F NOTTINGHAM, NOTTINGHAM, NG7 2 R D Recently Woodward and Hoffmann,lp2 Fukui? and Longuet-Higgins and Abrahamson4 have interpreted these concerted reactions in terms of the symmetry properties of the reactant and product energy levels. The definitive rules developed by Woodward and Hoffmann are of particular interest to the organic chemist for they not only explain the large majority of these reactions known at present, but permit predictions to be made on reactions hitherto not s t ~d i e d . ~, ~, ~ This Review is largely devoted to the qualitative aspects of orbital symmetry relationships, and in particular to examining the application of the Woodward-Hoffmann rules to concerted organic reactions. The rules are based on the results of extended Huckel calculations,6 and mark an important achievement of
A model protein, ribonuclease A (bovine pancreas), was examined for its ability to coordinate Ni2+ and promote selective oxidation. In the presence of a peracid such as monopersulfate, HSO5-, nickel induced the monomeric RNase A to form dimers, trimers, tetramers, and higher oligomers without producing fragmentation of the polypeptide backbone. Co2+ and to a lesser extent Cu2+ exhibited similar activity. The nickel-dependent reaction appeared to result from a specific association between the protein and Ni2+ that allowed for transient and in situ oxidation of the bound nickel to yield intermolecular tyrosine-tyrosine cross-links. Macrocylic nickel complexes that had previously been shown to promote guanine oxidation were unable to mimic the activity of the free metal salt. Amino acid analysis of the protein dimer confirmed the expected consumption of one tyrosine per polypeptide and formation of dityrosine. The presence of excess tyrosine efficiently inhibited formation of the protein dimer and produced instead a ribonuclease-tyrosine cross-link. In contrast, high concentrations of the hydroxyl radical quenching agent mannitol only partially inhibited ribonuclease dimerization. The polypeptide-mediated activation of nickel and its subsequent reactivity mimic a process that could contribute to the adverse effects of nickel in vivo.
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