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ABSTRACT: Spectroscopic and crystallographic studies reveal that Merocyanine 540 (MC 540), a well-known therapeutically important anionic cyanine dye, interacts with hen egg white lysozyme in ground state. The formation of the complex is validated by two isosbestic points in absorption spectra of lysozyme with varied concentration of MC 540 and appearance of an isodichroic point in induced CD spectra of MC 540 with lysozyme. The blue shift of fluorescence maximum of lysozyme in presence of MC 540 shows hydrophobic effect on Trp due to complex formation probably through cooperative binding. Above 1:3M stoichiometric ratio (lysozyme:MC 540) an additional fluorescence hump arises because of structural changes in protein, where MC 540 acts as self-denaturant, inducing non-linearity in Stern-Volmer plot. The van't Hoff isotherms with negative changes in enthalpy at lower concentration and positive changes in entropy for entire concentration range of MC 540 depict the binding forces as hydrogen bonding/van der Waal's and ionic/hydrophobic respectively. Finally X-ray crystallographic structure of the complex shows that MC 540 adopts two conformations, cis and trans, while it binds to lysozyme. Benzoxole moiety of MC 540 interacts with Trp123 through π-stacking and SO3(2-) group is stabilized by ionic interaction/H-bonding with Arg125 of lysozyme.
Journal of photochemistry and photobiology. B, Biology 02/2013; 121C:46-56. · 1.87 Impact Factor
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ABSTRACT: We aim to find out the extent of stability of the excimer of 9-aminoacridine hydrochloride hydrate (9AA), a prospective PDT drug, in different confined medium with varying cavity size. When confined in cetyl trimethyl ammonium bromide micelles, although at low concentration of 9AA only a single distinct peak (λmax at 460 nm) with a shoulder at 485 nm is observed in steady-state fluorescence spectrum, yet with increase in concentration the peak and the shoulder merge with simultaneous emergence of another peak at 535 nm, which is assigned for excimer. Similar behavior is also observed in Triton-X, crown ether, α-cyclodextrin, β-cyclodextrin and homogeneous aqueous medium. The formation of excimer, which reflects the extent of confinement of 9AA, is maximum in β- cyclodextrin followed by others. Steady-state and time resolved fluorescence studies along with TRES and TRANES analyses coupled with anisotropy data and transient absorption studies reveal the presence of monomer-dimer equilibrium of 9AA in the excited state. Molecular modeling indicate that the structure of excimer is stabilized by locking of the two monomeric species via four hydrogen bonds formed between the amino-H and imino-N of 9AA monomers, whereas, the dimer in the ground state has only two such hydrogen bonds.
The Journal of Physical Chemistry A 01/2013; · 2.95 Impact Factor
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ABSTRACT: In this article, we have investigated the interactions of three pyrimidine nucleic acid bases, cytosine (C), thymine (T), and uracil (U) with acridine (Acr), an N-heterocyclic DNA intercalator, through the changes in photophysics of Acr inside SDS micelles. Fluorescence of AcrH(+)* at 478 nm and its lifetime are quenched on addition of C, T, and U, while a concomitant increment of Acr* is observed only with C. However, the relative amplitude of Acr* increases with a simultaneous decrease in AcrH(+)* only with C. The fluorescence quenching of AcrH(+)* is explained by photoinduced electron transfer (PET), while changes in the relative contributions of Acr* and AcrH(+)* with C are due to associated excited-state proton transfer (ESPT). The rate of electron transfer (k(ET)) is maximum for T, followed by U and C. The associated ESPT from AcrH(+)* is the reason behind the reduced efficiency of PET with C. The lack of proton transfer with T and U as well as the higher k(ET) for T compared to U are explained by keto-enol tautomerization and subtle changes in the structure and geometry of the pyrimidine bases.
