Collision-induced dissociation of noncovalent complexes between vancomycin antibiotics and peptide ligand stereoisomers: evidence for molecular recognition in the gas phase

Abstract In solution, the antibiotics of the vancomycin group bind stereospecifically to peptides with the C-terminal sequence: – l -Lys– d -Ala– d -Ala. Substitution by a l -Ala at either of the two C-terminal residues causes a dramatic decrease in the binding affinity to the antibiotics. This solution behavior is clearly reflected in electrospray ionization (ESI) mass spectra obtained from equimolar mixtures of an antibiotic, an isotopically labelled peptide ligand and an unlabelled peptide stereoisomer. Using collision-induced dissociation (CID) we have probed the gas phase stability of isomeric (1:1) noncovalent complexes formed between vancomycin and tripeptide stereoisomers. In negative ion mode the CID results show that a complex formed between vancomycin and a – l -Ala– l -Ala ligand fragments more readily than a complex formed between vancomycin and a – d -Ala– d -Ala ligand. This difference in gas phase stability is in agreement with what would be expected if the noncovalent complexes had retained their structural specific interactions from solution to gas phase. In positive ion mode no significant difference in the gas phase stabilities of the isomeric complexes could be observed. We attribute this similarity in gas phase stability between isomeric positively-charged complexes to a protonation of the C-terminus of the peptide ligand which destroys the specific interaction between antibiotic and peptide ligand. Molecular recognition phenomena in the gas phase were investigated by CID of mixed cluster ions consisting of an antibiotic, a – l -Ala peptide, a – d -Ala stereoisomer, one ligand isotopically labelled. Upon CID of the negatively charged mixed cluster ions a stereoselective loss of the assumed “nonspecifically” bound – l -Ala ligand was observed.