Investigation of complexes between some glycopeptide antibiotics and bacterial cell-wall analogues by electrospray- and capillary zone electrophoresis/electrospray-mass spectrometry.

Complexation in aqueous solutions between vancomycin, ristocetin A, teicoplanin and two bacterial cell-wall analogues, Ac2-L-Lys-D-Ala-D-Ala (tripeptide) and Ac-D-Ala-D-Ala (dipeptide) has been examined by positive-ion electrospray mass spectrometry (ES-MS) and capillary zone electrophoresis (CZE)/ES-MS. The ES-MS data demonstrate that as well as complexes between monomeric antibiotics and either peptide, the investigated solutions contained complexes ranging from the simple homodimer of the antibiotic to a more complex association giving ions of the type [2(antibiotic) + 3(tripeptide) + 3H]3+. The same data also demonstrate that the homodimers of the investigated antibiotics are significantly suppressed in the presence of the tripeptide. The use of CZE/ES-MS made it evident that the complexes between the antibiotic and the tripeptide were present in the solution prior to their introduction into the ion source. The two sets of data are compared with existing UV difference spectroscopy and nuclear magnetic resonance data on complexation and dimerization.