CHEMICAL MODIFICATION OF METHIONINES OF RIBONUCLEASE A WITH o-BENZOQUINONE

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Abstract
The accessibility of methionines in [bovine] RNAse A to reaction with OBQ [O-benzoquinone] was studied at highly acidic pH. The differences between the rate constants of reactions of the methionine and methionines of RNAse A with OBQ is a reflection on the limited accessibility of methionines in the protein conformation. At sufficiently high OBQ concentration, all the 4 methionines of the enzyme can be modified. At lower concentration of OBQ, a derivative may be prepared in which a specific methionine is modified. The introduced chromophore ionizes at around pH 3 in this derivative. The derivative has partial activity towards RNA which is enhanced on addition of S-protein.