Steric effects

About: Steric effects is a research topic. Over the lifetime, 16112 publications have been published within this topic receiving 319615 citations. The topic is also known as: steric hindrance.

An assessment of the relative contributions of redox and steric issues to laccase specificity towards putative substrates

Abstract: Laccases catalyze the one-electron oxidation of a broad range of substrates coupled to the 4 electron reduction of O2 to H2O. Phenols are typical substrates, because their redox potentials (ranging from 0.5 to 1.0 V vs. NHE) are low enough to allow electron abstraction by the T1 Cu(II) that, although a relatively modest oxidant (in the 0.4–0.8 V range), is the electron-acceptor in laccases. The present study comparatively investigated the oxidation performances of Trametes villosa and Myceliophthora thermophila laccases, two enzymes markedly differing in redox potential (0.79 and 0.46 V). The oxidation efficiency and kinetic constants of laccase-catalyzed conversion of putative substrates were determined. Hammett plots related to the oxidation of substituted phenols by the two laccases, in combination with the kinetic isotope effect determination, confirmed a rate-determining electron transfer from the substrate to the enzyme. The efficiency of oxidation was found to increase with the decrease in redox potential of the substrates, and the Marcus reorganisation energy for electron transfer to the T1 copper site was determined. Steric hindrance to substrate docking was inferred because some of the phenols and anilines investigated, despite possessing a redox potential compatible with one-electron abstraction, were scarcely oxidised. A threshold value of steric hindrance of the substrate, allowed for fitting into the active site of T. villosa laccase, was extrapolated from structural information provided by X-ray analysis of T. versicolor lac3B, sharing an identity of 99% at the protein level, thus enabling us to assess the relative contribution of steric and redox properties of a substrate in determining its susceptibility to laccase oxidation. The inferred structural threshold is compatible with the distance between two phenylalanine residues that mark the entrance to the active site. Interaction of the substrate with other residues of the active site is commented on.

Electron Spin Resonance Spectra of Carbonyl Anion Radicals

Abstract: Electron spin resonance studies are reported on the anion radicals of single‐ring aromatic ompounds containing aldehyde, acetyl, or amide groups, as well as other substituents. The radicals were generated by electrolytic reduction in N,N‐dimethylformamide solution. Many of the radicals have spectra which indicate that the carbonyl group is locked in a conformation planar with the ring for times of the order of a microsecond or longer. The para dicarbonyls and the 3‐cyanoacetophenone anion were found to be present in both the cis and trans modifications. A simple modification of conventional molecular‐orbital theory has been used with considerable success to account for the loss of symmetry in the pi‐electron spin density for compounds with a locked carbonyl group, and the calculated energy differences for the cis and trans isomers are in good agreement with experiment. Molecular‐orbital calculations of spin densities were made for most of the radicals, often with excellent results, and comparisons are made with the predictions of valence‐bond theory. The benzaldehyde, acetophenone, and 4‐fluoroacetophenone anions have spectra with abnormally small ring‐proton splitting constants, and no satisfactory explanation of these anomalous results has been found. The appearance or nonappearance in all the radicals but these three of twofold symmetry in the pattern of splitting constants is interpreted qualitatively in terms of competing effects determined by the bond order of the bond between the ring and the carbonyl group, and steric factors in the neighborhood of the carbonyl group. A number of features of the experimentally determined spin‐density distributions have been correlated with the relative electron‐withdrawing effects of the substituents.