Interaction of 7-hydroxy-8-(phenylazo)1,3-naphthalenedisulfonate with bovine plasma albumin. Spectroscopic studies

Correlating enzyme density, conformation and activity on nanoparticle surfaces in highly functional bio-nanocomposites

The biological activity of the immobilized enzyme is crucial for the performance of different nanoparticle mediated enzymatic assays, where enzymatic conversion can be used for label-free analyte detection. In this article we have addressed two significant aspects of enzyme-nanoparticle interactions. First, we have developed copper sulfide (CuS) nanoparticles with an average diameter of 25 nm as a potential enzyme-interface using trypsin protease as a model enzyme. CuS nanoparticles showed high trypsin immobilization capacity of about 14.0 mg m(-2) with the significant retention of native enzymatic activity (75-98%) at room temperature, even beyond the calculated tightly packed monolayer coverage (which is around 4.1 mg m(-2)). Second, we report a quantitative correlation between the structure-functional relationship and the density of immobilized trypsin on a nanoparticle surface. The in situ conformation of immobilized trypsin could be efficiently analyzed by fluorescence, circular dichroism and FT-IR spectroscopic measurements because of the small size of the nanoparticles. Trypsin molecules appear to retain their close-native tertiary and secondary structural features (with a small loss of 1-2% of helical content) in the entire surface density range (2.0-14.0 mg m(-2)) on the CuS nanoparticles. However, interestingly, at a low surface coverage (2.0 mg m(-2)), immobilized trypsin retains almost 98% of its native enzymatic activity, leading to a highly functional bio-nanocomposite. However, at higher surface coverages, the enzyme activity decreases to 77%, indicating the influence of steric crowding. Furthermore, the high functionality of the immobilized trypsin at low surface density on CuS nanoparticle was also confirmed by determining the kinetic parameters of enzymatic activity.

Antihypertensive activity of a quinoline appended chalcone derivative and its site specific binding interaction with a relevant target carrier protein

The usefulness of heterocyclic chalcone derivative as a therapeutic target in controlling hypertension and its site specific binding interaction with model transport protein to get a clear picture about its delivery mechanism.

Role of the flavan-3-ol and galloyl moieties in the interaction of (-)-epigallocatechin gallate with serum albumin

The principal green tea polyphenol, (-)-epigallocatechin-3-O-gallate (EGCg), may provide chemoprotection against conditions ranging from cardiovascular disease to cancer. Binding to plasma proteins stabilizes EGCg during its transport to targeted tissues. This study explored the details EGCg binding to bovine serum albumin. Both fluorescence lifetime and intensity data showed that the hydrophobic pocket between subdomains IIA and IIIA is the binding site for EGCg. Fluorescence and circular dichroism were used to establish the roles of the flavan-3-ol and galloyl moieties of the EGCg in binding and to demonstrate a binding-dependent conformational change in the protein. Competitive binding experiments confirmed the location of binding, and molecular modeling identified protein residues that play key roles in the interaction. This model of EGCg-BSA interactions improves the understanding of the likely physiological fate of this green tea-derived bioactive polyphenol.

Modulation of accessibility of subdomain IB in the pH-dependent interaction of bovine serum albumin with Cochineal Red A: a combined view from spectroscopy and docking simulations

Our recent report on the binding of Cochineal Red A, a food dye, with HSA and BSA at pH 7.4 has revealed that electrostatic forces is the principal cause of interaction. In that study issues relating to complications arising out of modulation of dye binding affinity of BSA with pH had not been explored. Here we have further explored the interaction of Cochineal Red A with BSA in pH range 4.8-7.8. Surprisingly, this system behaves differently in the texture of interaction pattern at two extremes of studied pH range, unlike HSA. Importantly, the charge on the amino acid side chains in the binding pocket is likely to play a significant role.

