A mechanistic investigation into the microbial chiral inversion of 2-arylpropionic acids using deuterated derivatives of 2-phenylpropionic acid

Cordyceps militaris has been previously shown to invert the chirality of (R)-2-phenylpropionic acid to its antipode in the absence of any other biotransformation. To investigate the mechanism of this unusual biotransformation, (R,S)-[2-²H₁]-2-phenylpropionic acid, (R,S)-[2-²H,3,3,3-²H₃]-2-phenylpropionic acid and (R,S)-[3,3,3-²H₃]-2-phenylpropionic acid were synthesized and incubated with C. militaris. NMR spectroscopy showed that deuterium exchange of the α-methine proton occurred during the inversion process but there was no exchange of the β-methyl protons. There were no significant differences in the rates of chiral inversion of the three deuterated derivatives and the undeuterated compound, and with all the compounds attaining 84% enantiomeric excess of the (s)-enantiomer after 48 h incubation. The deuterium exchange is not therefore the rate limiting process in this biotransformation. The recovery profiles demonstrated that there was rapid metabolism of the β-methyl deuterated derivatives which did not occur for the undeuterated 2-phenylpropionic acid or the derivative which was only deuterated at the α-carbon. These studies clearly illustrate that C. militaris is a useful in vitro model of the chiral inversion of 2-arylpropionic acid derivatives in mammalian systems.