Tapan Ganguly

Fluorescence spectroscopic characterization of the interaction of human adult hemoglobin and two isatins, 1-methylisatin and 1-phenylisatin: a comparative study.

In this report, steady state and time-resolved fluorescence along with circular dichroism (CD) spectroscopic, FTIR, and anisotropy investigations were made to reveal the nature of the interactions between human adult hemoglobin (Hb) and the isatins, 1-methylisatin (1-MI) and 1-phenylisatin (1-PI). From the analysis of the steady state and time-resolved fluorescence quenching of Hb in aqueous solution in the presence of an isatin, i.e., 1-MI, it seemed that the nature of the quenching was of static type and a mixture of both static and dynamic nature for 1-PI. The primary binding pattern between isatins and Hb has been interpreted as a combined effect of hydrophobic association and electrostatic interaction for 1-MI. For 1-PI, this was the combined effect of hydrophobic association and ionic interactions and salt bridges or/and proton transfer. The pretwisted structure of 1-PI facilitates ionic interactions with Hb. The binding constants, number of binding sites, and thermodynamic parameters had been computed. The binding average distances between the Hb-1-MI and Hb-1-PI determined from Forsters theory were found to be 4.02 and 5.28 nm, respectively. CD, steady state, and time-resolved anisotropy measurements had been done in support.

A detailed spectroscopic study on the interaction of Rhodamine 6G with human hemoglobin.

UV-vis, time-resolved fluorescence and circular dichroism spectroscopic investigations have been made to reveal the nature of the interactions between xanthene dye Rhodamine 6G and the well known protein hemoglobin. From the analysis of the steady-state and time-resolved fluorescence quenching of Rhodamine 6G in aqueous solutions in presence of hemoglobin, it is revealed that the quenching is static in nature. The primary binding pattern between Rhodamine and hemoglobin has been interpreted as combined effect of hydrophobic association and electrostatic interaction. The binding constants, number of binding sites and thermodynamic parameters at various pH of the environment have been computed. The binding average distance between the energy donor Rhodamine and acceptor hemoglobin has been determined from the Forsters theory.

Interaction of bovine serum albumin and albumin-gold nanoconjugates with l-aspartic acid. A spectroscopic approach

The interaction of an essential transport protein bovine serum albumin (BSA) and albumin-gold nanoconjugates (BSA-GNPs) with amino acid l-aspartic (ASP) are investigated by steady state and time resolved spectroscopic techniques. In both the cases, static fluorescence quenching is observed indicating that a ground state complex is formed between the donor BSA/BSA-GNP with the acceptor ASP. High values of quenching constant suggest that energy transfer also occurred from BSA and BSA-GNPs to ASP. Distance between the fluorophore in the protein and the amino acid (ASP) is evaluated. Binding constants and the number of binding sites were determined in both the cases. The observed thermodynamic parameters suggest that the key interacting forces involved in both cases are hydrophobic interactions. Circular dichroism (CD) spectrum of BSA molecule suffers marginal change in the presence of ASP both in its pure as well as bio-nanoconjugate forms. As no structural deformation is occurred, the biological activity along with the activity of immune response of protein and the biocompatibility of protein-nanoconjugate remain as such.

Investigations on the interactions of aurintricarboxylic acid with bovine serum albumin: Steady state/time resolved spectroscopic and docking studies

In this paper, the nature of the interactions between bovine serum albumin (BSA) and aurintricarboxylic acid (ATA) has been investigated by measuring steady state and time-resolved fluorescence, circular dichroism (CD), FT-IR and fluorescence anisotropy in protein environment under physiological conditions. From the analysis of the steady state and time-resolved fluorescence quenching of BSA in aqueous solution in presence of ATA it has been inferred that the nature of the quenching originates from the combined effect of static and dynamic modes. From the determination of the thermodynamic parameters obtained from temperature-dependent changes in K(b) (binding constant) it was apparent that the combined effect of hydrophobic association and electrostatic attraction is responsible for the interaction of ATA with BSA. The effect of ATA on the conformation of BSA has been examined by analyzing CD spectrum. Though the observed results demonstrate some conformational changes in BSA in presence of ATA but the secondary structure of BSA, predominantly of α-helix, is found to retain its identity. Molecular docking of ATA with BSA also indicates that ATA docks through hydrophobic interaction.

