Evidence for two acyl group conformations in some furylacryloyl- and thienylacryloylchymotrypsins: resonance Raman studies of enzyme--substrate intermediates at pH 3.0.

Resonance Raman (RR) spectra, obtained by ultraviolet laser excitation, are reported for 10 acylchymoptrypsins at pH 3.0, in which the acyl groups are derivatives of furylacrylic and thienylacrylic acids. Spectra are also shown of the sodium dodecyl sulfate (NaDodSO4) denatured acyl enzymes and the acid and ester analogues of the acyl groups. For most of the native acyl enzymes, the RR spectral profiles in the carbonyl stretching region suggest that the acyl groups bound to Ser-195 adopt two conformations, which are characterized by having either strong hydrogen bonds to the carbonyl oxygen or a nonbonding hydrophobic environment about the C==O group. It is also likely that in solution the ester and acid analogues of the acyl group adopt more than one conformation about the acryloyl linkages. Thus, the measured spectral parameters, such as the ethylenic double bond stretching frequency v C==C). For a series of compounds based on a given acyl group a correlation exists between (v C==C) and the measured absorption maximum (lambda max). Possible explanations are given for the observed changes in (v C==C) and (lambda max) when the acyl groups bind to the active site. A band appears near 1260 cm-1 in the RR spectra of some of the native acyl enzymes; it is not observed in the spectra of the NaDodSO4-treated intermediates or in the spectra of any model compounds.