o-Benzoquinone-a reagent for determining conformational differences in related proteins.

OBQ reacts specifically with methionine residues of RNAase A at highly acidic pH. At pH 3, whereas RNAase A does not react, RNAase S, S-protein and S-peptide all react with OBQ. RNAase A derivatives in which some of the methionines have been initially modified at pH 1, further react with OBQ at pH 3. Thus, reaction with OBQ can be used to monitor structural transitions and conformational differences involving methionines in proteins.