Molecular weight-determination of biosynthetically modified monomeric and oligomeric muropeptides from Escherichia coli by plasma desorption-mass spectrometry

The presence of certain d-amino acids in the growth media of Escherichia coli results in the accumulation of 2 major and 3–5 minor new muropeptides in the murein sacculus. Preliminary data suggested that the major muropeptides correspond to a monomer and a cross-linked dimer with one residue of d-amino acid per molecule. We have analyzed several d-amino acid-modified muropeptides by plasma desorption-mass spectrometry. Our results confirmed that the general structures of the major modified muropeptides are: GlucNAc-MurNAc- l-Ala- d-Glu- m-A2pm- d-X, and GlucNAc-MurNAc- l-Ala- d-Glu- m-A2pm- d-Ala; GlucNAc-MurNAc- l-Ala- d-Glu- m-A2pm- d-X, being X a residue of the d-amino acid. These results corroborate the usefulness of this technique for the structural analysis of muropeptides.