Binding of a dimeric derivative of vancomycin to L-Lys-D-Ala-D-lactate in solution and at a surface

Background: The emergence of bacteria that are resistant to vancomycin (V), a glycopeptide antibiotic, results from the replacement of the carboxy-terminal D-Ala-D-Ala of bacterial cell wall precursors by D-Ala-D-lactate. Recently, it has been demonstrated that covalent dimeric variants of V are active against vancomycin-resistant enterococci (VRE). To study the contribution of divalency to the activities of these variants, we modeled the interactions of V and a dimeric V with L-Lys-D-Ala-D-lactate, an analog of the cell-wall precursors of the vancomycin-resistant bacteria. Results: A dimeric derivative of V (V-Rd-V) was found to be much more effective than V in inhibiting the growth of VRE. The interactions of V and V-R-d-V with a monomeric lactate ligand - diacetyl-L-Lys-D-Ala-D-lactate (Ac(2)KDADLac) - and a dimeric derivative of L-Lys-D-Ala-D-lactate (Lac-R-d'-Lac) in solution have been examined using isothermal titration calorimetry and UV spectroscopy titrations; the results reveal that V-R-d-V binds Lac-R-d'-Lac approximately 40 times more tightly than V binds Ac(2)KDADLac. Binding of V and of V-R-d-V to N-alpha-Ac-L-Lys-D-Ala-D-lactate presented on the surface of mixed self-assembled monolayers (SAMs) of alkanethiolates on gold indicates that the apparent off-rate for dissociation of V-R-d-V from the surface is much slower than that of V from the same surface. Conclusions: The results are compatible with the hypothesis that divalency is responsible for tight binding, which correlates with small values of minimum inhibitory concentrations of V and V-R-d-V.