Whole cells and cell-wall fractions of Staphylococcus aureus have been labeled by various combinations of [1-C-13]glycine, [N-15]glycine, L-[6-C-13]lysine, L-[6-N-15]lysine, D-[1-C-13]alanine, and D-[N-15]alanine. The resulting materials have been examined using C-13 and N-15 solid-state, magic-angle spinning NMR techniques including cross-polarization, double cross-polarization, and rotational-echo double resonance. The results of these measurements indicate that the peptidoglycan glycyl bridges are complete (five units long) and form cross-links between three-quarters of all peptide stems. The pentaglycyl bridges are immobilized in lyophilized cell-wall fractions in a compact conformation with inter-residue spacings comparable to those of an alpha helix. The bridges have a similar compact conformation in intact whole cells, regardless of whether the cells have been lyophilized or were hydrated and frozen at -10 degrees C. The bridges are also in a time-averaged compact conformation in whole cells at 0 degrees C but with sizable structural fluctuations associated with local mobility. A small fraction of bridges are in extended-chain conformations.
