Interplay between hydration and electrostatic attraction in ligand binding: direct observation of hydration barrier at reverse micellar interface.

The recognition of a charged biomolecular surface by an oppositely charged ligand is governed by electrostatic attraction and surface hydration. In the present study, the interplay between electrostatic attraction and hydration at the interface of a negatively charged reverse micelle (RM) at different temperatures has been addressed. Temperature-dependent solvation dynamics of a probe H33258 (H258) at the reverse micellar interface explores the nature of hydration at the interface. Up to 45 degrees C, the environmental dynamics reported by the interface-binding probe H258 becomes progressively faster with increasing temperature and follows the Arrhenius model. Above 45 degrees C, the observed dynamics slows down with increasing temperature, thus deviating from the Arrhenius model. The slower dynamics at higher temperatures is interpreted to be due to increasing contributions from the motions of the surfactant head groups, indicating the proximity of the probe to the interface at higher temperatures. This suggests an increased electrostatic attraction between the ligand and interface at higher temperatures and is attributed to the change in hydration. Densimetric and acoustic studies, indeed, show a drastic increase in the apparent specific adiabatic compressibility of the water molecules present in RMs after 45 degrees C, revealing the existence of a softer hydration shell at higher temperatures. Our study indicates that the hydration layer at a charged interface acts both as physical and energetic barrier to electrostatic interactions of small ligands at the interface.