以在气相中它们既不容易被氧化也难以被还原.

Characteristics of the one electron redox behavior of hydrophobic amino acids in gas phase were calculated with density functional theory at the B3LYP/DZP＋＋ level.For glycine,alanine,proline,valine,leucine,and isoleucine with small side chains,the computational results indicate that the negative charges are removed from the atoms of their amino,α-carbon,and carboxy moieties in one electron oxidation reactions.This yields large adiabatic ionization potentials（AIP） of 8.52-9.15 eV.The AIPs of cysteine,methionine,phenylalanine,tyrosine,and tryptophan decrease because of the larger amount of negative charge removed from the atoms in their side chains.The attachment of one electron to the molecules of hydrophobic amino acids leads to anions in which the extra electron is bound to the H atoms of the carboxyl or amino groups and to their valence orbitals,reflecting the double nature of the dipole-bound state and the valence state.The electron affinities（EA） for the amino acids are small and negative ranging from-0.08 to-0.63 eV.The molecules of the hydrophobic amino acids are oxidized or reduced with difficulty in gas phase because of their high VIPs and negative EAs.