REEXAMINATION OF THE H-1-NMR ASSIGNMENTS AND SOLUTION CONFORMATIONS OF L-CARNITINE AND O-ACETYL-L-CARNITINE USING C-2 STEREOSPECIFICALLY LABELED MONODEUTERATED ISOMERS

The two C-2 monodeuterated isomers of l-carnitine were synthesized by enzymatic hydration of crotonobetaine in D2O and by enzymatic proton exchange of l-[2-2H2] carnitine in H2O. These reactions, catalyzed by an induced Escherichia coli carnitine hydrolyase proceed stereospecifically. The two isomers of l-[2-2H]carnitine were examined by 1H NMR at 500 MHz, which allowed us to independently monitor the pD dependence and coupling constants of the H-2 protons. The results obtained indicate that there is little effect of the carboxyl charge on the conformational state(s) of l-carnitine about the C-2/C-3 bond. The NMR data obtained in this study do not support previous solution studies of the pH-dependent conformational changes for dl-carnitine nor the proposed conformation of O-acetyl-dl-carnitine in the crystalline state. © 1990.