The Journal of Physical Chemistry B 07/2012; · 3.70 Impact Factor
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Journal of Photochemistry and Photobiology A Chemistry 07/2012; 240:66-74. · 2.42 Impact Factor
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ABSTRACT: The binding mode between oxyhemoglobin (oxyHb) and adenosine triphosphate (ATP) has been studied using absorption and fluorescence spectroscopy. OxyHb forms a ground state complex with ATP supported by five isosbestic points which appear in absorption spectra of oxyHb in buffer solution on addition of ATP. Moreover, the changes in absorption spectra suggest an oxidative interaction between the particular interacting systems. The binding constant has been determined from the quenching of fluorescence of oxyHb in the presence of a varied concentration of ATP, and that is 3.8 × 10(3) M(-1) at 25 °C. The negative changes in entropy and enthalpy indicate that the binding is enthalpy driven and the hydrogen bond and van der Waals (stacking) interactions play a major role. The oxygen affinity of oxyHb decreases with simultaneous formation of metHb in the presence of ATP. ATP-induced structural changes have been affirmed using both circular dichroism spectroscopy and synchronous fluorescence. A theoretical docking study gives the molecular details about the binding site of ATP in oxyHb.
The Journal of Physical Chemistry B 05/2012; 116(21):6150-7. · 3.70 Impact Factor
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ABSTRACT: Photophysical studies on binding interactions of a negatively charged anti-tumor photosensitizer, Merocyanine 540 (MC 540), with serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA), have been performed using absorption and steady-state as well as time-resolved fluorescence techniques. Formation of ground state complex has been confirmed from the detailed studies of absorption spectra of MC 540 in presence of SAs producing isosbestic points. Binding between the proteins and MC 540, which perturbs the existing equilibrium between the fluorescent monomer and its non-fluorescent dimer, induces a remarkable enhancement in fluorescence anisotropy and intensity of MC 540 along with a red shift of its maximum. The binding stoichiometry of MC 540 and SAs are more than 1.0 which depicts that two types of complexes, i.e., 1:1 and 2:1 are formed with addition of varied concentration of protein. Both the steady-state and time-resolved fluorescence results show that in 2:1 complex one of the MC 540 molecules is exposed towards aqueous environment with a greater extent when bound with HSA compared to BSA due to the structural flexibility of that protein. Thermodynamic analyses using van't Hoff plot indicate that the binding between MC 540 and individual SA is an entropy-driven phenomenon. The probable hydrophobic binding site has been located by denaturation of proteins, micropolarity measurement and Förster resonance energy transfer and that is further supported by molecular docking studies. Changes in circular dichroism spectra of BSA in presence of MC 540 depict secondary structural changes of the protein. The induced-CD shows that BSA due to its rigid structure generates chirality in MC 540 much more efficiently compared to HSA.
Journal of photochemistry and photobiology. B, Biology 03/2012; 108:23-33. · 1.87 Impact Factor
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ABSTRACT: It has been spectroscopically monitored that a mononuclear nickel(II)-Schiff base complex {[NiL]·CH(3)OH=NSC} exhibits greater binding affinity for bovine serum albumin (BSA) than that of its human counterpart (HSA). Moreover the modes of binding of NSC with the two serum albumins also differ significantly. Docking studies predict a relatively rare type of 'superficial binding' of NSC at domain IIB of HSA with certain mobility whereas for BSA such phenomena has not been detected. The mobile nature of NSC at domain IIB of HSA has been well correlated with the spectroscopic results. It is to be noted that thermodynamic parameters for the NSC interaction also differ for the two serum albumins. Occurrence of energy transfer between the donor (Trp of BSA and HSA) and acceptor (NSC) has been obtained by means of Förster resonance energy transfer (FRET). The protein stability on NSC binding has also been experimented by the GuHCl-induced protein unfolding studies. Interestingly it has been found that NSC-HSA interaction enhances the protein stability whereas NSC-BSA binding has no such impact. Such observations are indicative of the fact that the conformation of NSC is responsible in recognizing the two serum albumins and selectively enhancing protein stability.
