Large-scale crystallization as an intermediate processing step in insulin downstream process: explored advantages and identified tool for process intensification

The rising global prevalence of diabetes and increasing demand for insulin, calls for an increase in accessibility and affordability of insulin drugs through efficient and cost-effective manufacturing processes. Often downstream operations become manufacturing bottlenecks while processing a high volume of product. Thus, process integration and intensification play an important role in reducing process steps and time, volume reduction, and lower equipment footprints, which brings additional process efficiencies and lowers the production cost. Manufacturers thrive to optimize existing unit operation to maximize its benefit replacing with simple but different efficient technologies. In this manuscript, the typical property of insulin in forming the pH-dependent zinc-insulin complex is explored. The benefit of zinc chloride precipitation/crystallization has been shown to increase the in-process product purity by reducing the product and process-related impurities. Incorporation of such unit operation in the insulin process has also a clear potential for replacing the high cost involved capture chromatography step. Same time, the reduction in volume of operation, buffer consumption, equipment footprint, and capabilities of product long time storage brings manufacturing flexibility and efficiencies. The data and capabilities of simple operation captured here would be significantly helpful for insulins and other biosimilar manufacturer to make progresses on cost-effective productions.

Development of a Systematic Strategy toward Promotion of α-Synuclein Aggregation Using 2-Hydroxyisophthalamide-Based Systems

Establishing a potent scheme against α-synuclein aggregation involved in Parkinson's disease has been evaluated as a promising route to identify compounds that either inhibit or promote the aggregation process of α-synuclein. In the last two decades, this perspective has guided a dramatic increase in the efforts, focused on developing potent drugs either for retardation or promotion of the self-assembly process of α-synuclein. To address this issue, using a chemical kinetics platform, we developed a strategy that enabled a progressively detailed analysis of the molecular events leading to protein aggregation at the microscopic level in the presence of a recently synthesized 2-hydroxyisophthalamide class of small organic molecules based on their binding affinity. Furthermore, qualitatively, we have developed a strategy of disintegration of α-synuclein fibrils in the presence of these organic molecules. Finally, we have shown that these organic molecules effectively suppress the toxicity of α-synuclein oligomers in neuron cells.

Emergence of Aggregation Induced Emission (AIE), Room-Temperature Phosphorescence (RTP), and Multistimuli Response from a Single Organic Luminogen by Directed Structural Modification

Multifunctional organic luminogens exhibiting simultaneous aggregation induced emission (AIE), room-temperature phosphorescence (RTP), and mechanochromism have recently attracted considerable attention owing to their potential applications in optoelectronics and bioimaging. However, a comprehensive correlation among these three distinguished properties is yet to be unveiled, which will help to decipher defined methodologies to design future generation multifunctional organic materials. Herein, we have demonstrated a route to obtain a multifunctional organic luminogen, starting from an ACQphore (TPANDI) by simple structural engineering. We have shown that a slight reduction in length of the planar acceptor moieties can effectively inhibit the undesirable π-π stacking interaction between molecules in the condensed state and thereby cause an ACQ to AIE type transformation from TPANDI to TPANMI and TPAPMI. Both TPANMI and TPAPMI exhibit RTP properties (even in ambient condition) because of the presence of a reasonably low singlet-triplet energy gap (ΔE_ST). In our study, these two luminogens were found to be mechano-inactive. Interestingly, an insertion of cyano-ethylene group and benzene linker in between the triphenylamine and phthalimide moieties introduced another luminogen TPACNPMI, which can simultaneously exhibit AIE, RTP, and mechanochromic properties.

