Reexamination of the ¹H 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.