Applications of Functionalized Carbon-Based Quantum Dots in Fluorescence Sensing of Iron(III).

Iron, an essential trace element exhibits detrimental effects on human health when present at higher or lower concentration than the required. Therefore, there is a pressing demand for sensitive and selective detection of Fe3+ in water, food etc. Unfortunately, in several instances, the traditional approaches suffer from a number of shortcomings like complicated procedures, limited sensitivity, poor selectivity and more expensive and time consuming. The scope of optical tuning and excellent photophysical properties of carbon- based nanomaterials like carbon dots (C-dots) and graphene dots (g-dots) have made them promising optical sensors of metal ions. Moreover, high surface area, superior stability of such materials contributes towards the fruitful development of sensors. The present review offered critical information on the fabrication and fluorimetric applications of these functional nanomaterials for sensitive and selective detection of Fe3+. An in-depth discussion on fluorescent C-dots made from naturally occurring materials and chemical techniques were presented. Effect of doping in C-dots was also highlighted in terms of improved fluorescence response and selectivity. In a similar approach g-dots were also discussed. Many of these sensors exhibited great selectivity, superior sensitivity, high quantum yield, robust chemical and photochemical stability and real-time applicability. Further improvement in these factors can be targeted to develop new sensors.

Multispectroscopic studies on the molecular interactions between bovine γ-globulin and borohydride-capped silver nanoparticles.

Interactions between bovine γ-globulin (BGG) and borohydride-capped silver nanoparticles (BAgNPs) were studied using dynamic light scattering (DLS) and spectroscopic techniques such as UV-vis spectroscopy, fluorescence, and circular dichroism. The results were compared with earlier reported interactions between γ-globulin and citrate-coated AgNPs (CAgNPs). BAgNPs were synthesized and characterized. Irrespective of the coating on AgNPs, nanoparticles had formed ground-state complexes with the protein. CAgNPs, as well as BAgNPs had caused static quenching of tryptophan (Trp) fluorescence of the protein. The change in the capping agent from citrate to borohydride weakened the binding of nanoparticles with the protein. But the same change in capping agent had increased the fluorescence quenching efficiency of AgNPs. Hydrogen bonding and van der Waals interactions were involved in BGG-BAgNPs complex similar to the CAgNPs complex with γ-globulin. Polarity of the Trp microenvironment in BGG was not altered using BAgNPs as opposed to CAgNPs, as supported using synchronous and three-dimensional fluorescence. Resonance light scattering experiments also suggested nano-bio conjugation. Far-UV and near-UV circular dichroism (CD) spectra respectively pointed towards changes in the secondary and tertiary structure of BGG by BAgNPs, which was not observed for CAgNPs.

Recent advances (2017-20) inthe detection of copper ion by using fluorescence sensors working through transfer of photo-induced electron (PET), excited-state intramolecular proton (ESIPT) and Förster resonance energy (FRET).

An essential trace element copper plays several physiological roles in living systems. But at excess concentration, it exerts toxicity and becomes associated with numerous disorders. In this article, we have reviewed the recent developments (from 2017 to 2020) in the field of fluorescence-based chemosensors for the detection of Cu2+ ion. The sensing probes which were built to work through transfer of photo-induced electron (PET), excited-state intramolecular proton (ESIPT) and Förster resonance energy (FRET) mechanisms have been included in this review. Emphasis is given on the design, sensitivity and response of the probe molecules for the detection of Cu2+ ion. Using suitable examples, applications of these three recognition mechanisms for the probing of copper ion have been addressed.

Elucidation of molecular interactions between human γD-crystallin and quercetin, an inhibitor against tryptophan oxidation.

Different post-translational changes in eye lens crystallin proteins contribute towards the development of cataract. We have studied in vitro oxidative modification of tryptophan (Trp) residues of human γD-crystallin (HGD) towards formation of N-formylkynurenine (NFK) associated with cataractogenesis. This oxidation was found to be inhibited by quercetin at relatively low concentration. Interactions between quercetin and HGD were further studied using fluorescence techniques. Binding and quenching constants were determined as ∼104 M-1. Static quenching of fluorescence due to HGD-quercetin complex formation at ground state was confirmed by finding excited state life time of Trp residues. Energy transfer occurred between the protein and quercetin. Hydrogen bonding and/or van der Waals interactions were involved between HGD and quercetin. Synchronous and three-dimensional fluorescence along with far-UV CD studies suggested no major conformational alterations occurred in HGD due to quercetin binding. Experimental observations were supported by the docking results.Communicated by Ramaswamy H. Sarma.

