The ligand/receptor pair consisting of a synthetic dimeric derivative of vancomycin (V), linked at the C terminus by p-xylylenediamine (V-CONHCH2C6H4CH2NHCO-V), and a dimeric derivative of L-Lys-D-Ala-D-Ala, [(CH2CONH)-H-epsilon(N-alpha-Ac)-L-Lys-D-Ala-D-Ala-CO2-](2), provides a new system with which to study the influence of divalency on the strength of binding. A competitive assay using affinity capillary electrophoresis (ACE) has been developed and used to estimate the dissociation constant of the divalent complex (K-d(d) approximate to 1.1 nM) and the enhancement in binding (similar to 10(3)) relative to the corresponding monomeric interaction between unmodified monomeric vancomycin and diacetyl-L-lys-D-Ala-D-Ala.