Abstract
Non-covalent complexes between the glycopeptide antibiotic vancomycin and the cell-wall precursor analog N-acetyl-D-Ala-D-Ala, as well as its deuterated enantiomer N-acetyl-L-Ala-L-Ala-d3 were probed using electrospray ionization mass spectrometry. In solution, the D,D-peptide interacts strongly with the antibiotic, whereas the L,L-peptide shows no observable complexation. Using a competition strategy, mass spectra were obtained from solutions containing vancomycin, N-acetyl-D-Ala-D-Ala, and N-acetyl-L-Ala-L-Ala-d3. With acetonitrile-water as the solvent, the ratio of free vancomycin to vancomycin complex was much higher than expected from solution interactions and the specific D,D-peptide complex and the non-specific L,L-peptide complex were detected in a 1.7 : 1 ratio. In contrast, sprayed from an ammonium acetate buffer solution both the amount and specificity of observed complex reflected solution interactions (only D,D-peptide complex was found). Thus, our results demonstrate that in electrospray mass spectrometry the conditions must carefully be chosen and control experiments must be carried out in order to exclude non-specific aggregation as a possible cause of complex formation.