Intrinsic-to-extrinsic emission tuning in luminescent Cu nanoclusters by in situ ligand engineering

Enhancement of the emission quantum yield and expansion of the emission tunability spectrum are the key aspects of an emitter, which direct the evolution of future generation light harvesting materials. In this regard, small molecular ligand-protected Cu nanoclusters (SLCuNCs) have emerged as prospective candidates. Herein, we report the broadband emission tunability in a SLCuNC system, mediated by in situ ligand replacement. 1,6-Hexanedithiol-protected blue emissive discrete Cu nanoclusters (CuNCs) and red emissive CuNC assemblies have been synthesized in one pot. The red emissive CuNC assemblies were characterized and found to be covalently-linked nanocluster superstructures. The blue emissive CuNC was further converted to a green-yellow emissive CuNC over time by a ligand replacement process, which was mediated by the oxidized form of the reducing agent used for synthesizing the blue emissive nanocluster. Steady-state emission results and fluorescence dynamics studies were used to elucidate that the ligand replacement process not only modulates the emission color but also alters the nature of emission from metal-centered intrinsic to ligand-centered extrinsic emission. Moreover, time-dependent blue to green-yellow emission tunability was demonstrated under optimized reaction conditions.

Proton-Coupled Electron Transfer in the Aqueous Nanochannels of Lyotropic Liquid Crystals: Interplay of H-Bonding and Polarity Effects

A molecular-level description of the aqueous nanochannels in lyotropic liquid crystals (LLCs) is crucial for their widespread utilization in diverse fields. Herein, the polarity and hydrogen bonding effects of LLC water molecules have been simultaneously explored using a single probe, 4'-N,N-dimethylamino-3-hydroxyflavone (DMA3HF), by the unique multiparametric sensitivity of the excited state proton-coupled electron transfer (PCET) phenomenon. The decreased ESIPT efficiency and the significantly retarded ESIPT dynamics (>20 times) of DMA3HF in the LLC phases suggests the dominant influence of strong hydrogen-bonded solute-solvent complexes that leads to a high activation barrier for ESIPT in the mesophases. The effects of hydrogen bonding on ESIPT have been elucidated by enhanced sampling techniques based on classical MD simulations of DMA3HF in explicit water. ESIPT via an extended hydrogen-bonded water wire is associated with a significantly high ESIPT activation barrier, substantiating the experimentally observed slow ESIPT dynamics inside the LLCs.

All in One: Stimuli-Responsive, Efficient Mitotracking, and Single Source White Light Emission

"All in one" type luminogens, possessing combined properties related to optical, materials, and biological implications, are of urgent demand today, mainly because of the combined application potential of such probes. To the best of our knowledge, until now, an "all in one" type white light emitter together with stimuli-responsive behavior and highly efficient mitochondrial-tracking ability has not been reported yet. In this contribution, for the first time, we have investigated a pair of luminogens exhibiting white light emission (CIE coordinates: 0.35, 0.35 (DPAEOA) and 0.29, 0.33 (DPAPMI)) with temperature-induced mechanochromic features of a centrosymmetrically packed probe (space group P-1). Most importantly, despite being neutral, our designed probe DPAEOA can specifically illuminate mitochondria with the highest Pearson coefficient value (0.93), which is rare, as almost all the commercially developed mitotrackers are cationic fluorophores. Thus, this study will pave a new avenue for the design of next generation "all in one" type organic luminogens exhibiting potential applications in notable optical, materials, and biological fields.

