As shown in the diagram below, penicillin is not a single compound but a group of closely related compounds, all with the same basic ring-like structure (a beta-lactam) derived from two amino acids (valine and cysteine) via a tripeptide intermediate. The third amino acid of this tripeptide is replaced by an acyl group (R in the diagram below), and the nature of this acyl group confers specific properties on different types of penicillin.