New probes of enzyme reaction mechanisms: Part I. Selenoxides as serine protease and esterase transition-state analogs. Part II. A new probe for radical intermediates in enzyme-catalyzed hydride transfers

These studies have focused on obtaining a better understanding of the mechanisms of action of enzymes. Enzymes are Nature's catalysts for the chemical reactions of life.;Part I introduces the concept of using selenoxide-containing compounds as transition-state (TS) analogs for serine proteases and a serine esterase. The selenoxide-containing compounds were found to be weak competitive inhibitors for these enzymes. An X-ray crystallographic study of the complex between p-amidinobenzyl phenyl selenoxide and trypsin was undertaken in collaboration with Professor James S. Remington. The expected covalent bond formation between the selenium of the selenoxide and the catalytically-essential active site serine, indicative of the selenoxide acting as a TS analog, was not observed. Possible explanations for this observation are presented.;The hydration of selenoxides has been studied extensively by NMR spectroscopy to aid in interpretation of the enzyme inhibition results. All the studies with different selenoxide molecules indicated that the formation of selenoxide hydrate is kinetically facile but not thermodynamically favored. It was discovered that solvent plays an important role in controlling the stability of alkyl selenoxides toward ;In this research, it was found that it is not feasible to incorporate a selenoxide in a peptide analog to serve as a substrate analog due to the inherent instability of this class of compounds. However, such compounds incorporating a selenoxide or sulfoxide into an amide (-C(=O)NHCH;In Part II, the mechanistic question of one step two-electron transfer vs. two sequential one-electron transfer for horse liver alcohol dehydrogenase (HLAD) catalyzed redox reactions was investigated. Quadricyclan-3-one and quadricyclan-3-ol were used as new probes for radical intermediates. No ring-opened products, diagnostic for a radical intermediate in catalysis, were detected. Model studies for quadricyclan-3-one under conditions involving one-electron reduction known to generate the putative radical intermediate were pursued as well. To this point, all the evidence favors direct hydride transfer for HLAD catalyzed redox reactions.