Recognition of cell‐wall peptide ligands by vancomycin group antibiotics: Studies using ion spray mass spectrometry

Non‐covalent binding of antibiotics to their target ligands represents a form of molecular recognition which is of considerable contemporary interest in bioorganic and bioanalytical chemistry. The vancomycin antibiotics, including vancomycin and ristocetin, are a family of complex glycopeptides which bind specifically to the C ‐terminal sequence X‐D‐Ala‐D‐Ala, where × is L ‐lysine, L ‐diaminopimelic acid, L ‐alanine or L ‐homoserine. It is shown that non‐covalent complexation of vancomycin and ristocetin with peptide ligands in solution, a key molecular recognition phenomenon in antibacterial chemotherapy, can be detected and analyzed in the gas phase by ionspray mass spectrometry. Using N α , N ϵ ‐diacetyl‐ L ‐Lys‐D‐Ala‐D‐Ala (Ac 2 KAA) as a representative ligand, it is further demonstrated that correlations of relative ion abundance with ligand concentrations in solution afford a direct method for measuring the binding constants of vancomycin and ristocetin complexes with target peptide sequences in bacterial cell wall. Results for ristocetin‐Ac 2 KAA ( K a = 6.25 × 10 5 l mol −1 ) and vancomycin‐Ac 2 KAA ( K a = 7.33 × 10 5 l mol −1 , are in reasonable agreement with previously values [ K a = 5.9 × 10 5 ] and 1.5 × 10 6 l mol −1 , respectively).