Site-specific incorporation of photoisomerizable azobenzene groups into ribonuclease S

Syntheses of S-peptide analogues bearing phenylazophenylalanine (Pap) residues at positions 4, 8, and 11 are described. Noncovalent reassociation of the Pap-4 and Pap-11 peptides with S-protein reconstitutes ribonuclease activity. Photoisomerization of the Pap-4 peptide is found to modulate the enzyme activity.