A Structure-Activity Relationship in the Binding of Multicharged Anionic Azo and Pyrene Dyes to Serum Albumin

The binding of 11 azo and pyrene dyes (which have 1 to 4 sulfonato groups) to bovine serum albumin at pH = 7.0 and 25 °C has been studied by spectrophotometry, equilibrium dialysis, and/or ultrafiltration method. Binding parameters were determined assuming one or two classes of independent binding sites. By combining with data reported before, we examined the effects of hydrophobicity and net charge number of ligands on the binding parameters for a total of 19 dyes. The ratio of the binding ability to the primary sites to the total binding ability, a measure of binding specificity, was found to increase with an increase in the anionic charge number of ligands. The values of binding constant for nonspecific binding of the monoanionic dyes and for the primary binding of the multicharged dyes were found to have a strong correlation to both the hydrophobicity and the anionic charge number of ligands. A multiple regression analysis has derived the equation: where K is a binding constant, P is the partition coefficient in 1-octanol/water system, and m and αi are respectively the number of anionic groups and the degree of dissociation of the i-th group. From this equation, the contribution of electrostatic interactions due to a unit net charge on ligands to the standard free energy change of binding was estimated to be −11.4 kJ mol−1. The role of electrostatic interactions has been discussed in detail.