Magnetic resonance studies of protein-small molecule interactions. Binding of N-trifluoroacetyl-D-(and L-)-tryptophan to α-chymotrypsin

A magnetic resonance technique has been developed for studying the competitive binding to proteins of two small molecules; the nmr spectrum of only one needs to be observed. This technique has been applied to study the competition between N-trifluoroacetyl-D-tryptophan and the L enantiomer for the active site of α-chymotrypsin from pH 5.0 to 8.0. The chemical shift for the fluorine nuclei of N-trifluoroacetyl-D-tryptophan bound to the enzyme is found to be the same as that for N-trifluoroacetyla-D -p fluorophenylalanine. The binding of both D- and L-tryptophan derivatives shows a marked dependence on deprotonation of a group on the free enzyme with pK_a = 6.6 (presumably His-57).