Prototropic Transformation and Rotational–Relaxation Dynamics of a Biological Photosensitizer Norharmane inside Nonionic Micellar Aggregates

Abstract

The effect of variation of the size of headgroup as well as the length of hydrocarbon tail of nonionic surfactants on the photophysics and rotational-relaxation dynamics of a promising biological photosensitizer, norharmane, (NHM) has been investigated. The series of nonionic micelles employed for the study belongs to Triton X family (allowing the variation in poly­(ethylene oxide) (PEO) chain length) and Tween family (providing access to vary the alkyl chain length of the surfactant tails). The spectral deciphering of the photophysics of the drug (NHM) within the series of the nonionic micelles reveals remarkable influence of binding of the drug with the micelles on the prototropic equilibrium which is notably favored toward the neutral species of the drug over the cationic counterpart. The strength of drug–micelle binding interaction as well as the extent of transformation of the cation ⇌ neutral prototropic equilibrium is found to be enormously governed by the variation of the headgroup size and the alkyl chain length of the surfactants. To this end, the equilibrium constant (Keq) and free energy change (ΔG) for cation ⇌ neutral prototropic transformation of the drug as a function of the micellar parameters have been meticulously explored from emission studies and comprehensively rationalized under the provision of the micellar hydration model, that is, the differential extents of water penetration to micellar units as a function of varying thickness of the palisade layer and hence a variation in the polarity of the micellar microenvironments. The significant enhancement in steady-state fluorescence anisotropy of NHM in micellar environments compared to that in bulk aqueous buffer phase substantiates the location of the drug in motionally constrained regions introduced by the nonionic micelles. Further, all these lines of arguments are effectively corroborated from time-resolved fluorescence experiments with particular emphasis on time-resolved anisotropy decay study of the drug within the micellar aggregates