Incorporation of double-labeled L-cystine and DL-valine in penicillin.

l-[3,3'-(3)H]cystine was incorporated into penicillin with retention of one tritium. This result can be explained by beta-lactam formation through ring closure between C3 of cysteine and NH of valine. No radioactivity of dl-[2,3-(3)H]valine was incorporated into penicillin. The loss of isotope at C2 occurs during the inversion of configuration. The loss of label at C3 is discussed in terms of possible intermediates for the formation of the thiazolidine ring of penicillin.