The conversion of the tripeptide (L-a-amino-6-adipoyl)-L-cysteinyl-D-valine (a-amino-6-adipoyl = 5-amino-5-carboxypentanoyl) into isopenicillin N by the enzyme isopenicillin N synthetase has been shown to proceed with complete retention of stereochemistry; a similar result was observed during penicillin formation with isoleucinyl and allo-isoleucinyl tripeptides.