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

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

Interplay of Multiple Interaction Forces: Binding of Norfloxacin to Human Serum Albumin

Herein, the binding interaction of a potential chemotherapeutic antibacterial drug norfloxacin (NOF) with a serum transport protein, human serum albumin (HSA), is investigated. The prototropic transformation of the drug (NOF) is found to be remarkably modified following interaction with the protein as manifested through significant modulations of the photophysics of the drug. The predominant zwitterionic form of NOF in aqueous buffer phase undergoes transformation to the cationic form within the protein-encapsulated state. This implies the possible role of electrostatic interaction force in NOF–HSA binding. This postulate is further substantiated from the effect of ionic strength on the interaction process. To this end, the detailed study of the thermodynamics of the drug–protein interaction process from isothermal titration calorimetric (ITC) experiments is found to unfold the signature of electrostatic as well as hydrophobic interaction forces underlying the binding process. Thus, interplay of more than one interaction forces is argued to be responsible for the overall drug–protein binding. The ITC results reveal an important finding in terms of enthalpy–entropy compensation (EEC) characterizing the NOF–HSA binding. The effect of drug-binding on the native protein conformation has also been evaluated from circular dichroism (CD) spectroscopy which unveils partial rupture of the protein secondary structure. In conjunction to this, the functionality of the native protein (in terms of esterase-like activity) is found to be lowered as a result of binding with NOF. The AutoDock-based docking simulation unravels the probable binding location of NOF within the hydrophilic subdomain IA of HSA. The present program also focuses on exploring the dynamical aspects of the drug–protein interaction scenario. The rotational-relaxation dynamics of the protein-bound drug reveals the not-so-common “dip-and-rise” pattern

Exploring the Nucleobase-Specific Hydrophobic Interaction of Cryptolepine Hydrate with RNA and Its Subsequent Sequestration

The study of the interactions of drug molecules with genetic materials plays a key role underlying the development of new drugs for many life-threatening diseases in pharmaceutical industries. Understanding their fundamental base-specific and/or groove-binding interaction is crucial to target the genetic material with an external drug, which can pave the way to curing diseases related to the genetic material. Here, we studied the interaction of cryptolepine hydrate (CRYP) with RNA under physiological conditions knowing the antimalarial and anticancer activities of the drug. Our experiments explicitly demonstrate that CRYP interacts with the guanine- and adenine-rich region within the RNA duplex. The pivotal role of the hydrophobic interaction governing the interaction is substantiated by temperature-dependent isothermal titration calorimetry experiments and spectroscopic studies. Circular dichroism study underpins a principally intercalative mode of binding of CRYP with RNA. This interaction is found to be drastically affected in the presence of magnesium salt, which has a strong propensity to coordinate with RNA nucleobases, which can in turn modulate the interaction of the drug with RNA. The temperature-dependent calorimetric results substantiate the occurrence of entropy–enthalpy compensation, which enabled us to rule out the possibility of groove binding of the drug with RNA. Furthermore, our results also show the application of host–guest chemistry in sequestering the RNA-bound drug, which is crucial to the development of safer therapeutic applications