Engineering topics
Publications and source records attributed to Hoffman, Brian M..
Active site FeV-cofactor of the V-nitrogenase and the EPR spectrum of the reduced cofactor showing 51 V-hyperfine coupling.
Significance In this multidisciplinary study, we investigated local and global structural responses upon carbon monoxide (CO) dissociation from the heme iron(II) in myoglobin in solution using ultrafast X-ray absorption and scattering. CO dissociation was optically triggered and pulsed X-rays probed structural and electronic changes as a function of time, revealing an iron(II) spin state transition and nuclear motions propagating from the active site throughout the entire protein. Quantum mechanical computational results were used to assign the iron K-edge absorption features, and to map out electronic and nuclear trajectories during Fe–CO bond elongation and heme macrocycle doming. The study has important implications for heme–protein function and the interaction of small molecule substrates with the metal active centers in enzymes.
Abstract Radical S ‐adenosyl‐ l ‐methionine (SAM) enzymes initiate biological radical reactions with the 5′‐deoxyadenosyl radical (5′‐dAdo . ). A [4Fe‐4S] + cluster reductively cleaves SAM to form the Ω organometallic intermediate in which the 5′‐deoxyadenosyl moiety is directly bound to the unique iron of the [4Fe‐4S] cluster, with subsequent liberation of 5′‐dAdo . . We present synthesis of the SAM analog S ‐adenosyl‐ l ‐ethionine (SAE) and show SAE is a mechanistically equivalent SAM‐alternative for HydG, both supporting enzymatic turnover of substrate tyrosine and forming the organometallic intermediate Ω. Photolysis of SAE‐bound HydG forms an ethyl radical trapped in the active site. The ethyl radical withstands prolonged storage at 77 K and its EPR signal is only partially lost upon annealing at 100 K, making it significantly less reactive than the methyl radical formed by SAM photolysis. Upon annealing above 77 K, the ethyl radical adds to the [4Fe‐4S] 2+ cluster, generating an ethyl‐[4Fe‐4S] 3+ organometallic species termed Ω E .
Abstract not provided
Three known nitrogenase isozymes, Mo-, V-, and Fe-, catalyze biological reduction of dinitrogen (N 2 ) to ammonia (NH 3 ). All three utilize the same reductive elimination mechanism: an intermediate with two metal-bound hydrides reductively-eliminates hydrogen gas (H 2 ) in a reaction coupled to binding and activation of N 2 . Nonetheless, the three isozymes show dramatically different relative rates of H 2 formation and N 2 reduction, revealing important differences in reactivity with substrates. Carbon monoxide (CO) has been characterized as both an inhibitor and substrate for Mo- and V-nitrogenases, but not for the Fe-nitrogenase. In this work, we present a comparative study of the reactivity of the three isozymes with CO, examining CO both as a substrate and as an inhibitor of proton (H + ) reduction under steady-state conditions. For Mo-nitrogenase, there is neither detectable reduction of CO nor inhibition of H + reduction. Fe- and V-nitrogenase show CO reduction and inhibition of H + reduction that depends on the CO partial pressure. For V-nitrogenase, ethylene (C 2 H 4 ) is the major reduction product with a maximum specific activity of ~7.5 nmol C 2 H 4 /nmol VFe protein/min at 1 atm CO. The major product of CO reduction for Fe-nitrogenase is methane (CH 4 ) with a maximum specific activity of ~4.8 nmol CH 4 /nmol FeFe protein/min at 0.05 atm CO. The rate of CH 4 production by Fe-nitrogenase progressively increases to a maximum at 0.05 atm CO and then declines to ~10% with increasing CO partial pressure up to 1 atm. CO does not inhibit proton reduction in Mo-nitrogenase but shows 16% inhibition for V-nitrogenase and 35% for Fe-nitrogenase.
Various forms of cobalt catalysts have been studied extensively for selective aerobic oxidation of hydrocarbons. However, it remains unclear whether cobalt can directly activate molecular oxygen under mild reaction conditions. Here we investigated the catalytic roles of cobalt in ethylbenzene oxidation with and without a hydroperoxide initiator. The contribution of different cobalt species was studied by varying the metal loading on Co-impregnated ZSM-5 samples. Quantitative EPR was used to determine the impact of cobalt catalysts on the free radical concentrations. Furthermore, this work provided strong evidence that cobalt, in several different forms, catalyzes hydrocarbon oxidation by facilitating peroxy bond cleavage, instead of direct oxygen activation.