Tulsi Mukherjee

Free radical scavenging behavior of folic acid: evidence for possible antioxidant activity.

The free radical scavenging properties and possible antioxidant activity of folic acid are reported. Pulse radiolysis technique is employed to study the one-electron oxidation of folic acid in homogeneous aqueous solution. The radicals used for this study are CCl(3)O(2)(*), N(3)(*), SO(4)(*-), Br(2)(*-), *OH, and O(*-). All these radicals react with folic acid under ambient condition at an almost diffusion-controlled rate producing two types of transients. The first transient absorption maximum is around 430 nm, which decays, and a simultaneous growth at around 390 nm is observed. Considering the chemical structure of folic acid, the absorption maximum at 430 nm has been assigned to a phenoxyl radical. The latter one is proposed to be a delocalized molecular radical. A permanent product has been observed in the oxidation of folic acid with CCl(3)O(2)(*) and N(3)(*) radicals, with a broad absorption band around 370-400 nm. The bimolecular rate constants for all the radical-induced oxidation reactions of folic acid have been measured. Folic acid is seen to scavenge these radicals very efficiently. In the reaction of thiyl radicals with folic acid, it has been observed that folic acid can not only scavenge thiyl radicals but can also repair these thiols at physiological pH. While carrying out the lipid peroxidation study, in spite of the fact that folic acid is considerably soluble in water, we observed a significant inhibition property in microsomal lipid peroxidation. A suitable mechanism for oxidation of folic acid and repair of thiyl radicals by folic acid has been proposed.

Effect of Solvent on the Excited‐state Photophysical Properties of Curcumin¶

Abstract Photophysical properties of curcumin, 1,7-bis-(4-hydroxy-3-methoxy phenyl)-1,6-heptadiene-2,5-dione, a pigment found in the rhizomes of Curcuma longa (turmeric) have been studied in different kinds of organic solvent and also in Triton X-100 aqueous micellar media using time-resolved fluorescence and transient absorption techniques having pico and nanosecond time resolution, in addition to steady-state absorption and fluorescence spectroscopic techniques. Steady-state absorption and fluorescence characteristics of curcumin have been found to be sensitive to the solvent characteristics. Large change (Δμ = 6.1 Debye) in dipole moments due to photoexcitation to the excited singlet state (S1) indicates strong intramolecular charge transfer character of the latter. Curcumin is a weakly fluorescent molecule and the fluorescence decay properties in most of the solvents could be fitted well to a double-exponential decay function. The shorter component having lifetime in the range 50–350 ps and percent contribution of amplitude more than 90% in different solvents may be assigned to the enol form, whereas the longer component, having lifetime in the range 500–1180 ps with less than 10% contribution may be assigned to the di-keto form of curcumin. Our nuclear magnetic resonance study in CDCl3 and dimethyl sulfoxide-D6 also supports the fact that the enol form is present in the solution by more than about 95% in these solvents. Excited singlet (S1) and triplet (T1) absorption spectrum and decay kinetics have been characterized by pico and nanosecond laser flash photolysis. Quantum yield of the triplet is low (ϕT ≤ 0.12). Both the fluorescence and triplet quantum yields being low (ϕf + ϕT < 0.18), the photophysics of curcumin is dominated by the energy relaxation mechanism via the internal conversion process.

Antioxidant properties of melatonin: a pulse radiolysis study.

Various one-electron oxidants such as OH*, tert-BuO*, CCl3OO*, Br2*- and N3*, generated pulse radiolytically in aqueous solutions at pH 7, were scavenged by melatonin to form two main absorption bands with lambda(max) = 335 nm and 500 nm. The assignment of the spectra and determination of extinction coefficients of the transients have been reported. Rate constants for the formation of these species ranged from 0.6-12.5x10(9) dm3 mol(-1) s(-1). These transients decayed by second order, as observed in the case of Br2*- and N3* radical reactions. Both the NO2* and NO* radicals react with the substrate with k = 0.37x10(7) and 3x10(7) dm3 mol(-1) s(-1), respectively. At pH approximately 2.5, the protonated form of the transient is formed due to the reaction of Br2*- radical with melatonin, pKa ( MelH* <=> Mel* + H+) = 4.7+/-0.1. Reduction potential of the couple (Mel*/MelH), determined both by cyclic voltammetric and pulse-radiolytic techniques, gave a value E(1)7 = 0.95+/-0.02 V vs. NHE. Repair of guanosine radical and regeneration of melatonin radicals by ascorbate and urate ions at pH 7 have been reported. Reactions of the reducing radicals e(aq)- and H* atoms with melatonin have been shown to occur at near diffusion rates.

Synthesis of nanosized silver colloids by microwave dielectric heating

Silver nanosized crystallites have been synthesized in aqueous and polyols viz., ethylene glycol and glycerol, using a microwave technique. Dispersions of colloidal silver have been prepared by the reduction of silver nitrate both in the presence and absence of stabilizer poly(vinylpyrolidone) (PVP). It was observed that PVP is capable of complexing and stabilizing Ag nanoparticles formed through the reduction of Ag+ ions in water and ethylene glycol. In the case of ethylene glycol, it has been shown that the use of PVP leads to particles with a high degree of stability. The colloids are stable in glycerol for months even in the absence of stabilizer.

