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1H-N.M.R. studies of a natural immunoadjuvant peptidoglycan monomer: proposed structure in solution in methyl sulfoxide.
The conformation in solution in methyl sulfoxide of the immunoadjuvant peptidoglycan monomer (PGM), obtained by digestion with lysozyme of the linear peptidoglycan polymer isolated from Brevibacterium divaricatum, was studied by 1H-n.m.r. spectroscopy. The temperature dependence of the chemical shift of the resonances of the amide protons suggested that the amino group of alanine-5 is involved in hydrogen bonding, most probably to the alpha-carbonyl of the isoglutamine which showed restricted rotation, as indicated by the large chemical shift non-equivalence for the resonances of the beta CH2 group. A cyclic structure is proposed for the C-terminal pentapeptide of PGM, which is further supported by various n.O.e. interactions involving the meso-diaminopimelic residue and the N-acetylmuramoyl group.
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