Spectrochimica Acta Part A Molecular and Biomolecular Spectroscopy 02/2012; 92:164-74. · 2.10 Impact Factor
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ABSTRACT: The protonation dynamics of the DNA base adenine (Ade) and its nucleoside 2'-deoxyadenosine (d-Ade) are investigated by monitoring the deprotonation kinetics of an N-heterocyclic DNA intercalator, acridine (Acr), in the confined environment of sodium dodecyl sulfate (SDS) micelles. Protonation of acridine (AcrH(+)) occurs at the hydrophilic interface and this species remains in dynamic equilibrium with its deprotonated counterpart (Acr) inside the hydrophobic core of SDS micelles. Quenching of the fluorescence of AcrH(+)* at 478 nm is observed after addition of Ade and d-Ade with Stern-Volmer constant (K(SV)) 298 and 75 M(-1), respectively, with a concomitant increment in Acr* at 425 nm. Time-resolved fluorescence studies reveal quenching in the lifetime of AcrH(+)*. The relative amplitude of AcrH(+)* decreases from 0.97 to 0.51 and 0.97 to 0.89 with equimolar addition of Ade and d-Ade, respectively. These observations are explained by excited-state proton transfer (ESPT) from AcrH(+)* to the bases. The reduced K(SV) value and negligible change in the relative amplitudes of AcrH(+)* with d-Ade infer that ESPT is hindered substantially by the presence of a 2'-deoxy sugar unit. Transient time-resolved absorption spectra of Acr reflect that Ade reduces the absorbance of (3)AcrH(+)*; however, d-Ade keeps it unaltered for more than a time delay of 2 μs. The optimized geometries calculated by quantum chemical methods reflect deprotonation of AcrH(+)* with protonation at the N1 position of Ade, while it remains protonated with d-Ade. The hindered ESPT between AcrH(+)* and d-Ade singles out the significance of the 2'-deoxy sugar moiety in controlling the deprotonation kinetics.
ChemPhysChem 02/2012; 13(2):525-34. · 3.41 Impact Factor
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ABSTRACT: Photophysical studies on binding interactions of a negatively charged anti-tumor photosensitizer, Merocyanine 540 (MC 540), with serum proteins, bovine serum albumin (BSA) and human serum albumin (HSA), have been performed using absorption and steady-state as well as time-resolved fluorescence techniques. Formation of ground state complex has been confirmed from the detailed studies of absorption spectra of MC 540 in presence of SAs producing isosbestic points. Binding between the proteins and MC 540, which perturbs the existing equilibrium between the fluorescent monomer and its non-fluorescent dimer, induces a remarkable enhancement in fluorescence anisotropy and intensity of MC 540 along with a red shift of its maximum. The binding stoichiometry of MC 540 and SAs are more than 1.0 which depicts that two types of complexes, i.e., 1:1 and 2:1 are formed with addition of varied concentration of protein. Both the steady-state and time-resolved fluorescence results show that in 2:1 complex one of the MC 540 molecules is exposed towards aqueous environment with a greater extent when bound with HSA compared to BSA due to the structural flexibility of that protein. Thermodynamic analyses using van’t Hoff plot indicate that the binding between MC 540 and individual SA is an entropy-driven phenomenon. The probable hydrophobic binding site has been located by denaturation of proteins, micropolarity measurement and Förster resonance energy transfer and that is further supported by molecular docking studies. Changes in circular dichroism spectra of BSA in presence of MC 540 depict secondary structural changes of the protein. The induced-CD shows that BSA due to its rigid structure generates chirality in MC 540 much more efficiently compared to HSA.
Journal of Photochemistry and Photobiology B: Biology. 01/2012; 108:23-33.
