Debosreeta Bose

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.

Photophysics and Rotational Dynamics of a β-Carboline Analogue in Nonionic Micelles: Effect of Variation of Length of the Headgroup and the Tail of the Surfactant

Effect of variation of length of nonionic surfactants in terms of the headgroup as well as the tail part on the photophysical and rotational dynamical properties of a beta-carboline analogue, 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a]quinolizine (AODIQ) has been investigated. Steady-state and time-resolved fluorescence and fluorescence anisotropy have been exploited for the purpose. The experiments revealed modification of the photophysics of AODIQ by the conjugate effect of polarity and rigidity of the micellar environments with varying poly(ethylene oxide) chain length in the case of Triton X series and the alkyl chain length in the case of Tween series surfactants. Fluorometric studies suggest that the fluorophore resides at the micelle-water interface in all these systems. The enhancements in the steady-state anisotropy in all the micellar media compared to those in pure aqueous solution reflect that the fluorophore is located in motionally restricted regions introduced by the nonionic micelles. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, they were found to be biexponential in the micellar environments. The rotational relaxation of AODIQ in the micellar environments has been discussed in light of the two-step and wobbling in a cone model. The model helps to evaluate different rotational parameters and to ascertain the location of the fluorophore in the micellar media. The significant feature is that the motional restriction decreases with an increase in the poly(ethylene oxide) chain length while it increases with an increase in the alkyl chain length. The difference in the extent of water penetration due to variation in the thickness of the palisade layer and therefore a variation in the micellar polarity with a variation of the length of poly(ethylene oxide) and alkyl chain has been argued to be responsible.

Probing the binding interaction of a phenazinium dye with serum transport proteins: a combined fluorometric and circular dichroism study

In the present investigation, an attempt has been made to study the interaction of phenosafranin (PSF), a cationic phenazinium dye with the transport proteins, bovine serum albumin (BSA) and human serum albumin (HSA), employing steady‐state and time‐resolved fluorometric and circular dichroism (CD) techniques. The photophysical properties of the dye are altered on binding with the serum proteins. An explicit study with respect to the modification of the fluorescence and fluorescence anisotropy upon binding, effect of denaturant, fluorescence lifetime and CD measurements reveal that the dye binds to both BSA and HSA with almost the same affinity. Far‐UV CD spectra indicate a decrease in the percentage of α‐helicity only for BSA upon binding with the probe. Near‐UV CD responses indicate an alteration in the tertiary structure of both the transport proteins because of binding.

Binding of a cationic phenazinium dye in anionic liposomal membrane: a spectacular modification in the photophysics

Interaction of a cationic phenazinium dye, phenosafranin (PSF), with the anionic liposomal vesicle/bilayer of dimyristoyl-L-alpha-phosphatidylglycerol (DMPG) has been demonstrated using steady state and time resolved fluorescence and fluorescence anisotropy techniques. The charge transfer emission spectrum of PSF shows a dramatic modification in terms of fluorescence yield together with an appreciable hypsochromic shift in the lipid environment. The blue shift indicates a lowering in polarity inside the vesicle as compared to that in bulk water. The fluorescence and fluorescence quenching studies and micropolarity determination reveal that the cationic fluorophore has a profound binding interaction with the anionic DMPG membrane. Anisotropy study indicates the imposition of a motional restriction on the probe inside the bilayer. The electrostatic interaction between the cationic dye and the anionic lipid membrane has been argued to be the reason behind all these observations. The results could be useful in analyzing membrane organization and heterogeneity in natural membranes exploiting PSF or alike compounds as fluorescent probes.