Inhibition of fibrillation of human γd-crystallin by a flavonoid morin.

To inhibit the formation of amyloid fibrils by human γd-crystallin (HGD), a series of four flavonoids (quercertin, rutin, morin and hesperetin) was tested. Only morin had demonstrated significant inhibition of HGD fibrillation. Results from fluorimetric assay techniques (using thioflavin T and ANS), FTIR, circular dichroism and microscopic imaging (fluorescence microscopy and transmission electron microscopy) confirmed HGD fibrillation inhibition by morin. HGD-morin complex formation at ground state resulted tryptophan fluorescence quenching through static mechanism, which was also confirmed by determining the excited-state life time of HGD tryptophan residues. Förster resonance energy transfer occurs from HGD to morin. Synchronous, three-dimensional fluorescence, FTIR and circular dichroism results suggest that major changes in HGD conformation did not occur on binding with morin. The interactions between HGD and morin involve hydrogen bonding and/or van der Waals forces. Docking predictions also support experimental results.Communicated by Ramaswamy H. Sarma.

2-Aryl benzazole derived new class of anti-tubercular compounds: Endowed to eradicate mycobacterium tuberculosis in replicating and non-replicating forms.

The high mortality rate and the increasing prevalence of Mtb resistance are the major concerns for the Tuberculosis (TB) treatment in this century. To counteract the prevalence of Mtb resistance, we have synthesized 2-aryl benzazole based dual targeted molecules. Compound 9m and 9n were found to be equally active against replicating and non-replicating form of Mtb (MIC(MABA) 1.98 and 1.66 µg/ml; MIC(LORA) 2.06 and 1.59 µg/ml respectively). They arrested the cell division (replicating Mtb) by inhibiting the GTPase activity of FtsZ with IC50 values 45 and 64 µM respectively. They were also capable of kill Mtb in non-replicating form by inhibiting the biosynthesis of menaquinone which was substantiated by the MenG inhibition (IC50 = 11.62 and 7.49 µM respectively) followed by the Vit-K2 rescue study and ATP production assay.

Inhibition of amyloid fibrillation of human γD-crystallin by gold nanoparticles: Studies at molecular level.

The capability of citrate-stabilized gold nanoparticles (AuNps) has been explored for the inhibition of amyloid fibrillation of human γD-crystallin (HGD), a major protein of eye lens. Citrate-capped AuNps were synthesized, characterized and used further for amyloid inhibition. The results from intrinsic and extrinsic (in the presence of Thioflavin T and ANS) fluorescence based assays and CD spectroscopy clearly suggest that AuNps at nanomolar concentrations can act as an effective inhibitor against fibrillation of HGD. Fluorescence microscopic and transmission electron microscopic images also supported this observation. Considering the inhibitory role of AuNps against HGD fibrillation, interactions between HGD and AuNps were studied to decipher the mechanism of amyloid inhibition. The binding and quenching constants were calculated as ~109 M-1 using the data of tryptophan fluorescence quenching of HGD by AuNps. Ground state complexation between the protein and nanoparticles was predicted. AuNps were not found to cause any major conformational changes in the native protein. Entropy-driven complexation process between the protein and nanoparticles indicates the interactions of AuNps with hydrophobic residues of HGD. Therefore, in the presence of AuNps, the exposure of the hydrophobic patches of HGD during its partial unfolding became restricted, which results inhibition in HGD fibrillation.

Studies on Molecular Interactions between Bovine ß-Lactoglobulin and Silver Nanoparticles.

BACKGROUND: Silver Nanoparticles (AgNPs) were found to modulate the fibrillation of Bovine Β-Lactoglobulin (BLG). OBJECTIVE: To gain an insight regarding the mechanism of BLG aggregation modulation by AgNPs at molecular level, studies on the interactions between BLG and AgNPs were carried out. METHODS: Protein-ligand interactions were studied based on Trp fluorescence quenching (at four different temperatures), synchronous and three-dimensional fluorescence and circular dichroism spectroscopy (far-UV and near-UV). RESULTS: Protein-nanoparticles association constant was in the range of 106 -1010 M-1 and the quenching constant was determined as ~107 M-1. Ground state complexation between the protein and nanoparticles was predicted. Change in polarity surrounding the Trp residue was not detected by synchronous and three-dimensional fluorescence spectroscopy. AgNPs caused a global change in the secondary and tertiary structure of the protein as revealed from far-UV and near-UV CD spectroscopy. Enthalpy driven complexation between the protein and nanoparticles indicates the involvement of hydrogen bonding and/or van der Waals interactions. CONCLUSION: Modulation of BLG aggregation by AgNPs is due to strong binding of the nanoparticles with BLG, which also causes structural perturbations of the protein.

