Effect of External Electric Field Upon Selected Proteogenic Amino Acids

Effect of external electric field (EEF) of 0.001, 0.005 and 0.01 a.u. upon molecular energy, charge distribution and dipole moments of non-dissociated and inner salt forms of alanine (Ala), arginine (Arg), aspartic acid Asp), cysteine (Cys), glutamic acid (Gln), glycine (Gly), isoleucine (Ile), leucine (Leu), lysine (Lys), methionine (Met), ornithine (Orn), proline (Pro), serine (Ser), threonine (Thr), tryptophan (Trp), tyrosine (Tyr), and valine (Val) were studied. For that purpose HyperChem 8.0 software was used together with the AM1 method for optimization of the conformation of the molecules in a computer vacuum. Based on the effect of EEF upon the charge density localized at the nitrogen atom of the α-amino group the acids were divided into two groups. They were Group I in which EEF increased the negative charge (Ala, Gly, Ile, Leu, Lys, Met, Phe, Pro, and Thr) and Group II in which EEF induced opposite effect (Cys, Ser, Tyr and Val). Generally, an increase in the EEF strength declined energy and increased dipole moments in non-ionized amino acids and in their inner salt forms. Energy of non-dissociated forms was more negative than these of corresponding zwitterions. Orientation of the molecules in EEF strongly depended on the EEF strength.