D-L conversion pathways between optical isomers of alanine: Applications of the scaled hypersphere search method to explore unknown reaction routes in a chiral system

D-L conversion pathways of alpha-alanine have been discovered by theoretical search of potential energy surfaces at the level of B3LYP/6-311++G(d,p). No direct conversion channel via only one transition state could be found between D and L optical isomers of alanine. It is demonstrated that there are four types of conversion channels in chiral amino acids. The interconversion of four functional groups (H, CH3, NH2, and COOH) around the asymmetric carbon atom can occur as rotation of a single-bonded pair of two groups involving either NH2 or COOH together with H or CH3.