Inhibition of copper-induced aggregation of human γD-crystallin by a diimine molecule and investigations on their molecular interactions.

Communicated by Ramaswamy H. Sarma.

Protective role of hesperetin against posttranslational oxidation of tryptophan residue of human γD-crystallin: A molecular level study.

Crystallin proteins undergo various posttranslational modifications with aging of eye lens. Oxidation of tryptophan (Trp) residues of a major γ-crystallin namely human γD-crystallin (HGD) was found to be inhibited by a naturally occurring flavonoid hesperetin at relatively low concentration mostly due to its antioxidant activity. Further the molecular interactions between HGD and hesperetin were elucidated on the basis of the quenching of Trp fluorescence of the protein by the flavonoid. Ground state complexation between HGD and hesperetin caused static quenching of the Trp fluorescence of HGD. Binding and quenching constants were in the order of (103- 104 M-1). Energy transfer from protein to hesperetin was suggested by FRET calculations. Thermodynamic parameters reveal significant hydrophobic association between the protein and hesperetin. Synchronous fluorescence and CD spectroscopic results had ruled out conformational changes in the protein due to binding of hesperetin. Docking studies suggested the proximity of hesperetin with Trp 42, which largely corroborates our experimental findings.

Inhibition of Amyloid Fibrillation by Small Molecules and Nanomaterials: Strategic Development of Pharmaceuticals Against Amyloidosis.

Amyloid fibrils are a special class of self-assembled protein molecules, which exhibit various toxic effects in cells. Different physiological disorders such as Alzheimer's, Parkinson's, Huntington's diseases, etc. happen due to amyloid formation and lack of proper cellular mechanism for the removal of fibrils. Therefore, inhibition of amyloid fibrillation will find immense applications to combat the diseases associated with amyloidosis. The development of therapeutics against amyloidosis is definitely challenging and numerous strategies have been followed to find out anti-amyloidogenic molecules. Inhibition of amyloid aggregation of proteins can be achieved either by stabilizing the native conformation or by decreasing the chances of assembly formation by the unfolded/misfolded structures. Various small molecules such as naturally occurring polyphenols, flavonoids, small organic molecules, surfactants, dyes, chaperones, etc. have demonstrated their capability to interrupt the amyloid fibrillation of proteins. In addition to that, in last few years, different nanomaterials were evolved as effective therapeutic inhibitors against amyloidosis. Aromatic and hydrophobic interactions between the partially unfolded protein molecules and the inhibitors had been pointed as a general mechanism for inhibition. In this review article, we are presenting an overview on the inhibition of amyloidosis by using different small molecules (both natural and synthetic origin) as well as nanomaterials for development of pharmaceutical strategies against amyloid diseases.

Crystallins and Their Complexes.

The crystallins (α, ß and γ), major constituent proteins of eye lens fiber cells play their critical role in maintaining the transparency and refractive index of the lens. Under different stress factors and with aging, ß- and γ-crystallins start to unfold partially leading to their aggregation. Protein aggregation in lens basically enhances light scattering and causes the vision problem, commonly known as cataract. α-crystallin as a molecular chaperone forms complexes with its substrates (ß- and γ-crystallins) to prevent such aggregation. In this chapter, the structural features of ß- and γ-crystallins have been discussed. Detailed structural information linked with the high stability of γC-, γD- and γS-crystallins have been incorporated. The nature of homologous and heterologous interactions among crystallins has been deciphered, which are involved in their molecular association and complex formation.

Inhibition of amyloid fibrillation and destabilization of fibrils of human γD-crystallin by direct red 80 and orange G.