Interaction of multimicrobial synthetic inhibitor 1,2-bis(2-benzimidazolyl)-1,2-ethanediol with serum albumin: spectroscopic and computational studies

The molecule, 1,2-Bis(2-benzimidazolyl)-1,2-ethanediol (BBE) is known to act as a selective inhibitor of poliovirus, rhinovirus, Candida albicans, several bacterial species, and is easily synthesized by Phillips reaction. The interaction of BBE with BSA and the effects of its binding on the conformation and unfolding/refolding pathways of the protein were investigated using multispectroscopic techniques and molecular modeling. The binding studies indicate that BSA has one high affinity BBE binding site with association constant 6.02±0.05×10(4) M(-1) at 298 K. By measuring binding at different temperatures, we determined the changes in enthalpy (ΔH = -15.13±2.15 kJ mol(-1)), entropy (ΔS = 40.87±7.25 J mol(-1) K(-1)) and free energy (ΔG( = )26.78±1.02) of interaction, which indicate that the binding was spontaneous and both enthalpically and entropically driven. Based on molecular modeling and thermodynamic parameters, we proposed that the complex formation involved mainly hydrophilic interaction such as hydrogen bonding between hydroxyl groups of ethane-1,2-diol fragment with Tyr410 and benzimidazole sp(2) nitrogen atom with Ser488 and hydrophobic interaction between phenyl ring of one benzimidazole of the ligand and hydrophobic residues namely, Ile387, Cys391, Phe402, Val432 and Cys437. The sequential unfolding mechanism of BSA, site-specific marker displacement experiments and molecular modeling showed that the molecule preferably binds in subdomain IIIA. The BBE binding to BSA was found to cause both secondary and tertiary structural alterations in the protein as studied by intrinsic fluorescence, near-UV and far-UV circular dichroism results. The unfolding/refolding study showed that BBE stabilized native to intermediate states (N⇌I) transition of the protein by ∼2 kJ mol(-1) without affecting the intermediate to unfolded states (I⇌U) transition and general mechanism of unfolding of BSA.

Optical spectroscopic exploration of binding of Cochineal Red A with two homologous serum albumins

Cochineal Red A is a negatively charged synthetic azo food colorant and a potential carcinogen. We present here the study of binding of Cochineal Red A with two homologous serum albumins, human (HSA) and bovine (BSA), in aqueous pH 7.4 buffer by optical spectroscopic techniques. Protein intrinsic fluorescence quenching by Cochineal Red A occurs through ground-state static interaction and its binding with BSA is stronger than with HSA. The magnitudes of thermodynamic parameters suggest that dye binding occurs principally via electrostatic complexation. Site-marker competitive binding shows that Cochineal Red A binds primarily to site I of serum albumins. Circular dichroic spectra indicate that dye binding results in some conformational modification of serum albumins. Increased ionic strength of the medium results in lowering of binding. This study provides an important insight into possible means of removal of dye toxicity.

Misconceptions over Förster resonance energy transfer between proteins and ANS/bis-ANS: Direct excitation dominates dye fluorescence

Our aim was to disprove the widespread misconception that Förster resonance energy transfer (FRET) is the only explanation for observing fluorescence from ANS (8-anilino-1-naphthalenesulfonic acid) and bis-ANS (4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonic acid, dipotassium salt) following excitation at 280nm in the presence of protein. From ultraviolet (UV) absorption spectra and fluorescence emission spectra of bis-ANS and ANS in buffer and ethanol, direct excitation at 280nm was found to be the dominant mechanism for the resulting dye fluorescence. Furthermore, Tyr/Trp quenching studies were performed for solutions of N-acetyl-l-tryptophanamide, heat-stressed immunoglobulin G (IgG), and bovine serum albumin (BSA) by monitoring changes in steady state fluorescence spectra and time-resolved fluorescence decays as a function of dye concentration. Stronger quenching of the intrinsic BSA and IgG fluorescence in steady state than in time-resolved fluorescence by bis-ANS and ANS pointed toward static quenching being the dominant mechanism in addition to dynamic quenching and/or FRET. In conclusion, one should consider the role of direct excitation of ANS and bis-ANS at 280nm to ensure a proper interpretation of fluorescence signals resulting from dye-protein interactions. When ANS or bis-ANS is to be used for protein characterization, we recommend selectively exciting the dyes at the higher absorption wavelength maximum (370 or 385nm, respectively).

