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

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Abstract
Changes in nine amino acid residues, which make up 73.5% of bovine serum albumin, were followed after irradiation in the solid state with 2-Mev electrons in vacuo. The sensitivity to radiation varies by a factor of 2.5. Although carbonyl groups were formed, no evidence of the breaking of the main chain could be detected. Other products measured included -SH groups, a new amino acid, [alpha]-mino-n-butyric acid, and an amide-like group which gave rise to ammonia on hydrolysis. There were pronounced changes in the reactivity of the side chains. The hidden -SH group was revealed, and the position of the tyrosine absorption maximum shifted, although its abnormally high pK remained unaffected. Some amino groups became screened, no longer reacting with 1-fluoro-2,4-dinitro-benzene except in 4 M guanidine hydrochloride. The changes in the amino acid residues could not explain the denaturation. It is concluded that denaturation is the result of the simultaneous rupture of a number of hydrogen bonds, and that the subsequent chemical changes measured here were not responsible.