Acid-base titrations of cowpea chlorotic mottle virus and its protein suggest that when the virus changes from the compact pH 6.0, 88 S form to the ‘swollen’ pH 7.0, 78 S form a series of changes in the dissociation constants of several amino acid residues is initiated. One such change probably occurs slowly and involves several basic amino acids and this is used to explain the hysteresis observed between acidic and basic titrations. Homoconjugate hydrogen-bonded carboxylic acid groups are probably involved in the swelling process and at least one of these on each sub-unit is detected by displacement of its single associated proton with magnesium. If magnesium is present, the hysteresis between acidic and basic titrations is not found. These observations are discussed in terms of the reversible configurational change undergone by the virus. Electrophoretic data suggest that the ratio of basic to acidic amino acids is greater on the surfaces of the protein subunits buried in the capsid rather than on the surface exposed externally on the capsid.


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