Photoinductive activity of humic acid fractions with the presence of Fe(III): the role of aromaticity and oxygen groups involved in fractions

Relationship between the photoinductive activity and the properties of humic acids (HA) fractions were investigated with and without Fe(III). Three fractions were separated based on the molecular weight (M(w)) and were obtained following the order of M(w): F(A)>F(B)>F(C). Compared to F(A) and F(B), photodegradation of atrazine under simulated sunlight was much faster in solution containing F(C), whose structure was dominated by greater aromaticity, more oxygen groups and fluorophores. The interaction of HA fractions and Fe(III) was studied using fluorescence spectrometry and F(C) had the largest quenching constant. The capacity of electron transfer, estimated from the amount of photoformed Fe(II), was also highest for F(C). Thus, the Fe(III)-F(C) complex was efficient in phototransformation of atrazine in nearly neutral aqueous solutions. These results suggest that the aromaticity and oxygen groups content of HA exert great influence on the binding ability of metals and on the fate of pollutants in natural waters.

Atrazine photodegradation in aqueous solution induced by interaction of humic acids and iron: photoformation of iron(II) and hydrogen peroxide

The photochemical formation of Fe(II) and hydrogen peroxide (H 2O 2) coupled with humic acids (HA) was studied to understand the significance of iron cycling in the photodegradation of atrazine under simulated sunlight. The presence of HA significantly enhanced the formation of Fe(II) and H 2O 2, and their subsequent product, hydroxyl radical ( (*)OH), was the main oxidant responsible for the atrazine photodegradation. During 60 h of irradiation, the fraction of iron presented as Fe(II) (Fe(II)/Fe(t)) decreased from 20-32% in the presence of the Fe(III)-HA complex to 10-22% after adding atrazine. The rate of atrazine photodegradation in solutions containing Fe(III) increased with increasing HA concentration, suggesting that the complexation of Fe(III) with HA accelerated the Fe(III)/Fe(II) cycling. Using fluorescence spectrometry, the quenching constant and the percentage of fluorophores participating in the complexation of HA with Fe(III) were estimated by the modified Stern-Volmer equation. Fourier transform infrared spectroscopy (FTIR) offered the direct evidence that Fe(III)-carboxylate complex could be formed by ligand exchange of HA with Fe(III). Based on all the information, a possible reaction mechanism was proposed.

Binding of bioactive phytochemical piperine with human serum albumin: A spectrofluorometric study

Piperine, the bioactive alkaloid compound of the spice black pepper (Piper nigrum) exhibits a wide range of beneficial physiological and pharmacological activities. Being essentially water-insoluble, piperine is presumed to be assisted by serum albumin for its transport in blood. In this study, the binding of piperine to serum albumin was examined by employing steady state and time resolved fluorescence techniques. Binding constant for the interaction of piperine with human serum albumin, which was invariant with temperature in the range of 17-47 degrees C, was found to be 0.5 x 10(5)M(-1), having stoichiometry of 1:1. At 27 degrees C, the van't Hoff enthalpy DeltaH degrees was zero; DeltaS degrees and DeltaG degrees were found to be 21.4 cal mol(-1) K(-1) and -6.42 kcal mol(-1). The binding constant increased with the increase of ionic strength from 0.1 to 1.0M of sodium chloride. The decrease of Stern-Volmer constant with increase of temperature suggested that the fluorescence quenching is static. Piperine fluorescence showed a blue shift upon binding to serum albumin, which reverted with the addition of ligands -triiodobenzoic acid and hemin. The distance between piperine and tryptophan after binding was found to be 2.79 nm by Förster type resonance energy transfer calculations. The steady state and time resolved fluorescence measurements suggest the binding of piperine to the subdomain IB of serum albumin. These observations are significant in understanding the transport of piperine in blood under physiological conditions.