1H NMR configurational correlation for retro-inverso dipeptides: application to the determination of the absolute configuration of "enkephalinase" inhibitors. Relationships between stereochemistry and enzyme recognition.

A stereospecific synthesis of thiorphan [N-[2(RS)-(mercaptomethyl)-1-oxo-3-phenylpropyl]glycine] and retro-thiorphan [3-[[1(RS)-(mercaptomethyl)-2-phenylethyl]amino]-3-oxopropanoic acid], two highly potent inhibitors of enkephalinase, a neutral endopeptidase involved in enkephalin metabolism, is reported. Due to a rapid isomerization process, derivatives of retro-thiorphan, which contains a 2-substituted malonyl moiety, cannot be separated by classical methods. However, a separation of the diastereoisomeric mixtures of these retro-thiorphan derivatives was achieved by HPLC. The absolute configuration of each isomer was determined by using an NMR configurational correlation. The inhibitory potency of the various inhibitors indicates that, in the thiorphan series, the affinity for enkephalinase is independent of the stereochemistry of the 2-(mercaptomethyl)-1-oxo-3-phenylpropyl moiety. In contrast, in the retro-thiorphan series a 100-fold difference in the inhibitory activity of the two enantiomers is observed. This indicates that there are large differences in the conformational behavior of the two series of inhibitors at the active site of the enzyme.