Thermodynamic insight into the interaction mechanism of an ESIPT drug with a serum transport protein: Slower solvent-relaxation dynamics explored through wavelength-sensitive fluorescence behavior

Abstract

Recently, 3,5-diiodosalicylic acid (3,5-DISA) has garnered enormous research attention owing to its wide-spread medicinal/biological applications, use as a model to study the role of halogen bonding in drug-protein interaction and so forth. However, the arena of the study of interaction of 3,5-DISA with relevant biological targets, particularly serum transport protein still starves for meticulous exploration of a number of fundamental aspects, such as, the thermodynamics and strength of binding, effect of drug binding on the native protein conformation and functionality, rotational and solvent-relaxation dynamics within the protein scaffolds etc. The present investigation endeavors to unveil these important aspects of 3,5-DISA:HSA interaction scenario. Our spectroscopic results demonstrate a remarkable modification of the excited-state intramolecular proton transfer (ESIPT) photophysics of 3,5-DISA upon interaction with HSA. A detailed isothermal titration calorimetry (ITC) study reveals that the interaction process is governed by favorable enthalpic (∆H0 < 0) and unfavorable entropic (T∆S0 < 0) contributions. The modulation of the hydration structure at the interfaces accompanying the binding phenomenon is also delineated in this context. Circular dichroism (CD) spectroscopy has been exploited to show the slight perturbation of the native protein conformation as a result of drug binding. In complementarity, the functionality of HSA (in terms of esterase-like activity) has also been shown to decrease with added 3,5-DISA. Our cumulative exploration of the wavelength-sensitive fluorescence parameters including red-edge excitation shift (REES) points out a remarkably slower rate of solvent-relaxation dynamics of the HSA-bound 3,5-DISA in the photoexcited state. The modification of the rotational-relaxation behavior of 3,5-DISA within the protein is also addressed and rationalized on the basis of the two-step and wobbling-in-cone model.