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

Dipeptide amides H-D-Leu-Phe-NH-R have been found to assume a conformation induced by the CH/pi 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 (K-1 3.6 x 10(-6) M), 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 (K-1 = 6.1 x 10(-7) M; this dipeptide is one of the most potent chymotrypsin inhibitors to date), H-1 NMR conformational analyses of these dipeptide amide derivatives show the CH/pi: 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 S-1 site, while the hydrophobic core constructed by D-Leu-Phe CH/pi interaction fits the chymotrypsin S-2 or S-1' site.