Theoretical study on the optical isomerization of amphoteric α-alanine and Na+ complexes in water-liquid phase environment.

The optical isomerization of amphoteric S-α-Ala·Na⁺ in water-liquid phase environment was investigated by using the M06-2X method based on density functional theory and the SMD model method based on self consistent reaction field (SCRF) theory. The results showed that the optical isomerization of S-α-Ala·Na⁺ has three channels a,b and c. In channel a, the proton transfers with amino nitrogen as a bridge after the isomerization of amphoteric S-α-Ala·Na⁺ to neutral S-α-Ala·Na⁺. And the proton is transferred merely using carbonyl oxygen as a bridge in channel b. In channel c, the proton transfers to α-C from protonation amino after α-H is transferred to carbonyl oxygen. The potential energy surface showed that the channel a has the most advantage under the effect of recessive solvent and the energy barrier of the rate-determining step is 234. 8 kJ·mol^-1; While channel b and c had a disadvantage, they both have the common energy barrier of the rate-determining step about 267. 9 kJ·mol^-1. Under the effect of explicit solvent, channel a turned to an inferior channel, and its energy barrier of the rate-determining step is between 155. 9 and 156. 5 kJ·mol^-1; channel b and c have a slight advantage and the energy barrier of the rate-determining step is between 132.2 and 138.6 kJ·mol^-1. The results showed that α-alanineNa (I) complex could only racemize at a extremely slow speed in the water-liquid environment. [ABSTRACT FROM AUTHOR]