Debarati Dey

Interactions of guanine and guanosine hydrates with quinones: a laser flash photolysis and magnetic field effect study.

Laser flash photolysis and an external magnetic field have been used to study the interaction of two quinone molecules, namely, 9,10-anthraquinone (AQ) and 2-methyl 1,4-naphthoquinone, commonly known as menadione (MQ), with one of the DNA bases, guanine (G) and its nucleoside guanosine hydrate (dG). In organic homogeneous medium, it has been observed that G undergoes a predominant hydrogen (H) abstraction reaction with both the quinones while dG supports photoinduced electron transfer (PET) along with H abstraction. On the other hand, in SDS medium, G supports PET with AQ but not with MQ. However, behavior of dG remains unperturbed toward AQ and MQ with the change in medium. All of these observations have been explained on the basis of stabilization of radical ion pair and difference in size of the quinones, which can affect the distance of approach among the interacting molecules.

Poly(ethylene glycol) vesicles: self-assembled site for luminescence generation.

Fluorescence in poly(ethylene glycol) (PEGs 400-12000) solutions is reported here for the first time. PEG solutions form a vesicular organization with the hydrophilic groups attached at both ends which arrange themselves beyond a particular concentration and offer electron-dense regions at the center of the vesicle. These vesicles provide an inherent site for fluorescence generation in PEG solutions. Fluorescence emission was observed at ~380 nm with an excitation wavelength of 300 nm. PEG of molecular weight 6000 was found to show maximum emission intensity at a particular concentration. The formation of PEG vesicles (~1 nm size) was confirmed by dynamic light scattering (DLS) and confocal laser microscopy. On addition of metal ions the polymeric vesicle breaks up to monomeric PEG, and hence, the fluorescence intensity decreases with a red shift. Fluorescence lifetime measurements indicate the nature of complexation of the metals with PEG. Since PEGs are used as one of the phases in aqueous biphasic systems (ABS) of liquid-liquid extractions, the nature of the fluorescence emission spectrum of the PEG phase after extraction was studied. Metal extraction in the PEG-rich phase of an ABS leads to quenching of fluorescence in PEG.

Influence of Heterogeneity of Confined Water on Photophysical Behavior of Acridine with Amines: A Time-Resolved Fluorescence and Laser Flash Photolysis Study

The photophysical behavior of acridine (Acr) shows facilitated water-assisted protonation equilibrium between its deprotonted (Acr* ∼ 10 ns) and protonated forms (AcrH(+*) ∼ 28 ns) within confined region of ordered water molecules inside AOT/H(2)O/n-heptane reverse micelles (RMs). The time-resolved-area-normalized-emission spectra confirm both Acr* and AcrH(+*), while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+*) with N,N-dimethylaniline (DMA) is a purely diffusion-controlled bimolecular quenching with linear Stern-Volmer (S-V) plot, while nonlinearity arises with triethylamine (TEA) that forms ground state complex with AcrH(+) (AcrH(+)··H(2)O··TEA) indicating both static and dynamic quenching. Transient intermediates, DMA(•+) and AcrH(•) infer photoinduced electron transfer from DMA to Acr, while those from AcrH(+)··H(2)O··TEA complex suggest water mediated excited-state proton transfer (ESPT) between AcrH(+) and TEA. The ESPT becomes faster in larger RMs due to enhanced mobility of hydronium ions in AcrH(+)··H(2)O··TEA, which reduces in smaller RMs as water becomes much more constrained owing to stronger complexation by excess confinement.

Exploring the mechanism of electron transfer between DNA and a ternary copper complex.

Photoinduced intramolecular electron transfer occurs in the triplet state within the complex [Htyr-Cu-phen](+) (Htyr = l-tyrosinato; phen = 1,10-phenanthroline) from tyrosine to phenanthroline. For this linked donor-acceptor system, a prominent magnetic field effect (MFE) is observed for the triplet-born radicals. The competitive binding study in the presence of ethidium bromide suggests that the complex interacts with calf thymus DNA (CT DNA) through partial intercalation. The photoexcited copper complex can oxidize DNA in a deoxygenated environment. Though the oxidation of tyrosine is thermodynamically more favorable than the oxidation of guanine, the primary electron transfer occurs from the DNA base to the phen ligand. A prominent MFE is observed for this noncovalently bound triplet-born guanine radical and phen radical anion. The process of partial intercalation of the copper complex within DNA is responsible for this rare observation.

