Surat Hotchandani

Binding sites of retinol and retinoic acid with serum albumins.

Retinoids are effectively transported in the bloodstream via serum albumins. We report the complexation of bovine serum albumin (BSA) with retinol and retinoic acid at physiological conditions, using constant protein concentration and various retinoid contents. FTIR, CD and fluorescence spectroscopic methods and molecular modeling were used to analyze retinoid binding site, the binding constant and the effects of complexation on BSA stability and secondary structure. Structural analysis showed that retinoids bind BSA via hydrophilic and hydrophobic interactions with overall binding constants of K(Ret)(-BSA) = 5.3 (±0.8) × 10(6) M(-1) and K(Retac-BSA) = 2.3 (±0.4) × 10(6) M(-1). The number of bound retinoid molecules (n) was 1.20 (±0.2) for retinol and 1.8 (±0.3) for retinoic acid. Molecular modeling showed the participation of several amino acids in retinoid-BSA complexes stabilized by H-bonding network. The retinoid binding altered BSA conformation with a major reduction of α-helix from 61% (free BSA) to 36% (retinol-BSA) and 26% (retinoic acid-BSA) with an increase in turn and random coil structures indicating a partial protein unfolding. The results indicate that serum albumins are capable of transporting retinoids in vitro and in vivo.

Modification of electrode surface with semiconductor colloids and its sensitization with chlorophyll a

Abstract The surface of an optically transparent electrode has been modified with colloidal particles of ZnO and CdS. Chlorophyll a electrodeposited on these modified electrode surfaces is capable of injecting electrons from its excited state into the conduction band of the semiconductor. The role of the coupled semiconductor in improving the photon to photocurrent conversion efficiency of the photosensitization process is described.

Photoelectrochemistry of Semiconductor ZnO Particulate Films

This paper reports on thin films of ZnO semiconductor that have been prepared on electrode surfaces by coating them with quantized ZnO colloids. The photoelectrochemical properties of semiconductor particulate films have been evaluated with both steady-state and laser pulse excitations. The ZnO film behaves like an n-type semiconductor with a flatband potential of {minus}0.6 V vs. SCE. The incident-photon conversion efficiency at 320 nm is 15%. The generation of photovoltage at these electrodes has been time-resolved with coulostatic laser-flash-photolysis experiments.

Photoelectrochemical behavior of coupled SnO2|CdSe nanocrystalline semiconductor films

A photoelectrochemical cell with a coupled SnO2|CdSe nanocrystalline semiconductor electrode has been prepared by sequential deposition of SnO2 and CdSe films onto an optically transparent electrode (OTE), and its photoelectrochemical behavior has been studied. The results show that the coupling of CdSe with SnO2 leads to an improvement in the performance of OTE|SnO2|CdSe over OTE|CdSe cells in terms of increased incident photon-to-current conversion efficiency, increased stability and smaller reversal of current. The favorable positioning of the energy bands of SnO2 and CdSe is responsible for the above observations. Various photoelectrochemical parameters of the OTE|SnO2|CdSe cell obtained for an incident light power of 0.31 mW cm−2 at 470nm, are as follows: Isc ≈ 25–30 μA cm−2, Voc ≈ 0.5–0.6 V, ƒƒ = 0.47 and a power conversion efficiency of about 2.25%.

Locating the Binding Sites of Pb(II) Ion with Human and Bovine Serum Albumins

Lead is a potent environmental toxin that has accumulated above its natural level as a result of human activity. Pb cation shows major affinity towards protein complexation and it has been used as modulator of protein-membrane interactions. We located the binding sites of Pb(II) with human serum (HSA) and bovine serum albumins (BSA) at physiological conditions, using constant protein concentration and various Pb contents. FTIR, UV-visible, CD, fluorescence and X-ray photoelectron spectroscopic (XPS) methods were used to analyse Pb binding sites, the binding constant and the effect of metal ion complexation on HSA and BSA stability and conformations. Structural analysis showed that Pb binds strongly to HSA and BSA via hydrophilic contacts with overall binding constants of KPb-HSA = 8.2 (±0.8)×104 M−1 and KPb-BSA = 7.5 (±0.7)×104 M−1. The number of bound Pb cation per protein is 0.7 per HSA and BSA complexes. XPS located the binding sites of Pb cation with protein N and O atoms. Pb complexation alters protein conformation by a major reduction of α-helix from 57% (free HSA) to 48% (metal-complex) and 63% (free BSA) to 52% (metal-complex) inducing a partial protein destabilization.