Synthesis of Pt, Pd, Pt/Ag and Pd/Ag nanoparticles by microwave-polyol method

Pt, Pd, Pt-Ag and Pd-Ag bimetallic nanoparticles were synthesized in ethylene glycol and glycerol using the microwave technique in the presence of a stabilizer poly(N-vinylpyrrolidone) (PVP). It has been observed that PVP is capable of complexing and stabilizing nanoparticles. Mixed clusters were formed by simultaneous reduction of the metal ions. The clusters were characterized using UV-Vis spectra, XRD and dynamic light scattering. To understand the mechanism of formation of mixed nanoparticles, several experimental parameters such asin situ irradiation of mixed metal salts and mixing of individual sols were attempted.

Photodynamics of the S1 state of some hydroxy- and amino-substituted naphthoquinones and anthraquinones

Photodynamics of the S1 state of the 1,4- and 1,8-disubstituted hydroxyquinones and aminoquinones have been studied in different organic solvents by absorption and fluorescence spectroscopy. The internal conversion process is the major deactivation path for the S1 state. The solvent and temperature dependence and deuterium isotope effect on the fluorescence dynamics suggest that hydrogen stretching vibrations in different intra- and inter-molecular hydrogen bonds are responsible for very fast non-radiative decay processes. Among the intra-molecular hydrogen-bonded molecules studied only the S1 state of 1,8-dihydroxy-9,10-anthraquinone showed the evidence of excited-state intramolecular proton-transfer process. Preliminary observations on the interaction of the S1 state of these quinones with benzene and other aromatic hydrocarbon solvents via the formation of an exciplex have also been reported.

Preparation, characterization and surface modification of Cu metal nanoparticles

Copper metal nanoparticles have been formed by irradiation with 253.7 nm light from a low pressure Hg-arc lamp in the presence of a protective agent gelatin. The nanoparticles were characterized by their absorption maxima and transmission electron micrographs. The quantum yields of formation of the metal nanoparticles increase in the presence of the stabilizer as well as in the presence of a photosensitizer, benzophenone (BP). Damping of copper plasmon absorption band was not observed in the presence of benzotriazole.

Triplet excited states and semiquinone radicals of 1,4-disubstituted anthraquinones

Abstract The triplet state properties of 1,4-dihydroxy-9,10-anthraquinone (quinizarin, QNZ), 1-amino-4-hydroxy-9,10-anthraquinone (AHAQ) and 1,4-diamino-9,10-anthraquinone (DAAQ) were investigated in cyclohexane and isopropanol solutions using nanosecond laser flash photolysis. TT absorption maxima, extinction coefficients, triplet quantum yields and kinetic parameters were measured. The corresponding properties of the neutral semiquinone radicals were also determined in isopropanol. Pulse radiolysis data were used to supplement the measurements. The possibility of dimerization in solution and its effect on the photophysics of the triplet state are discussed for these quinones.

Excess entropy scaling of transport properties of Lennard-Jones chains

Excess-entropy scaling relationships for diffusivity and viscosity of Lennard-Jones chain fluids are tested using molecular dynamics simulations for chain sizes that are sufficiently small that chain entanglement effects are insignificant. The thermodynamic excess entropy S(e) is estimated using self-associating fluid theory (SAFT). A structural measure of the entropy S(2) is also computed from the monomer-monomer pair correlation function, g(m)(r). The thermodynamic and structural estimators for the excess entropy are shown to be very strongly correlated. The dimensionless center-of-mass diffusivities, D(cm) (*), obtained by dividing the diffusivities by suitable macroscopic reduction parameters, are shown to conform to the excess entropy scaling relationship, D(cm) (*)=A(n) exp(alpha(n)S(e)), where the scaling parameters depend on the chain length n. The exponential parameter alpha(n) varies as -(1n) while A(n) varies approximately as n(-0.5). The scaled viscosities obey a similar relationship with scaling parameters B(n) and beta(n) where beta(n) varies as 1n and B(n) shows an approximate n(0.6) dependence. In accordance with the Stokes-Einstein law, for a given chain length, alpha(n)=-beta(n) within statistical error. The excess entropy scaling parameters associated with the transport properties therefore display a simple dependence on chain length.

Photochemical formation of copper nanoparticles in poly(N-vinylpyrrolidone)

Copper metal nanoparticles have been formed by irradiation with 253.7 nm light from a low pressure Hg-arc lamp in the presence of a protective agent poly(N-vinylpyrrolidone). The role of a photo-sensitizer, benzophenone (BP), in the formation of Cu metal particles was studied. The nanoparticles have been characterized by their absorption maxima and transmission electron micrographs. The average particle size for Cu was 15 nm. It appears that the presence of BP is essential for the photochemical preparation of copper nanoparticles. Laser spectroscopic studies have revealed that BP ketyl radical does not participate in the formation of Cu metal nanoparticles.