Differential Interaction of β-Cyclodextrin with Lipids of Varying Surface Charges: A Spectral Deciphering Using a Cationic Phenazinium Dye

Interaction of phenosafranin (PSF), a biologically potent cationic dye molecule, has been studied with zwitterionic and anionic lipid membranes of dimyristoyl-L-alpha-phosphatidylcholine (DMPC) and dimyristoyl-L-alpha-phosphatidylglycerol (DMPG), respectively. The effect of cyclic oligosaccharide, beta-cyclodextrin (beta-CD), on the stability of these probe-bound lipid bilayers has also been investigated exploiting steady state and time-resolved fluorescence, steady state fluorescence anisotropy, and dynamic light scattering techniques. An interpretation of membrane destabilization upon interaction of cyclodextrin with the lipids was drawn exploiting PSF as an extrinsic fluorescent probe. The fluorophore showed discernible interactions with DMPC and DMPG vesicles. Experimental results reveal that the extent of interaction of PSF with DMPG is greater compared to that with DMPC. Addition of beta-CD into the PSF-bound lipids showed a differential effect for the two lipids of varying surface charge characteristics. In the case of DMPC, addition of beta-CD resulted in a preferential interaction of the probe with CD. However, addition of beta-CD to PSF-bound DMPG resulted in the selective interaction of DMPG with the added CD leading to the release of the probe into the bulk aqueous medium.

Photophysics and rotational relaxation dynamics of a β-carboline based fluorophore in cationic alkyltrimethylammonium bromide micelles

Photophysics and rotational relaxation dynamics of a beta-carboline analog, 3-acetyl-4-oxo-6,7-dihydro-12H-indolo-[2,3-a] quinolizine (AODIQ) have been investigated in cationic alkyltrimethylammonium bromide (nTAB) micelles using steady-state and time-resolved fluorometric techniques. The study reveals modification of its photophysics by the conjugate effect of polarity and rigidity of the micellar environments with varying alkyl chain lengths of the surfactants. Furthermore, it suggests that the fluorophore resides at the micelle-water interfacial domain. Contrary to the single exponential nature of the fluorescence anisotropy decay of AODIQ in aqueous medium, the decay is found to be biexponential in all the micellar environments studied. The enhancements in the steady-state anisotropy and rotational relaxation time in the micellar media compared to that in pure aqueous solution reflect that the fluorophore resides in a motionally restricted environment introduced by the cationic micelles. The rotational correlation time increases marginally with an increase in the surfactant chain length. The rotational relaxation of AODIQ in the micellar environments has been discussed in the light of the two-step and wobbling in a cone model. The model helps in evaluating different rotational parameters and in ascertaining the location of the fluorophore in the micellar media. This technique provides valuable information regarding the rotational relaxations of the fluorophore within an organized assembly. When the lifetime measurements and orientational relaxation measurements are combined, significant inferences can be made regarding the partitioning of the probe in different regions of the micelles.

Photophysics and rotational relaxation dynamics of cationic phenazinium dyes in anionic reverse micelles: effect of methyl substitution

We present here, a detailed photophysical and rotational relaxation dynamical study of three structurally analogous cationic dyes, namely, phenosafranin (PSF), safranin-T (ST), and safranin-O (SO), carried out in well characterized, monodispersed biomimicking anionic reverse micellar nanocavities composed of sodium bis(2-ethylhexyl)sulfosuccinate (AOT)/heptane with increasing water contents. The dyes belong to the phenazinium family and they differ in terms of methyl substitution on the planar phenazinium skeleton. The objective of the present study is to investigate the modification in the photophysical and dynamical behavior of the dyes with the change in the size of the water pool of the reverse micelle and thereby to explore the role of methyl substitution. Steady state and time resolved emission and anisotropy studies have been exploited for the purpose. The dyes are found to exhibit a marked decrease in the fluorescence anisotropy with increasing water/surfactant mole ratio (w), i.e., the water pool size in the reverse micellar core, implying that overall motional restriction experienced by the molecules are decreased with increasing hydration. Some of the depth dependent fluorescence parameters such as fluorescence maximum, fluorescence anisotropy (r) have been monitored for exploring the microenvironment around the probes in the reverse micelles. Fluorescence studies suggest that at low w values, the probes do not penetrate into the reverse micellar core; rather it binds at the interfacial region. Estimates of the micropolarity at the binding sites of the probe molecule have been determined as a function of w. Finally, dynamic studies reveal that both the lifetime and rotational relaxation time decrease with an increase in w for all the three probes, the extent of the decrease being more for PSF than ST and SO. This indicates a stronger binding of the reverse micelle with ST and SO compared to that with PSF which is rationalized in terms of an increase in the hydrophobicity of the former two dyes because of the methyl substitution on the phenazinium moiety.

