1H NMR study of low-affinity binding of ibuprofen to human serum albumin at different pH

¹H nuclear magnetic resonance (NMR) chemical shift, relaxation and diffusion coefficient measurements were carried out to study the influence of pH (from 8.0 to 6.1) on the low-affinity binding of ibuprofen (IBP), a nonsteroidal anti-inflammatory drug, to human serum albumin (HSA). The study demonstrated that the binding affinity of IBP to HSA increases when the solution is lowered below the physiological values. The increased binding capacity of IBP to HSA at lower pH is attributed to an increase in the number of (likely hydrophobic) low-affinity binding sites, made available upon HSA base-to-neutral conformation transition. With a fast reversible and site-independent binding model, the number of binding sites of the IBP-HSA complex, calculated from the relaxation data, was 15±2 at pH 8.0 to 22±1 at pH 6.8.