Paulami Mandal

Fluorescence spectroscopic characterization of the interaction of human adult hemoglobin and two isatins, 1-methylisatin and 1-phenylisatin: a comparative study.

In this report, steady state and time-resolved fluorescence along with circular dichroism (CD) spectroscopic, FTIR, and anisotropy investigations were made to reveal the nature of the interactions between human adult hemoglobin (Hb) and the isatins, 1-methylisatin (1-MI) and 1-phenylisatin (1-PI). From the analysis of the steady state and time-resolved fluorescence quenching of Hb in aqueous solution in the presence of an isatin, i.e., 1-MI, it seemed that the nature of the quenching was of static type and a mixture of both static and dynamic nature for 1-PI. The primary binding pattern between isatins and Hb has been interpreted as a combined effect of hydrophobic association and electrostatic interaction for 1-MI. For 1-PI, this was the combined effect of hydrophobic association and ionic interactions and salt bridges or/and proton transfer. The pretwisted structure of 1-PI facilitates ionic interactions with Hb. The binding constants, number of binding sites, and thermodynamic parameters had been computed. The binding average distances between the Hb-1-MI and Hb-1-PI determined from Forsters theory were found to be 4.02 and 5.28 nm, respectively. CD, steady state, and time-resolved anisotropy measurements had been done in support.

A detailed spectroscopic study on the interaction of Rhodamine 6G with human hemoglobin.

UV-vis, time-resolved fluorescence and circular dichroism spectroscopic investigations have been made to reveal the nature of the interactions between xanthene dye Rhodamine 6G and the well known protein hemoglobin. From the analysis of the steady-state and time-resolved fluorescence quenching of Rhodamine 6G in aqueous solutions in presence of hemoglobin, it is revealed that the quenching is static in nature. The primary binding pattern between Rhodamine and hemoglobin has been interpreted as combined effect of hydrophobic association and electrostatic interaction. The binding constants, number of binding sites and thermodynamic parameters at various pH of the environment have been computed. The binding average distance between the energy donor Rhodamine and acceptor hemoglobin has been determined from the Forsters theory.

Does bimolecular charge recombination in highly exergonic electron transfer afford the triplet excited state or the ground state of a photosensitizer

The investigations were made on photoinduced electron transfer (ET) from the singlet excited state of rubrene (1RU*) to p-benzoquinone derivatives (duroquinone, 2,5-dimethyl-p-benzoquinone, p-benzoquinone, 2,5-dichloro-p-benzoquinone, and p-chloranil) in benzonitrile (PhCN) by using the steady state and time-resolved spectroscopies. The photoinduced ET produces solvent-separated type charge-separated (CS) species and the charge-recombination (CR) process between RU radical cation and semiquinone radical anions obeys second-order kinetics. Not only the CS species but also the triplet excited state of RU (3RU*) is seen in the transient absorption spectra upon laser excitation of a PhCN solution of RU and p-benzoquinone derivatives. The comparison of their time profiles clearly suggests that the CR process between RU radical cation and semiquinone radical anions to the ground state is independent from the deactivation of 3RU*. This indicates that the CR in a highly exergonic ET occurs at a longer distance with a large solvent reorganization energy, which results in faster ET to the ground state than to the triplet excited state that is lower in energy than the CS state. Photoinduced ET from 3RU* in addition from 1RU* also occurs when p-benzoquinone derivatives with electron-withdrawing substituents were employed as electron acceptors.

Formation of Two Different Types of Oligomers in the Early Phase of pH-Induced Aggregation of the Alzheimer A beta(12-28) Peptide

The early phase in the aggregation process of the Alzheimers peptide Aβ(12-28) with both protected and unprotected ends was studied by time-resolved infrared spectroscopy and circular dichroism spectroscopy. Aggregation in the time-resolved experiments was initiated by a rapid pH drop caused by the photolysis of 1-(2-nitrophenyl)ethyl sulfate (caged sulfate). The infrared spectra indicate aggregates from both versions of the Aβ(12-28) peptide. [corrected] They form fast (within 60 ms), presumably from initial aggregates, and their spectral signature is consistent with a β-barrel structure. The other type arises relatively slowly from unstructured monomers on the seconds-to-minutes time scale and forms at lower pH than the first type. These β sheets are antiparallel, planar, and large and show an absorption band at 1622 cm(-1) that shifts to 1617 cm(-1) in 12 min with most of the shift occurring in 10 s.

Investigations on the Photoreactions of Phenothiazine and Phenoxazine in Presence of 9-cyanoanthracene by Using Steady State and Time Resolved Spectroscopic Techniques

By using electrochemical, steady state and time resolved (fluorescence lifetime and transient absorption) spectroscopic techniques, detailed investigations were made to reveal the mechanisms of charge separation or forward electron transfer reactions within the electron donor phenothiazine (PTZH) or phenoxazine (PXZH) and well known electron acceptor 9-cyanoanthracene (CNA). The transient absorption spectra suggest that the charge separated species formed in the excited singlet state resulted from intermolecular photoinduced electron transfer reactions within the donor PTZH (or PXZH) and CNA acceptor relaxes to the corresponding triplet state. Though alternative mechanisms of via formations of contact neutral radical by H-transfer reaction have been proposed but the observed results obtained from the time resolved measurements indicate that the regeneration of ground state reactants is primarily responsible due to direct recombination of triplet contact ion-pair (CIP) or solvent-separated ion-pair (SSIP).

Sensing of Different Human Telomeric G-Quadruplex DNA Topologies by Natural Alkaloid Allocryptopine Using Spectroscopic Techniques

This article describes how a natural alkaloid allocryptopine (ALL) is able to differentiate two forms of biologically relevant human telomeric (htel22) G-quadruplex DNAs (GQ-DNA) depending on the presence of K+ and Na+ ions by steady-state and time-resolved spectroscopic techniques. For both interactions, predominant involvements of static-type quenching mechanism with the negligible influence of dynamic collision are established by UV-vis absorption and fluorescence emission study, which is further supported by fluorescence lifetime measurements. ALL exhibits appreciable affinity toward both GQ-DNAs. Both the mixed-hybrid (3 + 1) quadruplex structures in K+ ions and the basket-type antiparallel quadruplex structure under Na+ condition are converted to parallel types in the presence of ALL. Fluorescence intercalator displacement assay experiment revealed modest selectivity of ALL to both quadruplexes over duplex DNA along with higher selectivity for antiparallel types among the two quadruplexes via groove and/or loop binding, which is distinct from the conventional π-stacking of the ligands on external G-quartets. ALL stabilized both GQ-DNA topologies moderately. The differences in the dynamics of ALL within both DNA environments have been demonstrated vividly by time-resolved anisotropy measurements using the wobbling-in-cone model. These results suggest groove binding with antiparallel G-quartet with high affinity and moderate loop binding with mixed-hybrid G-quartet accompanied by the partial end stacking additionally in both of the cases. Our conclusions are further supported by steady-state anisotropy measurements and molecular docking. The present investigation can be used in the development of a biocompatible antitumour/anticancer agent targeting particular GQ-DNA conformation.