Sensing and modulation of amyloid fibrils by photo-switchable organic dots

Abnormal aggregation of amyloidogenic proteins (like Aβ 42, amylin, α-synuclein, insulin) and the deposition of these aggregates is believed to be associated with several diseases known as amyloidosis. The pathway of aggregation involves three distinct phases: the oligomeric, elongation and plateau phases. Among them, the oligomeric phase of Aβ 42 and α-synuclein involves the generation of transient oligomeric species suspected to cause several neurological disorders, including Alzheimer's and Parkinson's diseases. Over the past few years, scientists have devoted much more effort to devising new fluorescent molecular probes to estimate the mechanisms of formation, and have gained vital information about possible therapeutic routes for amyloidosis. However, such fluorescent probes face serious limitations because of self-quenching at high concentrations of the probe; therefore, they are inappropriate for quantitative analysis and bio-imaging experiments. Hence, smart biocompatible fluorescent probes are indispensable, as they not only overcome the drawbacks of conventional fluorescent probes, but also have the potential ability to fight amyloidosis through modulation of the pathways involved. In this work, for the first time we introduce a series of promising photo-switchable aggregation-induced emission (AIE) dots (DPAPMI, CPMI) and aggregation caused quenching (ACQ) dots (DMAPMI) which can detect amyloid fibrils in terms of switching and enhancing their fluorescence emission. Interestingly, the organic dots enhance the aggregation rate of insulin by speeding up the microscopic processes, specifically secondary nucleation (with rate constant k2) and the elongation process (with rate constant k+). Moreover, the comparison of kinetics studies with ThT suggests that our organic dots can sense pre-fibrillar aggregates of insulin during the aggregation process, which may be beneficial for the early detection of amyloid fibrils. In summary, our study indicates that these organic dots can be used for the imaging and early stage detection of amyloid fibril formation and the modulation of amyloid formation pathways.

Peculiar hydrogen bonding behaviour of water molecules inside the aqueous nanochannels of lyotropic liquid crystals

The hydrogen bonding abilities of the LLC water molecules and their effects on intramolecular hydrogen bonds of the target probe molecules.

Conformational and solution dynamics of hemoglobin (Hb) in presence of a cleavable gemini surfactant: Insights from spectroscopy, atomic force microscopy, molecular docking and density functional theory

Herein, we have explored the conformational alterations of hemoglobin (Hb) in presence of a cleavable gemini surfactant (C16-C4O2-C16). The concerned surfactant was found to induce significant structural perturbations in Hb. UV-vis spectroscopy, steady-state/time-resolved fluorescence, and other utilized techniques have authenticated the complexation of Hb with the gemini surfactant. CD has demonstrated the alterations in secondary structural elements (α-helicity, β-sheet, β-turn, and random coil) of Hb upon C16-C4O2-C16 addition. Atomic force microscopy (AFM) has revealed the existence of unique star-shaped gemini surfactant microstructures aligned to Hb in a necklace pattern. The ¹H NMR peak broadening and lower delta values hint at the binding of the concerned gemini surfactant to Hb. Molecular docking and DFT calculations have further substantiated the Hb-gemini complex formation and the involvement of electrostatic/hydrophobic forces therein. In future, these results might pave-the-way to construct self-assembled, sustainable, and green surfactant-protein mixtures for their end-use in industrial, engineering, biomedical, drug delivery, gene transfection, and other relevant excipient formulations.

Structural characteristics requisite for the ligand-based selective detection of i-motif DNA

A new approach has been explored to detect i-motif DNA structures over its complementary GQ DNA based on the hemi-protonated cytosine–cytosine (C⁺–C) base pairing recognition. This approach also shows its versatility by detecting various i-motif DNA structures with different chain lengths, molecularity and sizes, etc.

Silica nano-channel induced i-motif formation and stabilization at neutral and alkaline pH

Here, we have developed a new strategy to stabilize i-motif DNA in neutral and alkaline media by incorporating C-rich sequences inside silica nano-channels. Subsequently, the reversibility of this conformational transition has been achieved using a positively charged protein. Importantly, this entire conformational transition can be performed in multiple cycles, which offers an alternative way to control i-motif formation other than pH and thermal annealing.

Controlling anticancer drug mediated G-quadruplex formation and stabilization by a molecular container

Controlling of ligand mediated G-quadruplex DNA (GQ-DNA) formation and stabilization is an important and challenging aspect due to its active involvement in many biologically important processes such as DNA replication, transcription, etc. Here, we have demonstrated that topotecan (TPT), a potential anticancer drug, can instigate the formation and stabilization of GQ-DNA (H24 → GQ-DNA) in the absence of Na⁺/K⁺ ions via circular dichroism, fluorescence, NMR, UV melting and molecular dynamics (MD) simulation studies. The primary binding mode of TPT to GQ was found to be stacking at the terminal rather than binding to the groove. We have also reverted this conformational transition (GQ-DNA → H24) using a molecular container, cucurbit[7]uril (CB7), by means of the translocation of the drug (TPT) from GQ-DNA to its nanocavity. Importantly, we have carried out the detection of these conformational transitions using the fluorescence color switch of the drug, which is more direct and simple than some of the other methods that involve sophisticated and complex detection techniques.