Steady state, time resolved, and circular dichroism spectroscopic studies to reveal the nature of interactions of zinc oxide nanoparticles with transport protein bovine serum albumin and to monitor the possible protein conformational changes

In this paper, the interaction between bovine serum albumin (BSA) and zinc oxide (ZnO) nanoparticles was investigated by fluorescence quenching spectra, circular dichroism (CD), and synchronous spectra under physiological conditions. From the analysis of the steady state and time resolved fluorescence quenching of BSA in aqueous solution in presence of ZnO it was observed that the nature of the quenching is of static-type quenching. The Stern–Volmer quenching constant KS at different temperatures were determined and the thermodynamic parameters ΔH, ΔG, and ΔS were computed. The experiment revealed that the electrostatic interaction was the predominant force in stabilizing the complex. The effect of ZnO on the conformation of BSA has been analyzed by synchronous spectra and CD spectrum. Although the observed results demonstrate some conformational changes in BSA in presence of ZnO nanoparticles, the secondary structure of BSA, predominantly of α-helix, is found to retain its identity.

Studies on the photophysical properties of some 2,7- dimethoxycarbazoles in various environments by steady state and time resolved spectroscopic methods Part 1. Synthesis, absorption and fluorescence spectra at room temperature

Abstract The electronic spectroscopic and photophysical properties of 2,7-dimethoxy-N-methyl carbazole (DMNMC) and 2,7-dimethoxycarbazole (DMC) in various polar and non-polar environments at room temperature were compared with those of N-methylcarbazole (NMC). The synthesis and characterization of these compounds are described. The first two electronic transitions 1Lb ← 1A and 1La ← 1A which are well separated and characterized in NMC and carbazole are mixed in the absorption and fluorescence spectra of DMNMC and DMC. This “proximity effect” is responsible for the unidentified first absorption band system observed in solvents at room temperature, for a marked decrease in both the fluorescence quantum yields and lifetimes (NMC, τF = 15.1 ns; DMNMC, τF = 8.4 ns; DMC, τF = 3.9 ns in 3-methylpentane) and for a loss of the mirror-image between the absorption and the fluorescence spectra of these dimethoxy derivatives. The importance of the vibronic interactions between these two states is shown to be controlled (reduced) by N-methylation and/or the intermolecular hydrogen bonding formation in a proton-acceptor solvent such as ethanol.

Detailed spectroscopic investigations to reveal the nature of interaction of anionic porphyrin with calf thymus DNA

The interaction between anionic form of meso-tetrakis(4-carboxyphenyl) porphyrin (TCPP) and calf thymus deoxyribonucleic acid (CT DNA) is investigated by measuring UV-vis absorption, steady-state fluorescence, steady-state fluorescence anisotropy, time-resolved fluorescence, resonance light scattering (RLS), FT-IR and circular dichroism (CD) spectra along with the help of atomic force microscopy (AFM) under Tris-Borate-EDTA (TBE) buffer solution of pH 8.3. The static mode of fluorescence quenching of porphyrin by calf thymus deoxyribonucleic acid indicates the formation of a ground-state complex. The formation of ground-state complex is a spontaneous molecular interaction procedure in which outside groove binding through hydrogen bond or van der Waals force plays a major role. For biomedical application this investigation is very important as here TCPP, i.e. the anionic porphyrin does not bring any changes in the original structure of the CT DNA to selectively cleaving the nucleic acid to destroy the cancer or tumor cells whereas cationic porphyrin makes change in the protein structure significantly during the same process.

Steady state and time resolved spectroscopic studies of the photoinduced electron-transfer reactions between electron donor xylenol and acceptor tetracyanoquinodimethane in acetonitrile at room temperature

Abstract The present study was undertaken both by steady state and time correlated single photon counting techniques to reveal the nature of the electron transfer (ET) process within excited (or ground) xylenol donor and ground state (excited) well-known acceptor tetracyanoquinodimethane (TCNQ) in highly polar solvent acetonitrile (ACN) at 300 K. Observations of poor overlap between the emission of the donor and the electronic absorption spectra of the acceptor and the high negative value of ΔG, the driving energy (Gibbs free energy) when either donor or acceptor chromophore is excited seemingly indicate that the observed fluorescence quenching of the donor in presence of TCNQ is mainly due to highly exothermic electron transfer reactions and no significant energy transfer process is operative here. It is suggested that such highly exothermic ET reaction, which has been proposed to be of outersphere type, occurs at a long distance (⩾7 A ) between the fluorescer (donor) and the quencher (acceptor). Observations of slow electron transfer rate, kET (∼107 s−1) and −ΔG > λ (nuclear reorganization energy parameter) suggest that the reaction might occur in the Marcus inverted region. It has been proposed that in an ACN fluid solution the transient geminate ion-pair complex formed by ET reaction is of loose structure and due to the rapid dissociation of this complex stable anionic species TCNQ- ion is formed in the excited state. This anion is observed to emit at nearly 432 nm region. It is suggested that in the Marcus inverted region as the reaction becomes more exothermic, the necessary reorganization energy λ also increases.