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ABSTRACT: The photophysical behavior of acridine (Acr) shows a facilitated water assisted protonation equilibrium between its deprotonated (Acr* ∼ 3.4 ns) and protonated forms (AcrH(+)* ∼ 33 ns) within a confined environment of sodium dodecyl sulphate (SDS) micelles above the critical micellar concentration of 8 mM. The acidic interface of the micelles is capable of protonating Acr whereas deprotonated Acr is partitioned into the hydrophobic core. The time-resolved-area-normalized-emission spectra confirm the presence of both Acr* and AcrH(+)*, while time-resolved-emission spectra depict time evolution between them. Quenching of AcrH(+)* with triethylamine (TEA) results in a linear Stern-Volmer (S-V) plot, whereas non-linearity arises with N,N-dimethylaniline (DMA). Both steady-state and time-resolved quenching results with TEA are explained on the basis of excited state proton transfer (ESPT), however the reasons behind the quenching of excited Acr with DMA are proposed as ESPT followed by a photoinduced electron transfer. Partitioning of DMA at the interface makes it accessible for both Acr* and AcrH(+)* in hydrophobic and hydrophilic regions of micelles respectively. The rate of electron transfer at the interface is found to be slower compared to that in the hydrophobic core. Characterization of transient intermediates formed during ESPT and PET between Acr and amines by laser-flash photolysis also supports the observation obtained during fluorescence studies. The mode of interactions between Acr and amines inside micelles is controlled by the localization of the proton/electron donors and acceptors in different hydrophobic or hydrophilic regions of such nano-confined environments.
Physical Chemistry Chemical Physics 08/2011; 13(37):16821-30. · 3.57 Impact Factor
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Chemical Physics Letters 03/2011; · 2.34 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry A 01/2011; 115(2):128-35. · 2.95 Impact Factor
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ABSTRACT: Conventional spectroscopic tools such as absorption, fluorescence, and circular dichroism spectroscopy used in the study of photoinduced drug-protein interactions can yield useful information about ground-state and excited-state phenomena. However, photoinduced electron transfer (PET) may be a possible phenomenon in the drug-protein interaction, which may go unnoticed if only conventional spectroscopic observations are taken into account. Laser flash photolysis coupled with an external magnetic field can be utilized to confirm the occurrence of PET and authenticate the spin states of the radicals/radical ions formed. In the study of interaction of the model protein human serum albumin (HSA) with acridine derivatives, acridine yellow (AY) and proflavin (PF(+)), conventional spectroscopic tools along with docking study have been used to decipher the binding mechanism, and laser flash photolysis technique with an associated magnetic field (MF) has been used to explore PET. The results of fluorescence study indicate that fluorescence resonance energy transfer takes place from the protein to the acridine-based drugs. Docking study unveils the crucial role of Ser 232 residue of HSA in explaining the differential behavior of the two drugs towards the model protein. Laser flash photolysis experiments help to identify the radicals/radical ions formed in the due course of PET (PF(•), AY(•-), TrpH(•+), Trp(•)), and the application of an external MF has been used to characterize their initial spin-state. Owing to its distance dependence, MF effect gives an idea about the proximity of the radicals/radical ions during interaction in the system and also helps to elucidate the reaction mechanisms. A prominent MF effect is observed in homogeneous buffer medium owing to the pseudoconfinement of the radicals/radical ions provided by the complex structure of the protein.
The Journal of Physical Chemistry A 12/2010; 114(51):13313-25. · 2.95 Impact Factor
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ABSTRACT: The carcinogenic drug 4-nitroquinoline-1-oxide (4NQO) has been found to bind with the protein hen egg white lysozyme as evident from fluorescence quenching experiments. The binding constant and stoichiometry have been determined. The values of the thermodynamic parameters indicate that the interaction is an enthalpy-driven spontaneous phenomenon. The experimental value of change in free energy is similar to that obtained from the docking study. The far UV circular dichroism spectra show some changes in the secondary structure of protein. The high value of bimolecular quenching constant leads to the possibility of Förster resonance energy transfer (FRET). Along with FRET, the photoinduced electron transfer (PET) from tryptophan residue of protein to 4NQO has also been evident from the transient absorption spectra obtained in laser flash photolysis experiments. The simultaneous occurrence of FRET and PET is the key factor for quenching of intrinsic fluorescence of the protein as it binds with the drug.
