T. G. Traylor

Kinetics and mechanism studies in biomimetic chemistry: metalloenzyme model systems

To understand the biological transport and utilization of dioxygen at the level of mechanistic chemistry it has been necessary to synthesize small molecules (active site sections) which carry out the binding or catalytic function. We have prepared iron porphyrin compounds which mimic the dioxygen binding to myoglobin and the oxidations catalyzed by peroxidases and related enzymes. Using a combination of structural and environmental perturbations and millisecond to sub- picosecond laser photolysis methods of following bimolecular or cage reactions we have established many of the factors which control dioxygen binding to heme proteins as well as details of cage processes in ligand binding to small molecules. Additionally, effective catalysts for biomimetic oxidations have been developed and studied by kinetic methods. Studies of the effects of catalyst, oxidant and substrate structure and of environment on the rates of these catalytic processes have allowed mechanisms of epoxidation, hydroxylation and biomimetic suicide labelling to be determined.

A vanadium containing nitrogenase preparation: Implications for the role of molybdenum in nitrogen fixation☆

Abstract Cell free extracts containing significant amounts of non-dialyzable vanadium have been obtained by substitution of vanadium for molybdenum in the growth medium of nitrogen fixing Azotobacter vinelandii OP cells. These extracts exhibited much lower specific activity, total metal content, and binding affinity for acetylene than extracts obtained from cells grown on molybdenum. The results indicated that vanadium will replace molybdenum in nitrogenase and that molybdenum plays a role in the substrate binding and reduction site of the enzyme complex.

Sterically protected hemins with electronegative substituents: efficient catalysts for hydroxylation and epoxidation

Meso-tetra(2,6-dichlorophenyl)porphinatoiron(III)chloride and meso-tetra(pentachlorophenyl)porphinatioiron(III)chloride, which resist µ-oxo dimmer formation and oxidative destruction, are found to be unusually efficient catalysts for high-turnover, high-yield alkene epoxidation and alkane hydroxylation.

Model compound studies related to peroxidases.II: The chemical reactivity of a high valent protohemin compound

Abstract The chemical reactivity of the model analog to compound I of the peroxidases resulting from the reaction of “chelated protohemin” and m-chloroperbenzoic acid is examined. The model intermediate shows no H-atom abstraction or O insertion activity and substrate reactivity depends only on the E 1 2 value of the substrate. A Marcus theory treatment of the available kinetic data for HRP suggests that the oxidative pathway for substrate oxidations is an outer-sphere electron transfer. From the results of the model catalyzed oxidation of 1,4-cyclohexadiene to benzene, an alternate mechanism for cytochrome P-450 catalyzed hydroxylations is suggested.

Reactions of hydroperoxides with iron(III) porphyrins: Heterolytic cleavage followed by hydroperoxide oxidation

Abstract Reaction of α,α-dimethylphenethyl hydroperoxide with iron(III) tetramesitylporphyrin in protic solvent gives alkoxy-radical products and little epoxide. Such observations are usually interpreted as evidence for homolysis of the OO bond in the catalytic process. Yet the same products are now obtained under similar conditions from the reaction of this hydroperoxide with the high-valent oxene (Fe+O), generated unambiguously. Therefore such products are not necessarily evidence for homolysis but are consistent with heterolysis. Nevertheless, the solvent can affect the nature of O-O cleavage.

Photoelectron spectra of cyclopropylcarbinyltrimethyltin and allyltrimethyltin : A comparison of σ-σ and σ-π conjugation

Abstract The UV photoelectron spectrum of cyclopropylcarbinyltrimethyltin (I) has been compared with the spectra of allyltrimethyltin (II) and isobutyltrimethyltin. A σ-σ conjugation in (I) resulting in a 1.6 eV interaction between the CSn bond and the cyclopropane orbitals and a 2.2 eV σ-π interaction in (II) have been demonstrated.

A new method for the determination of ligand dissociation rate constant of carboxyhemoglobin.

Abstract Microperoxidase binds CO faster and more strongly than does hemoglobin. We have used this observation to determine the overall CO-dissociation rate constants, l, of carboxyhemoglobin. The results of our studies indicate that the rate constant l is not very different from the statistically corrected l 4 , suggesting that, compared to HbO 2 , cooperativity in the dissociation rate constants of carboxyhemoglobin is greatly reduced.

NMR studies of P-450 model systems: new structural probes for sulfur-containing hemoproteins.

Abstract A simple model for ferrous cytochrome P-450 has been investigated by proton and carbon-13 Fourier transform NMR. In the proton spectrum of the β-phenethyl mercaptan-protoheme-CO complex, the protons α and β to mercaptide sulfur are observed 2.62 and 0.62 ppm upfield of tetramethylsilane. The 13 CO spectra show characteristic shifts at 204.7 and 197.0 δ for neutral and deprotonated mercaptan complexes, respectively.

A novel hemin reducing system: Crown ether complexes of sodium dithionite and their uses as reducing agents in nonaqueous solvents

Abstract The preparation of an 18-crown-6-complex of sodium dithionite is described. This complex is soluble in DMF, DMSO, alcohols, and other solvents and allows dithionite to be used as a reducing agent in solvents in which it is otherwise insoluble. The complex easily reduces a new chelated protohemin in nonaqueous media and, in aprotic solvents, it selectively reduces the hemin in the presence of oxygen, yielding the oxyheme derivative. In addition, the reduction of both azobenzene and disulfide bonds in organic solvents further demonstrates the utility of the new complex.

Reversible oxygenation of protoheme-imidazole complex in aqueous solution (1,2)

Solutions of protohemin in aqueous buffer containing imidazole were reduced and exposed to carbon monoxide forming the carbon monoxide-imidazole complex similar to that in carboxyhemoglobin. This complex is stable for long periods in the presence of low pressures of oxygen and thus the standard flash photolysis methods can be used to determine rates of combination of the heme-imidazole complex with oxygen. Combination rates for both carbon monoxide and oxygen are faster than any on rates for hemoglobin and oxygen dissociation rates are also faster. But the equilibrium constant for binding of this isolated site is larger than that for hemoglobin.