The Pennsylvania State University - 1967
North Texas State University (Now University of North Texas) - 1963
North Texas State University(Now University of North Texas) - 1962
State University New York, Stony Brook - 1967
Outstanding Baylor University Professor
Baylor University - 1982
Baylor University - 1993
Studying the dynamics of chromium(III)- and cobalt(III)-amino acid complex ion reactions has been a continuing theme in our laboratory. Kinetics and stoichiometric studies ranging from oxidation-reduction to general acid hydrolyses to spontaneous and catalytic processes characterize this work. Fundamental to an understanding of metallo-enzyme redox chemistries are the nature and effects of amino acid or peptide ligands in simple metal ion oxidation-reduction reactions. For example, questions of the type, "Is the mechanism of the inner-sphere or outer-sphere type?", "What role, if any, does the chirality of the complex ion play?", "What is the effect of the intervening bond system between metal centers?", and "What, if any, effect(s) do the nonbridging ligands have on the redox reaction rates?" The issue of chirality of the metal ion complex in facilitating electron transfer has been addressed.
There are numerous chiral amino acid cobalt(III) and chromium(III) complexes which have been well-characterized, providing a fertile set of model small molecule oxidants for inquiry. And the problem of metal ion cooperativity in electron transfer events has enormous import for metalloenzyme systems, especially respiratory enzymes. We have recently begun studies of oxo-centered trinuclear, mixed metal complexes which we (and others) have characterized via single crystal x-ray structure determinations and spectral (IR and VIS/UV) analyses. The basic compositions are M2M'(µ3-O)(µ2-O2CR)6(OH2)3n+, where the three metal ions form an equilateral triangle about the central oxygen atom; each pair of metal ions is bridged via two carboxylates, one canted above the M3O plane and the other below; the three metal ions then have one terminal water group bound to complete the octahedral structure about each metal center.
A major difficulty with acid assisted decomposition was encountered with these complex ions in aqueous media, but the decomposition process has afforded a new method for generating oxo-bridged, binuclear metal complex ions. Our second major thrust into reaction mechanisms lies in the study of processes which accelerate the substitution rates on inert chromium(III) complex ions. Specific acid hydrolyses, general acid/general base hydrolyses, chromium(II) catalysis, anion effects, and macrocyclic ligand effects--all have been observed. General acids (e.g. acetic acid and its chloro derivatives) smoothly cleave chromium-sulfur, nitrogen, and oxygen bonds. Remarkably, the three systems which we have investigated to date have yielded different rate laws, and hence involve different mechanisms, for chromium-sulfur bond cleavage. Results from the above noted studies may provde an impetus for a return to investigate the "so-called" Glucose Tolerance Factor, or Cr-GTF, which was found to contain amino acids as simple ligands (or a polypeptide) on the chromium(III) center.
The one study which led to the curtailment of this quest was a claim that the GTF activity had been separated from the Cr. However, the separation procedures in that study involved lengthy chromatographic procedures where the Cr-GTF was exposed to buffer systems containing general acids and bases, where hydrolysis of amino acid- (or petptide)-chromium bonds may have occurred.