Chymotrypsin inhibitory conformation induced by amino acid side chain–side chain intramolecular CH/π interaction

Dipeptide amides H-D-Leu-Phe-NH-R have been found to assume a conformation induced by the CH/π interaction and to inhibit chymotrypsin strongly. A series of benzyl amide derivatives H-D-Leu-Phe-NH-[CH2]n-C6H5(n= 0–4) have been assayed for chymotrypsin. They inhibit the enzyme in a competitive manner and the highest inhibition is achieved by the amide of n= 1 (Ki= 3.6 × 10–6M). The activity enhancement is dependent upon the length of methylene chain, not upon the increase in molecular hydrophobicity, indicating the presence of an optimal distance between dipeptide backbone and C-terminal phenyl group for chymotrypsin inhibition. The C-terminal phenyl group has been found to interact with chymotrypsin stereospecifically. The R-isomer of H-D-Leu-Phe-NH-CH(CH3)-C6H5 is as active as the benzyl amide, while the S-isomer is about twenty-fold less active. When the fluorine atom is introduced at a para-position of the C-terminal phenyl group, the resulting dipeptide H-D-Leu-Phe-NH-CH2-C6H4F-p exhibits about six-times increased inhibitory activity (Ki= 6.1 × 10–7M; this dipeptide is one of the most potent chymotrypsin inhibitors to date). 1H NMR conformational analyses of these dipeptide amide derivatives show the CH/π interaction between D-Leu-isobutyl and Phe-phenyl as a key structural element for chymotrypsin inhibition. These structural examinations strongly suggest that in the inhibitory conformation the C-terminal phenyl group fits the chymotrypsin S1 site, while the hydrophobic core constructed by D-Leu-Phe CH/π interaction fits the chymotrypsin S2 or S1′ site.