THERMODYNAMICS AND KINETICS OF DISSOCIATION OF LIGAND-INDUCED DIMERS OF VANCOMYCIN ANTIBIOTICS

The thermodynamics of dissociation of vancomycin and ristocetin dimers in the presence and absence of specific ligands has been studied by d irect microcalorimetry over a range of temperature, pH and ionic stren gth conditions in H2O and D2O. Dimerization of these antibiotics is ex othermic with large temperature dependence (Delta C-p) and consequent entropy-enthalpy compensation effects that may be consistent with solv ation changes associated with burial of non-polar surfaces during macr omolecular association. For vancomycin, no significant ionic strength effects are observed, so non-specific electrostatic contributions are probably discounted, but pH and buffer effects on the thermodynamic pa rameters are consistent with hydrogen ion uptake and pK shift in the d imerization process. Vancomycin dimerization is significantly enhanced in the presence of specifically binding ligands: acetate, N-acetyl-D- Ala, and N-a,N-g-diacetyl-Lys-D-Ala-D-Ala, in increasing order of effe ctiveness. The dipeptide ligand N-acetyl-D-Ala-D-Ala promotes higher o ligomerization and crystallization of the complex. Ristocetin, in cont rast, displays no such ligand effects; it shows a slight reduction in dimerization in the presence of strongly binding N,, N, diacetyl-lys-D -Ala-D-Ala. This difference may reflect the need for flexibility in th e antibiotic structure to allow ligand-induced aggregation. Eremomycin dimerizes strongly even in the absence of ligand. Dissociation of the vancomycin-N-a, N-g-diacetyl-Lys-D-Ala-D-Ala dimer complex is slow (k (diss) ca. 0.005 s(-1)) and kinetics can be measured by conventional U V difference techniques.