Photocatalytic reduction of azo dyes Naphthol Blue Black and Disperse Blue 79

Photocatalytic reduction of two textile azo dyes, Naphthol Blue Black (NBB) and Disperse Blue 79 (DB79) has been carried out in colloidal WO3 and TiO2 suspensions. Under bandgap excitation of the semiconductor colloids these dyes undergo irreversible reduction as they react with the trapped electrons. The quantum efficiency for the photocatalytic reduction of these dyes were 5.4% and 4.8% for NBB and DB79 respectively. The kinetics and mechanism of the interfacial charge transfer in these colloidal suspension has been elucidated with transient absorption spectroscopy. The reaction between the dye and trapped electrons is diffusion limited and occurs with rate constants of 1.1×108 M−1s−1 and 4.0×107 M−1s−1 for NBB and DB79 respectively.

Analysis of dark current‐voltage characteristics of Al/chlorophyll a/Ag sandwich cells

The analysis of dark current‐voltage (I‐V) characteristics of Al/chlorophyll a/Ag cells at room temperature with respect to the elucidation of conduction mechanisms and evaluation of cell parameters is presented. It is seen that the presence of series and shunt resistances can considerably affect the I‐V plots. It is therefore important to remove their effects for correct and meaningful analysis of the I‐V curves. The results suggest that for Al/Chl a/Ag cells with microcrystalline Chl a∼3000 A thick, the conduction mechanism for voltages between 0.53 and 1 V can be described by a modified Shockley equation from which the values of Rs, Rsh, n, and I0 obtained are 3.2×104 Ω, 1.7×109 Ω, 1.74, and 2.4×10−15 A, respectively. Rs is most likely due to the combined effect of bulk Chl a and the electrodes, particularly the insulating layer of Al2O3 that is formed as a result of oxidation of Al in air. For higher forward biases, i.e., between 1 and 2 V, the current transport is due to the space‐charge‐limited curr...

Enhanced charge separation in chlorophyll a solar cell by gold nanoparticles

An efficient organic photoelectrochemical cell based on chlorophyll a (Chla) and gold nanoparticles is constructed. The enhanced performance of this cell is due to the beneficial role of gold nanoparticles in accepting and shuttling the photogenerated electrons in Chla to the collecting electrode. This produces a long-distance charge-separated state, resulting into an enhancement in charge separation efficiency.

Redox characteristics of Schiff base manganese and cobalt complexes related to water-oxidizing complex of photosynthesis

In an effort to obtain synthetic analogues of water-oxidizing complex (WOC) of photosystem II (PS II) of plant photosynthesis, a Schiff base manganese and a cobalt complex, employing Niten, a SALEN type ligand, have been prepared. Cyclic and square wave voltammetric measurements have been performed to assess their redox characteristics. Both complexes undergo several reduction processes in cathodic negative potential region at more or less similar potentials. In view of these reductions being independent of the nature of the metal, they are thought to be ligand-localized. Although similar in negative region, a marked difference in the behavior of the complexes is observed in anodic region. While the cobalt complex is electrochemically inactive in the positive potentials up to +1.0 V vs. Ag/AgCl, the manganese complex displays two oxidation waves at +0.25 and +0.5 V vs. Ag/AgCl. The presence of oxidation wave in manganese complex at +0.5 V vs. Ag/AgCl or +0.7 V vs. NHE suggests that this complex can catalyze the oxidation of water and can, thus, simulate the WOC of PS II.

Fluorescence and photoelectrochemical behavior of chlorophyll a adsorbed on a nanocrystalline SnO2 film

Fluorescence and photoelectrochemical studies of chlorophyll a (Chl a) adsorbed on nanocrystalline SnO2 film were carried out. The results of fluorescence and incident photon to current conversion efficiency (IPCE) as a function of applied bias suggest that the fluorescence quenching and the photocarrier generation are interrelated. Fluorescence quenching has thus been utilized to determine the photogeneration efficiency, η(e), of charges in a SnO2/Chl a based photoelectrochemical cell. A value of 0.75 was obtained for η(e) for unbiased cells. With an IPCE of 13%, η(e) of 75%, and a light harvesting efficiency of 70%, the charge collection efficiency of ∼23% was evaluated. These results suggest that the losses due to the charge recombination are a major factor that limit the efficiency of the cells.