Developing the structure-property relationship to design solid state multi-stimuli responsive materials and their potential applications in different fields

Establishing the structure–property relationship for multi-stimuli responsive mechanochromic materials based on charge transfer luminogens.

Prediction of multi-stimuli responsive behavior in newly developed luminogens is an appealing yet challenging puzzle, since no concrete design strategy has been developed so far. In this article, we demonstrate a potent strategy to gain a deep understanding of the structure–property relationship to design multi-stimuli responsive mechanochromic materials. To achieve our goal, a variety of new isoindolinone core based charge transfer luminogens exhibiting aggregation-induced emission (AIE) have been prepared through C–H bond activation using a cost-effective ruthenium (Ru) metal catalyzed one-pot synthetic strategy. We have shown that slight tuning of the donor moiety is found to be highly effective in controlling molecular packing and metastable energy states in solid states, and thus, optical properties and multi-stimuli responsive behaviors. The flexibility and twisting of donor moieties afford a loosely bound ‘herringbone’ packing, enabling reversible transformation under multiple mechanical stimuli. The cyclized derivative of the donor exhibits a completely different packing mode (i.e., cross packing), and subsequently, does not give rise to mechanochromism. The Hirshfeld surface analysis from a single crystal infers that non-covalent interactions (specifically C–H···π and π···π) are extremely important to yield mechanochromism under external force. Correlating all solid-state behavior with the molecular structure, we conclude that the synergistic effect between the twisting and conformational flexibility of donor moieties along with numerous non-covalent interactions gives rise to multi-stimuli responsive behaviors. Finally, the newly designed molecules are found to be highly emissive in solution and potentially applicable in fluorescence thermometer construction, lighting up cells, acid–base sensors and rewritable devices.

Ionic liquid induced G-quadruplex formation and stabilization: spectroscopic and simulation studies

Among different polymorphs of DNA, G-quadruplex (GQ) formation in guanine rich sequences has received special attention due to its direct relevance to cellular aging and abnormal cell growths. To date, smaller ions like Na⁺, K⁺, Li⁺, and NH₄⁺ are the best possible selective GQ stabilizing materials. Herein, we report that an ionic liquid (IL), i.e. guanidinium tris(pentafluoroethyl)trifluorophosphate, can not only instigate the GQ formation in the absence of conventional GQ forming ions (like Na⁺, K⁺, NH₄⁺, etc.), but also stabilizes the GQ structure. This conformational transition has been confirmed through different spectroscopic tools and molecular dynamics (MD) simulation studies. MD simulation shows that one of the guanidinium cations resides in the G-tetrad core, while bulky anions prefer to stay near the GQ surface resulting in GQ formation and stabilization. This study thus brings out a special type of ionic liquid that acts as a GQ stabilizer. The origin of GQ stabilization by IL presented here may also help in the future design of IL for GQ formation and stabilization.

Dynamics of different steps of the photopyrolytic cycle of an eminent anticancer drug topotecan inside biocompatible lyotropic liquid crystalline systems

Dynamics of different steps of photopyrolytic processes of an eminent anticancer drug topotecan have been investigated inside different lyotropic liquid crystalline systems.

