Irradiation of Proteins in the Solid State: II. Chemical Changes Produced in Bovine Serum Albumin

Changes in nine amino acid residues, which make up 73.5% of bovine serum albumin, have been followed after irradiation of this protein in the solid state with 2-Mev electrons in vacuo. The sensitivity to radiation varies by a factor of 2.5. Although carbonyl groups are formed, no evidence of the breaking of the main chain could be detected. Other products measured included - SH groups, a new amino acid, alpha -amino-n-butyric acid, and an amidelike group which gives rise to amnnonia on hydrolysis. There are pronounced changes in the reactivity of the side changas. The hidden - SH group is revealed, and the position of the tyrosine absorption maximum is shifted, though its abnormally high pK remains unaffected. Some of the amino groups become screened so that they no longer react with 1-fluoro-2,4-dinitrobenzene except in 4 M guandine hydrochloride. It is concluded that the changes in the amino acid residues cannot explain the demturation as shown by changes in the reactivity of the side chains and by the physicochemical changes described in an earlier paper. This investigation supports the hypothesis that denaturation is the result of and that the subsequent chemical changes measured here are not responsible.