Nitin Chattopadhyay

Binding Interaction of Cationic Phenazinium Dyes with Calf Thymus DNA: A Comparative Study

Absorption, steady-state fluorescence, steady-state fluorescence anisotropy, and intrinsic and induced circular dichroism (CD) have been exploited to explore the binding of calf thymus DNA (ctDNA) with three cationic phenazinium dyes, viz., phenosafranin (PSF), safranin-T (ST), and safranin-O (SO). The absorption and fluorescence spectra of all the three dyes reflect significant modifications upon interaction with the DNA. A comparative study of the dyes with respect to modification of fluorescence and fluorescence anisotropy upon binding, effect of urea, iodide-induced fluorescence quenching, and CD measurements reveal that the dyes bind to the ctDNA principally in an intercalative fashion. The effect of ionic strength indicates that electrostatic attraction between the cationic dyes and ctDNA is also an important component of the dye-DNA interaction. Intrinsic and induced CD studies help to assess the structural effects of dyes binding to DNA and confirm the intercalative mode of binding as suggested by fluorescence and other studies. Finally it is proposed that dyes with bulkier substitutions are intercalated into the DNA to a lesser extent.

Effect of cyclodextrine cavity size on twisted intramolecular charge transfer emission: dimethylamino benzonitrile in β -cyclodextrine

Abstract Emission spectra of p -N,N-dimethylamino benzonitrile (DMABN) in aqueous β-cyclodextrine (β-CD) solutions are reported. In β-CD two distinct kinds of emission are observed, one with enhanced nonpolar emission (at 355 nm) and the other exhibiting enhanced TICT emission (at 480 nm). The former is attributed to those DMABN molecules which are totally inside the β-CD cavity while the latter is due to DMABN molecules only partially enclosed in the cavity.

Photophysics in Motionally Constrained Bioenvironment: Interaction of Norharmane with Bovine Serum Albumin¶

Steady‐state photophysics of norharmane (NHM), a bioactive alkaloid, has been studied in the presence of a model transport protein, bovine serum albumin (BSA). The emission spectrum undergoes a remarkable change upon addition of BSA to the aqueous solution of NHM in buffer. Addition of BSA leads to a marked increase in the fluorescence anisotropy of the neutral species of NHM, although the fluorescence anisotropy for the cationic species is almost invariant to BSA addition, suggesting that the neutral species is located in a motionally restricted environment of BSA, whereas the cationic species remains in the bulk aqueous phase. The binding constant (K) and free energy change (ΔG) for the probe‐protein binding have been calculated from the fluorescence data. Light has been thrown on the action of urea on protein‐bound NHM. The denaturation study suggests that the protein, in its native form, binds with NHM. Polarity of the microenvironment around the probe has been determined from a comparison of the fluorescence properties of the two prototropic species of NHM in water‐dioxane mixture with varying composition.

Spectroscopic Exploration of Mode of Binding of ctDNA with 3-Hydroxyflavone: A Contrast to the Mode of Binding with Flavonoids Having Additional Hydroxyl Groups

Binding interaction of 3-hydroxyflavone (3HF), a bioactive flavonoid, with calf-thymus DNA (ctDNA) has been explored exploiting various experimental techniques. The dual fluorescence of 3HF resulting from the excited state intramolecular proton transfer (ESIPT) is modified remarkably upon binding with the biomacromolecule. The determined binding constant, fluorescence quenching experiment, circular dichroism (CD) study, comparative binding study with the known intercalative binder ethidium bromide and thermometric experiment relating to the helix melting of ctDNA confirm the groove binding of 3HF with the DNA. This is in contrast to two other members of the flavonoid group, namely, fisetin and quercetin, where the bindings are established to be intercalative. The structural difference of 3HF from the other two probes with respect to the absence/presence of the additional hydroxyl groups is ascribed to be responsible for the difference in the mode of binding.

Role of rotamerisation and excited state intramolecular proton transfer in the photophysics of 2-(2′-hydroxyphenyl)benzoxazole, 2-(2′-hydroxyphenyl)benzimidazole and 2-(2′-hydroxyphenyl)benzothiazole: a theoretical study

Semiempirical (AM1-SCI) calculations have been performed to rationalise the experimental findings in relation to the photophysics of 2-(2′-hydroxyphenyl)benzoxazole (HBO), 2-(2′-hydroxyphenyl)benzimidazole (HBI) and 2-(2′-hydroxyphenyl)benzothiazole (HBT). The calculations reveal that, while for HBO and HBI, two rotameric isomers are present in the ground state, there is only one stable species in the S0 state of HBT. Excited state intramolecular proton transfer (ESIPT) reaction is, however, operative in the lowest excited singlet (S1) and triplet (T1) states for all the three molecular systems; resulting altogether three fluorescence bands for HBO and HBI and two for HBT. The excitation, fluorescence and phosphorescence bands have been assigned theoretically. The calculated results agree well with the existing experimental reports. The potential energy surfaces (PES) have been generated for the intramolecular proton transfer (IPT) reactions. The PES reflect that although the IPT process is not favourable in the ground state, the ESIPT process is feasible, both thermodynamically as well as kinetically, for all the three molecular systems in the S1 as well as T1 states.

Photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine: emission from two states

Steady state photophysics of 3-acetyl-4-oxo-6,7-dihydro-12H indolo-[2,3-a] quinolizine (AODIQ) has been studied in different pure and mixed solvents. The fluorometric behaviour of AODIQ is modified dramatically with a change in the solvent polarity. The fluorescence quantum yield, when plotted against the solvent polarity parameter (E T (30)), passes through a maximum. Plot of the fluorescence parameters in a graded series of dioxane-water mixture against E T (30) shows segmented linearity with two independent slopes; reflecting a lower sensitivity in the less polar region and a much higher sensitivity in higher polarity range. The observations lead to the proposition that the fluorescence of AODIQ originates from two different states, viz., a locally excited (LE) and a charge transfer (CT) one. Studies of the phosphorescence and heavy atom quenching of the fluorescence reveal that the CT state lies above the lowest triplet state even in a very polar environment.

Effect of Surfactant Chain Length on the Binding Interaction of a Biological Photosensitizer with Cationic Micelles

Steady-state and time-resolved fluorometric techniques have been exploited to study the photophysical and distribution behavior of an efficient cancer cell photosensitizer, norharmane (NHM), in well-characterized, biomimicking nanocavities formed by cationic micelles with varying surfactant chain length. Amphiphiles like dodecyl trimethyl ammonium bromide (DTAB), tetradecyl trimethyl ammonium bromide (TTAB), and cetyl trimethyl ammonium bromide (CTAB) have been used for the purpose. Emission behavior of NHM is very much dependent on the surfactant concentration as well as their hydrophobic chain length. The binding constant (K) and free-energy change (DeltaG) for the interaction of NHM with the cationic micelles have been determined from the fluorescence data. Polarity of the microenvironment around the probe has been determined in the cationic micellar environments from a comparison of the variation of fluorescence properties of the two-prototropic species of the probe in water/dioxane mixture with varying composition. Experimental results demonstrate that the variation in the cationic surfactant chain length plays an important role in promoting a specific prototropic form of the probe molecule. Fluorescence decays are biexponential in all the micelles indicating that the probe molecules are distributed between the two distinct regions of the micelles. The population of the component with a longer lifetime corresponds to the probe in the head group site, while the short-lived component comes from the probe bound to the core region of the micelles. On the basis of the lifetime measurements, the partitioning behavior of the chromophore in the head group and in the core regions in the micelles has been determined.

Effect of cyclodextrin complexation on excited state proton transfer reactions

Steady state and time-resolved fluorometric investigations of the excited state proton transfer reactions of carbazole and 2-naphthylamine in the presence of β-cyclodextrin are reported in this paper. The results, together with earlier reports, reveal that the prototropic reaction depends not only on the microenvironment of the molecule, which is imposed by cyclodextrin, but also on the nature of the molecule itself. Thus, in comparison with the bare chromophore, the deprotonation rate of the cyclodextrin inclusion complex is enhanced when the guest molecule is carbazole, whereas it is decreased for guests like naphthylamine or naphthols.

Deciphering the perturbation of serum albumins by a ketocyanine dye: a spectroscopic approach.

Steady state and time resolved fluorometric and circular dichroism (CD) techniques have been exploited to explore the binding interaction of a ketocyanine dye, namely, 2-[3-(N-methyl-N-phenylamino)-2-propenylidene] indanone (MPAPI) with transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA). The emission spectrum of buffered solution of the dye is found to be perturbed remarkably upon binding with the proteins. An explicit study with respect to modification of fluorescence and fluorescence anisotropy upon binding, effect of denaturant, fluorescence lifetime and CD measurements reveal that the dye binds with both BSA and HSA; the binding being stronger with the latter. Denaturation and CD studies reveal that stability of the proteins increases upon binding with the dye. The probable binding sites of the dye in the proteinous environments have been assessed from fluorescence resonance energy transfer (FRET) study. The probe is argued to be located in the inter domain cleft region of HSA (near Trp-214). From the similarity in the fluorescence behavior of the dye in BSA and HSA it is inferred that in BSA environment the probe is located near Trp-212 rather than Trp-132.

Vibrational spectrum and molecular structure of triphenylamine monomer: A combined matrix-isolation FTIR and theoretical study

Theoretical optimization of triphenylamine geometry, carried out at DFT(B3LYP) level using 6-31G** and aug-cc-pVDZ basis sets, predicted a propeller-like structure of the compound with D3 overall symmetry. In this structure, the central NCCC atoms are coplanar and the phenyl rings are symmetrically twisted from this plane by 41.5° (6-31G**) or 41.6° (aug-cc-pVDZ). The experimental FTIR spectrum of triphenylamine monomers isolated in an argon matrix was measured and interpreted by comparison with theoretical spectra calculated at the DFT(B3LYP) level with 6-31G** or aug-cc-pVDZ basis sets. The good agreement between the experimental and theoretical spectra allowed a positive assignment of the observed infrared absorption bands. Conformational flexibility of triphenylamine was investigated by carrying out a series of theoretical scans of the potential energy hypersurface of the system. Special attention was granted to the minimal energy pathway between the left-hand rotating and right-hand rotating symmetry identical structures of the compound. A route conserving a C2 symmetry axis was identified as implying an energy barrier of 20 kJ mol−1 only, whereas the calculated barrier for the concerted twist of all the phenyl rings (the route with conservation of the C3 symmetry axis) was as high as 54 kJ mol−1.