Loading of an anti-cancer drug into mesoporous silica nano-channels and its subsequent release to DNA

Mesoporous silica nano-channel (MCM-41) based molecular switching of a biologically important anticancer drug, namely, ellipticine (EPT) has been utilized to probe its efficient loading onto MCM-41, and its subsequent release to intra-cellular biomolecules, like DNA. By exploiting various spectroscopic techniques (like, steady state fluorescence, time-resolved fluorescence and circular dichroism), it has been shown that EPT can be easily translocated from MCM-41 to DNA without using any external stimulant. Blue emission of EPT in a polar aprotic solvent, i.e., dichloromethane (DCM), completely switches to green upon loading inside MCM-41 due to the conversion from a neutral to a protonated form of the drug inside nano-pores. Powder X-ray diffraction (PXRD), N₂ gas adsorption and confocal fluorescence microscopy results confirm the adsorption of EPT inside the nano-pores of MCM-41. Here, the lysozyme (Lyz) protein has been utilized as a pore blocker of MCM-41 in order to prevent premature drug release. Interestingly, EPT is released to DNA even from the EPT-MCM-Lyz composite system, and results in intensification of green fluorescence. Electron microscopy results reveal the formation of a distinctive garland kind of morphology involving MCM-41 and DNA probably through non-covalent interactions, and this is believed to be responsible for the DNA assisted release of drug molecules from silica nano-pores. Confocal laser scanning microscopy (CLSM) imaging revealed that EPT-MCM is successfully internalized into the HeLa cervical cancer cells and localized into the nucleus. Cell viability assay results infer that EPT-MCM and EPT-MCM-Lyz showed much improved efficacy in HeLa cancer cells compared to free ellipticine.

Topological Influence of Lyotropic Liquid Crystalline Systems on Excited-State Proton Transfer Dynamics

In the present work, we have investigated the excited-state proton transfer (ESPT) dynamics inside lipid-based reverse hexagonal (HII), gyroid Ia3d, and diamond Pn3m LLC phases. Polarized light microscopy (PLM) and small-angle X-ray scattering (SAXS) techniques have been employed for the characterization of LLC systems. Time-resolved fluorescence results reveal the retarded ESPT dynamics inside liquid crystalline systems compared to bulk water, and it follows the order HII < Ia3d < Pn3m < H2O. The slower solvation, hampered "Grotthuss" proton transfer process, and most importantly, topological influence, of the LLC systems are believed to be mainly responsible for the slower and different extent of ESPT dynamics. Interestingly, recombination dynamics is found to be faster with respect to bulk water and it follows the order H2O < Pn3m < Ia3d < HII. Faster recombination dynamics arises due to lower dielectric constant and different channel diameters of these LLC systems. However, the dissociation dynamics is found to be slower than bulk water and it follows the order HII < Ia3d < Pn3m < H2O. Differences in critical packing parameter of LLC systems are believed to be the governing factors for the slower dissociation dynamics in these liquid crystalline systems.

RGO enveloped vertically aligned Co3O4 nanowires on carbon fabric: a highly efficient prototype for flexible field emitter arrays

RGO enveloped Co₃O₄ nanowires on flexible carbon fabric exhibit a splendid field emission performance with remarkably enhanced current density.

Solvation Dynamics in Different Phases of the Lyotropic Liquid Crystalline System

Reverse hexagonal (HII) liquid crystalline material based on glycerol monooleate (GMO) is considered as a potential carrier for drugs and other important biomolecules due to its thermotropic phase change and excellent morphology. In this work, the dynamics of encapsulated water, which plays important role in stabilization and formation of reverse hexagonal mesophase, has been investigated by time dependent Stokes shift method using Coumarin-343 as a solvation probe. The formation of the reverse hexagonal mesophase (HII) and transformation to the L2 phase have been monitored using small-angle X-ray scattering and polarized light microscopy experiments. REES studies suggest the existence of different polar regions in both HII and L2 systems. The solvation dynamics study inside the reverse hexagonal (HII) phase reveals the existence of two different types of water molecules exhibiting dynamics on a 120-900 ps time scale. The estimated diffusion coefficients of both types of water molecules obtained from the observed dynamics are in good agreement with the measured diffusion coefficient collected from the NMR study. The calculated activation energy is found to be 2.05 kcal/mol, which is associated with coupled rotational-translational water relaxation dynamics upon the transition from "bound" to "quasi-free" state. The observed ∼2 ns faster dynamics of the L2 phase compared to the HII phase may be associated with both the phase transformation as well as thermotropic effect on the relaxation process. Microviscosities calculated from time-resolved anisotropy studies infer that the interface is almost ∼22 times higher viscous than the central part of the cylinder. Overall, our results reveal the unique dynamical features of water inside the cylinder of reverse hexagonal and inverse micellar phases.

