A systematic survey of the structural requirements for biosynthetic incorporation of non- natural residues into a polypeptide is presented. Relative translation efficiencies for a series of 12 semi-synthetic acylated suppressor tRNAs ranged from 0 to 91% depending on the structure of the residue incorporated. A systematic survey of the structural requirements for biosynthetic incorporation of non-natural residues into a polypeptide is presented. ...