Cover Picture (Angew. Chem. Int. Ed. Engl. 6/1995)

The cover picture shows a schematic representation of the X‐ray structure (green) of the staphylococcal nuclease mutant Glu → homoglutamate (yellow) in the foreground. This mutant containing one unnatural amino acid was prepared biosynthetically by using the new method of oligonucleotide‐directed mutagenesis. The procedure introduces defined, site‐specific structural changes that facilitate physicalorganic studies on this important class of macromolecules. In the background are examples of several of the over one hundred non‐proteinogenic amino acids that have been successfully incorporated into proteins, including aspartate aminotransferase, methionine aminopeptidase, ribonucleotide reductase, ras p21, staphylococcal nuclease, and T4 Iysozyme. More about the methodology and its application is reported by P. G. Schultz et al. on pages 620 ff.