Excited state proton transfer dynamics of an eminent anticancer drug, ellipticine, in octyl glucoside micelle

Photophysics and proton transfer dynamics of an eminent anticancer drug, ellipticine (EPT), have been investigated inside a biocompatible octyl-β-D-glucoside (OBG) micellar medium using steady state and time-resolved fluorescence spectroscopic techniques. EPT exists as protonated form in aqueous solution of pH 7. When EPT molecules are encapsulated in OBG micelles, protonated form is converted to neutral form in the ground state due to the hydrophobic effect of the micellar environment. Interestingly, steady state fluorescence results indicate the existence of both neutral and protonated forms of EPT in the excited state, even though neutral molecules are selectively excited, and it is attributed to the conversion of neutral to protonated form of EPT by the excited state proton transfer (ESPT) process. A clear isoemissive point in the time-resolved area normalized emission spectra (TRANES) further supports the excited state conversion of neutral to protonated form of EPT. Notably, this kind of proton transfer dynamics is not observed in other conventional micelles, such as, SDS, Triton-X and CTAB. Therefore, the observed ESPT dynamics is believed to be an outcome of combined effects of the local dielectric constant felt by EPT and the local proton concentration at the OBG micellar surface.

A Green Solvent Induced DNA Package

Mechanistic details of DNA compaction is essential blue print for gene regulation in living organisms. Many in vitro studies have been implemented using several compaction agents. However, these compacting agents may have some kinds of cytotoxic effects to the cells. To minimize this aspect, several research works had been performed, but people have never focused green solvent, i.e. room temperature ionic liquid as DNA compaction agent. To the best of our knowledge, this is the first ever report where we have shown that guanidinium tris(pentafluoroethyl)trifluorophosphate (Gua-IL) acts as a DNA compacting agent. The compaction ability of Gua-IL has been verified by different spectroscopic techniques, like steady state emission, circular dichroism, dynamic light scattering and UV melting. Notably, we have extensively probed this compaction by Gua-IL through field emission scanning electron microscopy (FE-SEM) and fluorescence microscopy images. We also have discussed the plausible compaction mechanism process of DNA by Gua-IL. Our results suggest that Gua-IL forms a micellar kind of self aggregation above a certain concentration (≥1 mM), which instigates this compaction process. This study divulges the specific details of DNA compaction mechanism by a new class of compaction agent, which is highly biodegradable and eco friendly in nature.

Ultrafast dynamics of a molecular rotor in chemical and biological nano-cavities

Excited state dynamics of CCVJ are investigated inside the nano-cavities of CD and HSA using steady-state and femtosecond up-conversion techniques.

Fluorescence switching of sanguinarine in micellar environments

Conversion from iminium to alkanolamine form of Sanguinarine, a key member of benzylisoquinoline alkaloid family, is monitored in micellar environments by fluorescence switch.

pH responsive translocation of an anticancer drug between cyclodextrin and DNA

Herein, the pH triggered translocation of EPT in between γ-cyclodextrin and DNA is monitored using fluorescence switch of the drug.

An anticancer drug to probe non-specific protein-DNA interactions

A visible fluorescence switch of an eminent anti-carcinogen, ellipticine has been used to probe non-specific protein-DNA interaction. The unique pattern of protein-DNA complexation is depicted for the first time through field emission scanning electron microscopy (FE-SEM) images and spectroscopic techniques.

