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The stereospecificity of peptide utilization in Escherichia coli K-12 4212, a methionine auxotroph, was investigated using diastereomers of trimethionine and trimethionine methyl ester. Of the eight stereoisomers examined, only l-Met-l-Met-l-Met, l-Met-l-Met-d-Met, and d-Met-l-Met-l-Met and the corresponding methyl esters serve as growth substrates. Triornithine-resistant mutants of strain 4212 were isolated which failed to transport d-Met-l-Met-l-Met. These results provide evidence that an oligopeptide containing a d residue at its amine terminus can enter E. coli by the oligopeptide transport system.

A stereochemical investigation, by circular dichroism, of a synthetic nonapeptide (cyclolinopeptide A) in several organic and organic-sulfuric acid solvents is presented. From this examination, and results found for a conformationally rigid model compound, 1,7,7-trimethyl-3-azabicyclo [2.2.1] heptan-2-one(camphorolactam), it is concluded that cyclolinopeptide A may exist in several conformations in solution. None of these conformations is believed to be stabilized by intramolecular hydrogen bonds. Some details on an x-ray analysis of the cyclic nonapeptide are also presented.