Development of a method allowing high resolution X-ray crystallographic analysis of proteins by site-specific heavy-atom labeling

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This research project includes synthesis of amino acids containing heavy atoms that allow high resolution X-ray crystallographic analysis of a protein labeled with a few heavy atoms at specific positions in the molecule. Although the introduction of a chalcogen atom such as selenium or tellurium into tryptophan was unsuccessful, bromine atom has been incorporated in the 5-bromotryptophan derivative (1). Because the bromine atom in 1 is much larger than hydrogen atom, there remains a need to replace the nitrogen atom in the indole ring with an atom in the same group as nitrogen if biosynthetic labeling of a protein is required : the preparation of arsenotryptophan is now in progress. An approach using semisynthesis of a protein from a small synthetic peptide containing a heavy-atom and a large peptide fragment of native origin has also been developed for the case with 1.The reliability of this method could be enhanced strongly by an algorithm allowing the calculation of the lowest-energy structure of a protein precisely. The study of tertiary-structure prediction of the S-peptide of ribonuclease A by Monte Carlo simulated annealing has proved promising because not only the experimental evidence about this peptide could be reproduced but also the electrostatic effect predicted for the side-chain groups has been supported by experiments including CD and NMR.These features evolved from this project could possibly make the structural analyses of proteins faster and more precise in resolution, although the key amino acid containing an adequate heavy atom for this purpose has not yet be prepared.