Studies on photophysical properties of some 2,7-dimethoxycarbazoles in various environments by steady state and time-resolved spectroscopic methods Part 2. INDO/S molecular orbital calculations, fluorescence lifetime measurements and polarization studies at 77 K

Abstract Fluorescence excitation and polarization spectra and fluorescence and phosphorescence polarization spectra are presented for 2,7-dimethoxycarbazole (DMC) and 2,7-dimethoxy- N -methylcarbazole (DMNMC) at 77 K in 3-methylpentane (3MP) and ethanol (EtOH) glassy matrices. Nanosecond time-resolved fluorescence spectra have also been recorded for these systems. Semiempirical INDO/S-CI molecular orbital calculations are presented to correlate with the observed spectral changes caused by the methoxy substitution. The results are interpreted in terms of overlapping ππ* electronic transitions 1 L a ← 1 A and 1 L b ← 1 A. Both molecules exhibit fluorescence polarization and time-resolved fluorescence consistent with 1 L b as the lowest excited singlet state. Both molecules exhibit dual fluorescence emission at 77 K whose average lifetime approximately equals the room temperature fluorescence lifetime in any solvent. A reversible two-state kinetic model has been used to interpret the double exponential decay of DMC and DMNMC in low temperature matrices. At any given temperature, the fluorescence lifetimes observed in these systems do not correspond to any of the “pure” electronic state lifetimes. The dual fluorescence observed in these dimethoxy molecules is mainly due to the fact that the radiationless process ( k ab ≈ 10 8 s −1 ) becomes relatively forbidden. A low density of states isoenergetic with the 1 L a state is presumably the reason for this forbidden reaction.

Studies on quenching reactions in the excited electronic states of tetrahydronaphthols both at the ambient temperature as well as at 77 K

Abstract The present investigation was carried out at the ambient temperature as well as at 77 K both by steady-state and time-resolved luminescence techniques to reveal the mechanisms of non-radiative transitions of the lowest excited singlet (S1) and triplet (T1) states of tetrahydronaphthol donors 5,6,7,8-tetrahydro-1-naphthol (THN1OH) and 5,6,7,8-tetrahydro-2-naphthol (THN2OH) in presence of the electron acceptor 2-nitrofluorene (2NF) in solvents of different polarity. At room temperature large negative values of ΔG ° (~ −1.9 eV), Gibbs free energy for photo-induced electron transfer (ET) reaction, indicate highly exothermic ET reaction occurs within the present donor-acceptor systems possibly in the Marcus inverted region. At this temperature evidence of concurrent occurrence of Forsters-type singlet-singlet (SD1→SA1) energy transfer process is also found. In presence of the acceptor 2NF, the fluorescence of the donor molecule is strongly quenched and the large dynamic (as there is absence of ground-state complex) fluorescence quenching rate constant kq, of the order of 1012 M−1s−1 is observed. This quenching has been ascribed mainly due to two concurrent processes: non-radiative energy transfer and ET. Radiative energy transfer is found to have negligible role. In 2NF, apart from the quenching in fluorescence spectra of the present donor molecules a longer wavelength band of broad nature was observed due to formation of contact charge transfer (CT) complex in non-polar solvent. In highly polar acetonitrile (ACN) this band was absent but instead another broad band at a far longer wavelength region due to formation of donor anion (confirmed from metallic sodium experiment and excitation spectra) was found. It was suggested that this anion was produced from its initially formed cationic species, resulted from ET reaction with 2NF. The reaction scheme showing this mechanism is given. At 77 K it seems the Forster mechanism of singlet-singlet energy transfer is still operative resulting the observed fluorescence quenching of the donor in presence of the acceptor 2NF. A reaction mechanism has been proposed for the observed lowering of donor phosphorescence intensity in presence of the acceptor. The possibility of the occurrence of photo-induced ET reaction between excited (T1) donor and ground-state acceptor is hinted from time-resolved phosphorescence measurements and thermodynamic considerations.