Determination of Binding Constants of Polyethylene Glycol Vancomycin Derivatives to Peptide Ligands Using Affinity Capillary Electrophoresis

Vancomycin (Van) from Streptomyces orientalis has been derivatized with polyethylene glycol [PEG; PEG-550 (1), 750 (2), 1,100 (3), 2,000 (4), 5,000 (5), and 8,000 (6) g mol⁻¹] at the N-terminus of the glycopeptide backbone and their binding to d-Ala-d-Ala terminus peptides assessed using affinity capillary electrophoresis (ACE). Utilizing ACE, a plug of Van-PEG and non-interacting standards are injected and electrophoresed. Analysis of the change in the relative migration time ratio of the Van-PEG species, relative to the non-interacting standards, as a function of the concentration of peptide, yields a value for the binding constant (K _b). Values of K _b for N-acetyl-d-Ala-d-Ala, 7 to the Van-PEG derivatives are weaker than those for N _α,N _ε-diacetyl-Lys-d-Ala-d-Ala, 8 (for example, values of K _b for 7-1 and 8-1 are 1.8 and 47.7 × 10³ M⁻¹, respectively). These results demonstrate that derivatization of Van with PEG has little effect on the affinity of d-Ala-d-Ala peptide ligands to it. The findings further prove the versatility of ACE and its ability to estimate binding parameters of ligands to antibiotics.