Inhibition of amyloid fibril formation by a lens protein namely human γD-crystallin (HGD) under stressful conditions was targeted by using some small molecules like direct red 80 (DR), orange G (OG) and rhodamine B (RH). The protein itself was found to form matured fibrils after 48h of incubation at pH 3.0 at 37°C. Various fluorescence based assays (thioflavin T assay, ANS binding assay, intrinsic Trp fluorescence determination), circular dichroism and microscopic imaging techniques were used in the inhibition studies. Above studies unequivocally proved that DR had acted as the most potent inhibitor among these molecules and it was little better efficient than OG. RH had shown a moderate inhibition of HGD fibrillation. Microscopic images from fluorescence microscopy and transmission electron microscopy also substantiated our spectroscopic observations. These small molecules were not only capable to restrict the fibrillation, but they were also able to disassemble the mature and premature fibrils of HGD. Hydrophobic and aromatic interactions between the inhibitor molecules and partially unfolded HGD are likely to be responsible for exhibiting inhibition of protein fibrillation.

Inhibition of copper-mediated aggregation of human γD-crystallin by Schiff bases.

Protein aggregation, due to the imbalance in the concentration of Cu2+ and Zn2+ ions is found to be allied with various physiological disorders. Copper is known to promote the oxidative damage of ß/γ-crystallins in aged eye lens and causes their aggregation leading to cataract. Therefore, synthesis of a small-molecule 'chelator' for Cu2+ with complementary antioxidant effect will find potential applications against aggregation of ß/γ-crystallins. In this paper, we have reported the synthesis of different Schiff bases and studied their Cu2+ complexation ability (using UV-Vis, FT-IR and ESI-MS) and antioxidant activity. Further based on their copper complexation efficiency, Schiff bases were used to inhibit Cu2+-mediated aggregation of recombinant human γD-crystallin (HGD) and ß/γ-crystallins (isolated from cataractous human eye lens). Among these synthesized molecules, compound 8 at a concentration of 100 µM had shown ~95% inhibition of copper (100 µM)-induced aggregation. Compound 8 also showed a positive cooperative effect at a concentration of 5-15 µM on the inhibitory activity of human αA-crystallin (HAA) during Cu2+-induced aggregation of HGD. It eventually inhibited the aggregation process by additional ~20%. However, ~50% inhibition of copper-mediated aggregation of ß/γ-crystallins (isolated from cataractous human eye lens) was recorded by compound 8 (100 µM). Although the reductive aminated products of the imines showed better antioxidant activity due to their lower copper complexing ability, they were found to be non-effective against Cu2+-mediated aggregation of HGD.

Interaction of different prototropic species of an anticancer drug ellipticine with HSA and IgG proteins: multispectroscopic and molecular modeling studies.

Studies on interactions between an anticancer alkaloid, ellipticine, and various carrier proteins in blood serum show tangible results to gain insight into the solubility and transport of the drug under physiological conditions. In this report, we extensively studied the interactions of different prototropic species of ellipticine with two prominent serum proteins namely human serum albumin (HSA) and immunoglobulin G (IgG) in their native and partially unfolded states using steady state and time resolved fluorescence spectroscopy, molecular docking and circular dichroism (CD). Both the fluorescence techniques and molecular modeling studies elucidate that only neutral species of ellipticine binds to HSA in the sudlow site II. Unlike HSA, IgG in the native state mostly binds to cationic species of ellipticine. However, in partially unfolded configuration, IgG binds to the neutral ellipticine molecules. Molecular docking studies indicate the prevalence of electrostatic interactions involving charged residues in the binding process of cationic species of ellipticine with native IgG in its Fab region. In native conformation, the hydrophobic residues of the Fab region are found to be buried completely by the ligand. This implies that the hydrophobic interaction will be favored by unfolding of IgG through which the hydrophobic pocket will be more accessible to neutral species of ellipticine. The circular dichroism measurements reveal that upon interaction with ellipticine, heat and acid treated HSA resumes its α-helical content. This conclusive comparative study on interactions of IgG and HSA with ellipticine yields the result that native HSA is responsible for transport of neutral species of ellipticine whereas IgG carries cationic ellipticine in its native form.

Involvement of non-polyalanine (polyA) residues in aggregation of polyA proteins: Clue for inhibition of aggregation.

Presence of polyalanine (polyA) stretches in some proteins is found to be associated with their aggregation, which causes disorders in various developmental processes. In this work, inherent propensities towards aggregation of some residues, which are not part of the polyA stretches, have been identified by using the primary sequences of seven polyA proteins with the help of Betascan, PASTA and Tango programs and explored unambiguously. This provides a basis for proposing molecular mechanism of this type of aggregation. Reported suppression of aggregation of polyA proteins by chaperones like HSP40 and HSP70 is substantiated through molecular docking. The hydrophobic residues of identified aggregating region are found to be interacting with hydrophobic surface of chaperones. This suggests a crucial clue for possible way to inhibit the aggregation of such proteins.

