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 an tibiotics, including vancomycin and ristocetin, are a family of comple x glycopeptides which bind specifically to the C-terminal sequence X-D -Ala-D-Ala, where X 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 recog nition phenomenon in antibacterial chemotherapy, can be detected and a nalyzed in the gas phase by ionspray mass spectrometry. Using N-alpha, N-epsilon-diacetyl-L-Lys-D-Ala-D-Ala (Ac(2)KAA) as a representative li gand, it is further demonstrated that correlations of relative ion abu ndance with ligand concentrations in solution afford a direct method f or measuring the binding constants of vancomycin and ristocetin comple xes with target peptide sequences in bacterial cell wall. Results for ristocetin-Ac(2)KAA (K-a = 6.25 x 10(5) l mol(-1)) and vancomycin-Ac(2 )KAA (K-a = 7.33 x 10(5) l mol(-1)) are in reasonable agreement with p reviously reported values [K-a = 5.9 x 10(5) and 1.5 x 10(6) 1 mol(-1) , respectively).