Evidence of heterogeneous 1-anilinonaphthalene-8-sulfonate binding to β-lactoglobulin from fluorescence spectroscopy

Steady-state and dynamic fluorescence titrations show that: (a) the complex between β-lactoglobulin (BLG) and 1-anilinonaphthalene-8-sulfonate (ANS) displays a heterogeneous equilibrium with large changes in the binding strength vs. pH and ion concentration; and (b) the fluorescence response of bound ANS reveals two separate lifetimes that suggest two different sites (or binding modes). While steady-state fluorescence titrations yield effective values of the binding constant and of the bound ANS quantum efficiency, it is shown that, by combining steady-state fluorescence and lifetime decay of ANS, it is possible to give quantitative estimates of the association constants for each site. When heading from the acid (pH~2) to the native state (pH~6) the main result is a very large reduction of the effective binding constant. This and the results of titrations vs. ionic strength suggest that electrostatic interactions are a major contribution to ANS binding to BLG.