Inherent aggregation propensity of flanking residues attached to polyglutamines: implication to aggregation inhibition.

Expansion of polyglutamine (polyQ) sequence in some proteins leads to their aggregation, which is responsible for neurodegenerative diseases like Huntington's disease, ataxia etc. A flanking domain is usually fused at the N-terminal to polyQ in these proteins. On linking the flanking residues to polyQ, they accelerate aggregation of the proteins, which initiates from the flanking residues. In this report the inherent propensity of the flanking residues towards aggregation in six aggregating proteins has been elucidated from their primary sequences with the help of Betascan and PASTA programs and explored unambiguously. This will provide a molecular mechanism of this process. Suppression of aggregation using chaperones like αB-crystallin by masking the exposed hydrophobic surface of flanking residues is also documented through molecular docking, which could be applied for inhibition of aggregation of this type of proteins.

An investigation into the altered binding mode of green tea polyphenols with human serum albumin on complexation with copper.

Green tea is rich in several polyphenols, such as (-)-epicatechin-3-gallate (ECG), (-)-epigallocatechin (EGC), and (-)-epigallocatechin-3-gallate (EGCG). The biological importance of these polyphenols led us to study the major polyphenol EGCG with human serum albumin (HSA) in an earlier study. In this report, we have compared the binding of ECG, EGC, and EGCG and the Cu(II) complexes of EGCG and ECG with HSA. We observe that the gallate moiety of the polyphenols plays a crucial role in determining the mode of interaction with HSA. The binding constants obtained for the different systems are 5.86 ± 0.72 × 104 M⁻¹ (K ECG-HSA), 4.22 ± 0.15 × 104 M⁻¹ (K ECG-Cu(II)-HSA), and 9.51 ± 0.31 × 104 M⁻¹ (K EGCG-Cu(II)-HSA) at 293 K. Thermodynamic parameters thus obtained suggest that apart from an initial hydrophobic association, van der Waals interactions and hydrogen bonding are the major interactions which held together the polyphenols and HSA. However, thermodynamic parameters obtained from the interactions of the copper complexes with HSA are indicative of the involvement of the hydrophobic forces. Circular dichroism and the Fourier transform infrared spectroscopic measurements reveal changes in α-helical content of HSA after binding with the ligands. Data obtained by fluorescence spectroscopy, displacement experiments along with the docking studies suggested that the ligands bind to the residues located in site 1 (subdomains IIA), whereas EGC, that lacks the gallate moiety, binds to the other hydrophobic site 2 (subdomain IIIA) of the protein.

An investigation of the molecular interactions of diacetylcurcumin with ribonuclease A.

Curcumin is a natural product with diverse pharmacological activities. Studies of curcumin and its structural derivatives have been a subject of growing interest as a result of their diverse biological activities. We report the interaction of diacetylcurcumin (DAC) with Ribonuclease A (RNase A). The binding constant of DAC with RNase A was found to be of the order of 10(4) M(-1). The intrinsic fluorescence of RNase A was quenched by DAC with a quenching constant of 2.2 x10(4) M(-1). The distance between the fluorophore of RNase A and DAC was found to be 2.6 nm, calculated from a Förster type fluorescence resonance energy transfer (FRET). Secondary structural changes of RNase A after binding were analyzed from circular dichroism and Fourier transform infrared studies. Protein-ligand docking studies were conducted to determine the residues involved in the interaction of RNase A with DAC and changes in the accessible surface of the interacting residues were calculated accordingly.

A spectroscopic investigation into the interactions of 3'-O-carboxy esters of thymidine with bovine serum albumin.

Binding studies of 3'-O-carboxy esters of thymidine, reported inhibitors of ribonucleases, with bovine serum albumin (BSA) have been explored in this report. Fluorescence spectroscopy in combination with Fourier transform infrared (FTIR) and circular dichroism (CD) spectroscopy have been used to determine the nature and mode of binding. The binding and quenching parameters were determined from tryptophan fluorescence quenching by Scatchard plots and modified Stern-Volmer plots. The association constants are of the order of 10(4) M(-1) for both the ligands. Thermodynamic parameters suggest that apart from an initial hydrophobic association, hydrogen bonding and van der Waals interactions play a decisive role during protein-ligand complex formation. Minor changes were observed in the secondary structures of human serum albumin (HSA) as revealed by FTIR and CD. Docking studies suggest that the ligands are close to Trp 213, which causes fluorescence quenching.