Photochemistry and Photobiology 09/2010; 86(6):1237-46. · 2.41 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry B 07/2009; 113(25):8689-94. · 3.70 Impact Factor
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ABSTRACT: Laser flash photolysis and an external magnetic field have been used for the study of the interaction of two quinone molecules, namely, 9,10-anthraquinone (AQ) and 2-methyl 1,4-naphthoquinone (or menadione, MQ) with a DNA base, cytosine (C) and its nucleoside cytidine (dC) in two media, a homogeneous one composed of acetonitrile/water (ACN/H(2)O, 9:1, v/v) and a SDS micellar heterogeneous one. We have applied an external magnetic field for the proper identification of the transients formed during the interactions in micellar media. Cytosine exhibits electron transfer (ET) followed by hydrogen abstraction (HA) while dC reveals a reduced ET compared to C, with both quinones in organic homogeneous medium (ACN/H(2)O). Due to a higher electron affinity, AQ supports more faciler ET than MQ with dC in ACN/H(2)O but observations in SDS have been just the reverse. In SDS, ET from dC is completely quenched and a dominant HA is all that could be discerned. This work reveals two main findings: first, a drop in ET on addition of a ribose unit to C, which has been attributed to a role of keto-enol tautomerism in inducing ET from electron-rich nucleus and second, the effect of medium in controlling reaction mechanism by favoring HA with AQ although it is intrinsically more prone towards ET.
Biophysical chemistry 01/2009; 140(1-3):62-8. · 2.28 Impact Factor
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ABSTRACT: 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 the RNA base uracil (U) and two of its derivatives, 1,3-dimethyluracil (dmU) and uridine (dU). We have conducted our studies in homogeneous organic and heterogeneous micellar media in order to investigate the effect of media on the molecules and any change in reactivity on account of substitution. In organic homogeneous medium, both the quinones have behaved similarly with the bases. Here U has undergone both electron transfer (ET) and hydrogen (H) transfer, while dU and dmU have failed to exhibit any ET. Failure to support ET has been attributed to keto-enol tautomerism, which has been found to have a significant role in determining the occurrence of ET from these pyrimidine bases. However, in SDS micelles some variations regarding the reactivity of these molecules have been discerned. The variations are 2-fold. Here ET from U has been found to get completely eclipsed by a dominant H abstraction with both the quinones, and AQ reveals a difference in the extent of H abstraction with the bases in SDS. With U and dU, the prevailing H abstraction with AQ has succeeded in formation of only AQH(*), while dmU has produced both AQH(*) and AQH(2), the latter being formed by two successive H abstraction. Explanations of this intriguing behavior with U and its derivatives with quinone molecules have been the main concern in this work.
The Journal of Physical Chemistry A 12/2008; 112(47):12045-53. · 2.95 Impact Factor
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ABSTRACT: Laser flash photolysis has been used for the study of the interaction of 9,10-anthraquinone (AQ) with the DNA base, adenine (A) and its corresponding nucleoside, 2'-deoxyadenosine (dA). This study has provided two very important observations. AQ has been found to support electron transfer in different categories of media, acetonitrile/water on one hand and SDS micelles on other. While in our earlier work 2-methyl 1,4-naphthoquinone was found to undergo a switchover in reactivity (J. Am. Chem. Soc. 126 (2004) 10589-10593). Again A and dA are found to behave differently on account of an extra sugar unit, which not only affects the rate of reaction but the reaction pathway has been found to be modified too.
Biophysical Chemistry 08/2008; 136(1):59-65. · 2.20 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry A 07/2008; 112(22):4914-20. · 2.95 Impact Factor
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ABSTRACT: 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.
The Journal of Physical Chemistry A 06/2008; 112(17):3943-6. · 2.95 Impact Factor
