Dimerization of A82846B, vancomycin and ristocetin: influence on antibiotic complexation with cell wall model peptides.

The thermodynamics of glycopeptide antibiotic dimerization have been studied by means of sedimentation equilibrium, using A82846B, vancomycin, ristocetin and complexes formed with several cell wall model peptides. These results indicate that vancomycin dimerization can be strongly promoted in two ways: i) stabilization of the antibiotic conformation in which the carbonyl group of residue three is on the back face of the molecule and ii) preferential interaction of the dimer with the lysine residue of N,N'-diacetyl-lysyl-D-alanyl-D-alanine. This effect was not found in ristocetin. A82846B forms stable dimers at very low antibiotic concentration. Two conformational forms have been found for complexed A82846B by 1H NMR. However, calorimetric binding experiments have shown that all its binding sites are thermodynamically equivalent. The affinity of the A82846B dimer for the tripeptide has been estimated to be about 3kJ x mol-1 higher than that of the vancomycin monomer and about -2.6kJ x mol-1 lower than that of dimeric vancomycin. The possible role of dimerization in the biological activity of glycopeptide antibiotics is discussed further on the basis of present thermodynamic data.