Direct Determination of Solution Binding Constants for Noncovalent Complexes between Bacterial Cell Wall Peptide Analogues and Vancomycin Group Antibiotics by Electrospray Ionization Mass Spectrometry

We report a method based on electrospray ionization (ESI) mass spectrometry to determine solution association constants (K_A) for complexes between glycopeptide antibiotics (vancomycin and ristocetin) and several peptide ligands. The measured K_A values are in good agreement with previously reported values obtained by standard spectroscopic titration techniques. The pH stability of the ristocetin-diacetyl-L-rysyl-D-alanyl-D-alanine complex was investigated by ESI mass spectrometry and circular dichroism (CD) spectroscopy. The two methods produce very similar results, demonstrating that the ESI mass spectra reflect the pH stability of the complex. In solution, the antibiotics bind stereospecifically to peptides containing a C-terminal D-Ala-D-Ala sequence, whereas no complex formation is observed with peptides containing the L-Ala-L-Ala stereoisomer. To investigate whether electro-spray ionization is able to reflect the structurally specific interaction between antibiotics and D-Ala-D-Ala peptide ligands, an equimolar mixture of vancomycin, diacetyl-L-tysyl-D-alanyl-D-alanine, and the deuterium-labeled stereo-isomer diacetyl-L-lysyl-L-alanyl-L-alanine-d₆ was analyzed by ESI mass spectrometry. In agreement with solution-phase behavior, only ions corresponding to a complex between diacetyl-L-lysyl-D-alanyl-D-alanine and vancomycin are observed.