The three tripeptides δ-L-α-aminoadipoyl-L-cysteinyl-D-propargylglycine, δ-L-α- aminoadipoyl-L-cysteinyl-D-cyanoalanine and δ-L-α-aminoadipoyl-L-cysteinyl-D-[4, 4, 5, 5- 2H4]-norvaline were synthesised and incubated with the enzyme Isopenicillin N Synthase (IPNS). All incubation mixtures contained biologically active products, and led to the isolation of four new penicillins (β-ethyl, α-acetylenic, α-and β-nitrile) following purification ...