Excited-State-Proton-Transfer-Triggered Fluorescence Resonance Energy Transfer: from 2-Naphthylamine to Phenosafranin

Excited-state proton transfer (ESPT) and fluorescence resonance energy transfer (FRET) have been linearly coupled leading to an efficient pH-sensitive energy transfer from 2-naphthylamine (2NA) to a potentially bioactive cationic phenazinium dye, phenosafranin (PSF). The prototropic product produced exclusively from the photoexcited 2NA in the presence of added alkali serves as the donor for the energy transfer process. The energy transfer process is turned on at pH > or = 12, whereas the process is turned off at a pH lower than that. Within the range of pH 12 to 13, the energy transfer efficiency (E) has been shown to follow a linear relation with the solution pH establishing the governing role of pH of the solution on the energy transfer process. The energy transfer follows a long-range dipole-dipole interaction mechanism. The critical energy transfer distance (R0) and the distance between the acceptor and the donor (r) have been determined for the ESPT-promoted FRET process at an optimum pH of 13. The present study involving the coupled processes is simple but has its implication due to its potential to be exploited for designing a pH-sensitive molecular switch.

Studies of Triton X-165–β-cyclodextrin interactions using both extrinsic and intrinsic fluorescence

The interaction of beta-cyclodextrin with the non-ionic micelle-forming surfactant Triton X-165 (TX-165) has been studied using steady state fluorescence and fluorescence anisotropy techniques. Both extrinsic and intrinsic fluorescence have been exploited for the purpose. Phenosafranin (PSF), a cationic phenazinium dye, has been used as the extrinsic probe while fluorescence of TX-165 has served as the intrinsic one. PSF shows discernible interactions with both TX-165 and beta-CD. The experimental results reveal that the extent of interaction of PSF with TX-165 is greater than with beta-CD. However, addition of beta-CD to a micellar solution of TX-165 containing PSF leads to a disruption of the micelles whereby the fluorophore is released from the micellar environment to the bulk aqueous phase. It has been substantiated that an inclusion complex is formed between the non-ionic surfactant and the cyclodextrin. A 1:1 stoichiometry of the TX-165-beta-CD inclusion complex has been proposed. Such a complexation between TX-165 and beta-CD results in an inhibition in the micellization process of TX-165 leading to an enhancement in the apparent CMC value. The inferences are drawn from a series of experiments, viz., binding studies, determination of micropolarity, heavy-ion quenching studies and steady state fluorescence anisotropy experiments monitoring both extrinsic and intrinsic fluorescences.

Multiple emissions of benzil at room temperature and 77 K and their assignments from ab initio quantum chemical calculations

Multiple emissions have been observed from benzil under different conditions in solutions at room temperature as well as in low temperature glass matrices at 77 K. Low temperature emission has been monitored in rigid matrices frozen under different conditions of illumination. Steady state and time-resolved results together with the ab initio quantum chemical calculations provide, for the first time, the assignments of the different fluorescence bands to the different geometries and∕or electronic states of the fluorophore molecule. It is revealed that the skew form of benzil emits from the first (S(1)) as well as the second excited singlet (S(2)) states depending on the excitation wavelength, while the relaxed transplanar conformer fluoresces only from the S(1) state. The yet unexplored emission band peaking at around 360 nm has been assigned to originate from the S(2) state. Ab initio calculations using the density functional theory at B3LYP∕6-31G∗∗ level corroborate well with the experimental observations.