Brotati Chakraborty

Magnetic Field Effect Corroborated with Docking Study to Explore Photoinduced Electron Transfer in Drug-Protein Interaction

Conventional spectroscopic tools such as absorption, fluorescence, and circular dichroism spectroscopy used in the study of photoinduced drug-protein interactions can yield useful information about ground-state and excited-state phenomena. However, photoinduced electron transfer (PET) may be a possible phenomenon in the drug-protein interaction, which may go unnoticed if only conventional spectroscopic observations are taken into account. Laser flash photolysis coupled with an external magnetic field can be utilized to confirm the occurrence of PET and authenticate the spin states of the radicals/radical ions formed. In the study of interaction of the model protein human serum albumin (HSA) with acridine derivatives, acridine yellow (AY) and proflavin (PF(+)), conventional spectroscopic tools along with docking study have been used to decipher the binding mechanism, and laser flash photolysis technique with an associated magnetic field (MF) has been used to explore PET. The results of fluorescence study indicate that fluorescence resonance energy transfer takes place from the protein to the acridine-based drugs. Docking study unveils the crucial role of Ser 232 residue of HSA in explaining the differential behavior of the two drugs towards the model protein. Laser flash photolysis experiments help to identify the radicals/radical ions formed in the due course of PET (PF(•), AY(•-), TrpH(•+), Trp(•)), and the application of an external MF has been used to characterize their initial spin-state. Owing to its distance dependence, MF effect gives an idea about the proximity of the radicals/radical ions during interaction in the system and also helps to elucidate the reaction mechanisms. A prominent MF effect is observed in homogeneous buffer medium owing to the pseudoconfinement of the radicals/radical ions provided by the complex structure of the protein.

Excimer of 9-Aminoacridine Hydrochloride Hydrate in Confined Medium: An Integrated Experimental and Theoretical Study

We aim to find out the extent of stability of the excimer of 9-aminoacridine hydrochloride hydrate (9AA), a prospective PDT drug, in different confined media with varying cavity size. When confined in cetyltrimethyl ammonium bromide micelles, although at low concentration of 9AA, only a single distinct peak (λ(max) at 460 nm) with a shoulder at 485 nm is observed in steady-state fluorescence spectrum, yet with increase in concentration the peak and the shoulder merge with simultaneous emergence of another peak at 535 nm, which is assigned to excimer. Similar behavior is also observed in Triton-X, crown ether, α-cyclodextrin, β-cyclodextrin, and homogeneous aqueous medium. The formation of excimer, which reflects the extent of confinement of 9AA, is maximum in β-cyclodextrin followed by others. Steady-state and time-resolved fluorescence studies along with TRES and TRANES analyses coupled with anisotropy data and transient absorption studies reveal the presence of monomer-dimer equilibrium of 9AA in the excited state. Molecular modeling indicates that the structure of excimer is stabilized by locking of the two monomeric species via four hydrogen bonds formed between the amino-H and imino-N of 9AA monomers, whereas the dimer in the ground state has only two such hydrogen bonds.

Spectroscopic exploration of drug–protein interaction: a study highlighting the dependence of the magnetic field effect on inter-radical separation distance formed during photoinduced electron transfer

In this article, we study the binding interaction of acridone (AD) with human serum albumin (HSA) using conventional spectroscopic techniques and then decipher the latent occurrence of photoinduced electron transfer (PET) using the laser flash photolysis (LFP) technique in conjunction with a weak magnetic field (MF). The experimental observations are further substantiated by docking results. An absorption study reveals the formation of a ground state complex between AD and HSA while a circular dichroism study implies that AD brings about a substantial change in the secondary and tertiary structures of the protein. A fluorescence study mainly helps in evaluation of the binding and thermodynamic parameters. This is one of the rare reports which utilize time-resolved emission spectra and time-resolved area normalized emission spectra to study drug–protein interactions. The detection of the occurrence of PET would be ignored if only the observations of conventional spectroscopic techniques had been considered. LFP detects the occurrence of PET from the tryptophan residue of HSA to AD and MF effect authenticates the triplet origin of the radical ions involved. Furthermore, the actual reaction pathway is elucidated with the help of the MF effect. Although the MF effect is generally observed in a restricted medium, in the present case the complex structure of the protein offers pseudo-confinement to the radical pairs or radical ion pairs resulting in observation of a substantial MF effect. The significance of maintaining the condition of “pseudo-confinement” or the optimum distance between radical ions originating from the acceptor and donor moieties in the observation of the MF effect has been specifically highlighted in this work by integrating the findings of the present study with our previous reports.

Acridone in a biological nanocavity: detailed spectroscopic and docking analyses of probing both the tryptophan residues of bovine serum albumin

Spectroscopic and docking analyses reveal that acridone (AD) interacts with both the tryptophan (Trp) residues of bovine serum albumin (BSA) (both Trp 134 and Trp 212) in contrast to other organic ligands including other acridine derivatives which generally prefer to interact with Trp 212. The use of fluorescence spectroscopy, specifically the unusual time-resolved area normalized spectra depicting two isoemissive points with different times of evolution, confirms that AD “unusually” interacts with both the Trp residues present in the model protein. Upward curvature of the Stern Volmer plot suggests the interaction of AD with both the Trp residues present in varying microenvironments within BSA and possibly also indicates the denaturation of the protein. Ground state interaction of AD and BSA is explored using absorption spectroscopy, whereas strong perturbation in secondary and tertiary structures of the model protein on binding with the ligand is divulged from the observation of circular dichroism spectroscopy. Femtosecond fluorescence up-conversion kinetics implies that a photoinduced electron transfer reaction takes place from the Trp residue of the protein to AD, which has been authenticated using laser flash photolysis via identification of the radical ions. Binding as well as thermodynamic parameters associated with AD–BSA interaction are obtained from fluorescence studies. The prime deduction from the detailed spectroscopic and docking analyses is that AD initially interacts with Trp 212 present in the crevice of hydrophobic domain IIA of the protein and then perturbs the structure of BSA to bring about conformational changes such that it can gain access to Trp 134 housed in hydrophilic domain IB, which is possibly facilitated by hydrogen bonding.