A new method has been developed that makes it possible to site-specifically incorporate unnatural amino acids into proteins. Synthetic amino acids were incorporated into the enzyme beta-lactamase by the use of a chemically acylated suppressor transfer RNA that inserted the amino acid in response to a stop codon substituted for the codon encoding residue of interest. Peptide mapping localized the inserted amino acid to a single peptide, and enough enzyme could be generated for purification to homogeneity. The catalytic properties of several mutants at the conserved Phe66 were characterized. The ability to selectively replace amino acids in a protein with a wide variety of structural and electronic variants should provide a more detailed understanding of protein structure and function.

An experimental determination of the thermodynamic stabilities of a series of amyloid fibrils reveals that this structural form is likely to be the most stable one that protein molecules can adopt even under physiological conditions. This result challenges the conventional assumption that functional forms of proteins correspond to the global minima in their free energy surfaces and suggests that living systems are conformationally as well as chemically metastable.

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Methodology is described for the synthesis and chemical aminoacylation of the hybrid dinucleotide 5'-phospho-2'-deoxyribocytidylylriboadenosine (pdCpA). Ligation of aminoacylated pdCpA to a truncated amber suppressor tRNACUA (-CA) using T4 RNA ligase generates an aminoacylated suppressor tRNA which can be used for site-specific incorporation of unnatural amino acids into proteins. Both the ligation and in vitro suppression efficiencies are the same when either pCpA or pdCpA is used. The use of deoxycytidine simplifies the chemistry involved in the synthesis of the dinucleotide pCpA. In addition, these results demonstrate that ribocytidine is not required for recognition of the aminoacylated tRNA during protein synthesis.