Structural Elucidation of the Bispecificity of A Domains as a Basis for Activating Non‐natural Amino Acids

Many biologically active peptide secondary metabolites of bacteria are produced by modular enzyme complexes, the non‐ribosomal peptide synthetases. Substrate selection occurs through an adenylation (A) domain, which activates the cognate amino acid with high fidelity. The recently discovered A domain of an Anabaenopeptin synthetase from Planktothrix agardhii (ApnA A₁) is capable of activating two chemically distinct amino acids (Arg and Tyr). Crystal structures of the A domain reveal how both substrates fit into to binding pocket of the enzyme. Analysis of the binding pocket led to the identification of three residues that are critical for substrate recognition. Systematic mutagenesis of these residues created A domains that were monospecific, or changed the substrate specificity to tryptophan. The non‐natural amino acid 4‐azidophenylalanine is also efficiently activated by a mutant A domain, thus enabling the production of diversified non‐ribosomal peptides for bioorthogonal labeling.     

A systematic ab initio study of the water dimer in hierarchies of basis sets and correlation models

A systematic, high-level ab initio investigation of the water dimer has been performed. The oxygen-oxygen bond distance has been estimated to be around 2.90 ?, about 0.05 ? shorter than the experimentally estimated distance, challenging the accuracy of the latter. The interaction energy has been obtained at −5.0±0.1 kcal/mol, which compares favourably with the experimentally estimated value of −5.4±0.7 kcal/mol. The importance of employing basis sets that include diffuse functions in correlated calculations on hydrogen-bonded systems is confirmed. In correlated calculations on the water dimer and the hydrogen fluoride dimer, the counterpoise-corrected interaction energies converge considerably slower towards the basis set limit than do the uncorrected energies, provided that the correlation-consistent basis sets are augmented with diffuse functions. Received: 12 February 1997 / Accepted: 5 June 1997