Investigation of a 17β-estradiol-monoclonal antiestradiol antibody binding mechanism using dilute solutions of organic solvents

Abstract Quantitative understanding of steroid hormone transport and receptor-mediated action requires knowledge of the bonding forces involved in each steroid-protein complex and the effects of a biological environment on these forces. An approach to these problems using dilute solutions of water-miscible organic solvents, with a range of polarity, dielectric and hydrogen bonding properties, was tested on an estradiol-antiestradiol antibody binding system on the basis that comparing the effects of the solvents would both permit the importance of hydrophobic and hydrogen bonding to be differentiated and give information on the effects of the environment on the reaction. The results were compared with thermodynamic measurements. All the solvents reduced the Gibbs free energy of binding as a function of their concentration in the medium. The decreases were virtually a monotonic function of their dielectric constant, indicating reduced hydrogen bonding. Analysis of the decreases in terms of the solvents' hydrogen bonding and polarity properties supported this. Thermodynamic measurement showed the binding reaction was enthalpy-driven with, overall, a slightly unfavorable entropy contribution. This again showed the hydrophobic effect was not the main bonding force. The most deleterious solvent, iso -propanol, not only decreased the enthalpic contribution to binding but rendered the entropic contribution more favorable. This approach still does not allow the relative importance of hydrogen bonding and van der Waals contacts in the actual binding to be differentiated but it does give indications on how a biological environment may affect a steroid-protein binding reaction in vivo .