Aromatic amino acids in high selectivity bismuth(III) recognition

The three aromatic amino acids, tyrosine, tryptophan and phenylalanine, play different physiological roles in life processes. Metal ions capable of binding these amino acids may aid in the reduction of effective concentration of these amino acids in any physiological system. Here we have studied the efficacy of some heavy metals for their complexation with these three amino acids. Bismuth has been found to bind selectively with these aromatic amino acids and this was confirmed using spectrofluorimetric, spectrophotometric and cyclic voltammetric studies. The series of heavy metals has been chosen because each of these metals remains associated with the others at very low concentration levels and Bi(III) is the least toxic amongst the other elements. So, selective recognition for Bi(III) would also mean no response for the other heavy elements if contaminants are present even at low concentration levels. The affinity towards these amino acids has been found to be in the order tryptophan < phenylalanine < tyrosine. The association constants of these amino acids have been calculated using Benesi-Hildebrand equations and the corresponding free energy change has also been calculated. The values of the association constants obtained from BH equations using absorbance values corroborate with the Stern-Volmer constants obtained from fluorimetric studies. The evidence for complexation is also supported by the results of cyclic voltammetry.

Unexplored analytics of some novel 3d–4f heterometallic Schiff base complexes

Three heterometallic Schiff-base complexes of Cu having Pr, Nd and Sm as the heteroatoms have been synthesized. The compounds have also been characterized by their IR spectra and CHN analysis. The single crystal structures of these compounds have been studied from the X-ray crystallographic data. To the best of our knowledge the article describes the possibility of application of these compounds in the field of species dependent anion sensing for the first time. Amongst a number of anionic species, certain sulphur species were found to have greater reactivity towards a Schiff-base complex as they can incur probable changes in the molecular complexity. The S2O82−and S2O32− species could modify the spectral features of the Schiff-base complex containing Nd as the heteroatom. This particular complex was found to exhibit changes in its absorbance and fluorescence spectral features upon interaction with the anionic species S2O82−and S2O32−. The results give a strong platform for the Schiff base complexes for their analytical applications.

Laser flash photolysis and magnetic-field-effect studies on interaction of thymine and thymidine with menadione: role of sugar in controlling reaction pattern

Abstract The magnetic field effect (MFE) in conjunction with laser flash photolysis has been used for the study of the interaction of one of the small drug like quinone molecules, 2-methyl, 1,4-naphthoquinone, commonly known as menadione (MQ), with one of the DNA bases, thymine (THN), and its corresponding nucleoside, thymidine (THDN), in acetonitrile (ACN) and sodium dodecylsulfate (SDS) micelles. It has been observed that THN undergoes electron transfer (ET) and hydrogen (H) abstraction with MQ, while THDN undergoes only H abstraction in both the media. However, our earlier studies showed that a purine base, adenine (ADN), and its nucleoside, 2′-deoxyadenosine (ADS), undergo ET in ACN and H abstraction in SDS. Here we have attempted to explain the differences in the reactions of these DNA bases with MQ. We also reveal the crucial role of a sugar unit in altering the behavior of purine and pyrimidine bases with respect to ET and H abstraction.

Magnetic field effect on photoinduced electron transfer between [Cu(phen)2]2+ and DNA.

The magnetic field effect (MFE) on the photoinduced electron transfer (PET) reaction between the [Cu(phen)2]2+ complex and DNA has been studied in homogeneous buffer medium and in reverse micelles. The copper complex on photoexcitation can oxidize DNA in a deoxygenated environment. A prominent MFE is found even in a homogeneous aqueous medium for the triplet born radicals. The process of partial intercalation of [Cu(phen)2]2+ complex within DNA is responsible for such a rare observation. In reverse micelles, the MFE is not very much prominent because of the large separation distance between the component radicals of the geminate radical ion pairs generated through PET.

Magnetic Field Effect: An Efficient Tool To Investigate The Mechanism Of Reactions Using Laser Flash Photolysis Technique

Magnetic field effect combined with laser flash photolysis technique have been used to study the mechanism of interactions between two drug‐like quinone molecules, Menadione (1,4‐naphthoquinone, MQ) and 9, 10 Anthraquinone (AQ) with one of the DNA bases, Adenine in homogeneous acetonitrile/water and heterogeneous micellar media. A switchover in reaction mode from electron transfer to hydrogen abstraction is observed with MQ on changing the solvent from acetonitrile/water to micelle; whereas, AQ retains its mode of interaction towards Adenine as electron transfer in both the media due to its bulky structure compared to MQ.