Role of Mg²⁺ ions in flavin recognition by RNA aptamer

The role of Mg(2+) ion in flavin (flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN)) recognition by RNA aptamer has been explored through steady state and time-resolved fluorescence, circular dichroism (CD), thermal melting (TM) and isothermal titration calorimetry (ITC) studies. A strong quenching of flavin emission is detected due to stacking interaction with the nucleobases in the mismatched region of aptamer, and it enhances manifold with increasing Mg(2+) concentrations. A comparatively lower binding affinity toward FAD compared to FMN is attributed to the presence of intramolecular 'stack' conformer of FAD, which cannot participate in the intermolecular stacking interactions with the nucleobases. CD and TM studies predict that flavin detection causes structural reformation of RNA aptamer. ITC results indicate that flavin detection is thermodynamically feasible and highly enthalpy driven.

Loading of an anti-cancer drug onto graphene oxide and subsequent release to DNA/RNA: a direct optical detection

Graphene oxide based molecular switching of ellipticine (E) has been utilized to probe its efficient loading onto graphene oxide (GO) and subsequent release to intra-cellular biomolecules like DNA/RNA. The green fluorescence of E switches to blue in GO and switches back to green with polynucleotides. The intensified blue emission of the ellipticine-GO (E-GO) complex with human serum albumin (HSA), switches to a bluish green upon addition of dsDNA. Electron microscopy reveals the formation of distinctive 3D assemblies involving GO and biomolecule(s) probably through non-covalent interactions and this is primarily responsible for the biomolcule(s) assisted fluorescence-switching of E. To our knowledge, such morphological patterning of a GO-DNA complex is very unusual, reported here the first time and could find applications in the fabrication of biomedical devices. Moreover, our approach of direct optical detection of drug loading and releasing is very cheap, appealing and will be useful for clinical trial experiments once the cytotoxicity of GO is duly taken care.

Urea induced unfolding dynamics of flavin adenine dinucleotide (FAD): spectroscopic and molecular dynamics simulation studies from femto-second to nanosecond regime

Here, we investigate the effect of urea in the unfolding dynamics of flavin adenine dinucleotide (FAD), an important enzymatic cofactor, through steady state, time-resolved fluorescence spectroscopic and molecular dynamics (MD) simulation studies. Steady state results indicate the possibility of urea induced unfolding of FAD, inferred from increasing emission intensity of FAD with urea. The TCSPC and up-conversion results suggest that the stack-unstack dynamics of FAD severely gets affected in the presence of urea and leads to an increase in the unstack conformation population from 15% in pure water to 40% in 12 M urea. Molecular dynamics simulation was employed to understand the nature of the interaction between FAD and urea at the molecular level. Results depict that urea molecules replace many of the water molecules around adenine and isoalloxazine rings of FAD. However, the major driving force for the stability of this unstack conformations arises from the favorable stacking interaction of a significant fraction of the urea molecules with adenine and isoalloxazine rings of FAD, which overcomes the intramolecular stacking interaction between themselves observed in pure water.

Comparative study of flavins binding with human serum albumin: a fluorometric, thermodynamic, and molecular dynamics approach

Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are derivatives of riboflavin (RF), a water-soluble vitamin, more commonly known as vitamin B(2). Flavins have attracted special attention in the last few years because of the recent discovery of a large number of flavoproteins. In this work, these flavins are used as extrinsic fluorescence markers for probing the microheterogeneous environment of a well-known transport protein, human serum albumin (HSA). Steady-state and time-resolved fluorescence experiments confirm that both FMN and FAD bind to the Sudlow's site-1 (SS1) binding pocket of HSA, where Trp214 resides. In the case of RF, a fraction of RF molecules binds at the SS1, whereas the major fraction of RF molecules remains unbound or surface bound to the protein. Moreover, flavin(s)-HSA interactions are monitored with the help of isothermal titration calorimetry, which provides free energy, enthalpy, and entropy changes of binding along with the binding constants. The molecular picture of binding interaction between flavins and HSA is well explored by docking and molecular dynamics studies.