Caffeine and sulfadiazine interact differently with human serum albumin: A combined fluorescence and molecular docking study

Molecular Insights into the Conformational and Binding Behaviors of Human Serum Albumin Induced by Surface-Active Ionic Liquids

Extensive research has been carried out to investigate the stability and function of human serum albumin (HSA) when exposed to surface-active ionic liquids (SAILs) with different head groups (imidazolium, morpholinium, and pyridinium) and alkyl chain lengths (ranging from decyl to tetradecyl). Analysis of the protein fluorescence spectra indicates noticeable changes in the secondary structure of HSA with varying concentrations of all SAILs tested. Helicity calculations based on the Fourier transform infrared (FTIR) data show that HSA becomes more organized at the micellar concentration of SAILs, leading to an increased protein activity at this level. Small-angle neutron scattering (SANS) data confirm the formation of a bead-necklace structure between the SAILs and HSA. Atomistic molecular dynamics (MD) simulation results identify several hotspots on the protein surface for interaction with SAIL, which results in the modulation of protein conformational fluctuation and stability. Furthermore, fluorescence resonance energy transfer (FRET) experiments with the intramolecular charge transfer (ICT) probe trans-ethyl p-(dimethylamino) cinnamate (EDAC) demonstrate that higher alkyl chain lengths and SAIL concentrations result in a significantly increased energy transfer efficiency. The findings of this study provide a detailed molecular-level understanding of how the protein structure and function are affected by the presence of SAILs, with potential implications for a wide range of applications involving protein-SAIL composite systems.

Elucidating the interaction between equisetin and human serum albumin: A comprehensive study using spectroscopy, microcalorimetry and molecular docking approaches

Equisetin, a bioactive compound of marine origin, offers compelling inhibitory properties against HIV-1 transcriptase. To gain comprehensive insights into the interaction of Equisetin with human serum albumin (HSA), we utilized a multipronged approach involving spectroscopy, isothermal titration calorimetry (ITC) and molecular docking. Our fluorescence analyses confirmed that the interaction between Equisetin and HSA results in a significant quenching of HSA's fluorescence, primarily achieved through a dynamic mechanism aided by hydrogen bonding and van der Waals forces. Isothermal titration calorimetry (ITC) measurements revealed an impressive binding affinity of Equisetin for HSA, quantified to be 4.3 × 107 mol L-1. Molecular docking studies illustrated that Equisetin binds at site III of HSA, with specific amino acid residues, GLN-104 and LYS-106, playing a pivotal role. Further, our study discovered that the interaction induces slight unfolding of HSA's polypeptide chain and significant alterations in its secondary structure, thereby triggering the exposure of previously concealed hydrophobic regions. This comprehensive study enhances our understanding of Equisetin's interaction with serum proteins, potentially influencing its pharmacokinetics and pharmacodynamics, and opening avenues for future research and therapeutic applications.

Binding Affinity and Mechanism of Six PFAS with Human Serum Albumin: Insights from Multi-Spectroscopy, DFT and Molecular Dynamics Approaches

Per- and Polyfluoroalkyl Substances (PFAS) bioaccumulate in the human body, presenting potential health risks and cellular toxicity. Their transport mechanisms and interactions with tissues and the circulatory system require further investigation. This study investigates the interaction mechanisms of six PFAS with Human Serum Albumin (HSA) using multi-spectroscopy, DFT and a molecular dynamics approach. Multi-spectral analysis shows that perfluorononanoic acid (PFNA) has the best binding capabilities with HSA. The order of binding constants (298 K) is as follows: "Perfluorononanoic Acid (PFNA, 7.81 × 106 L·mol-1) > Perfluoro-2,5-dimethyl-3,6-dioxanonanoic Acid (HFPO-TA, 3.70 × 106 L·mol-1) > Perfluorooctanoic Acid (PFOA, 2.27 × 105 L·mol-1) > Perfluoro-3,6,9-trioxadecanoic Acid (PFO3DA, 1.59 × 105 L·mol-1) > Perfluoroheptanoic Acid (PFHpA, 4.53 × 103 L·mol-1) > Dodecafluorosuberic Acid (DFSA, 1.52 × 103 L·mol-1)". Thermodynamic analysis suggests that PFNA and PFO3DA's interactions with HSA are exothermic, driven primarily by hydrogen bonds or van der Waals interactions. PFHpA, DFSA, PFOA, and HFPO-TA's interactions with HSA, on the other hand, are endothermic processes primarily driven by hydrophobic interactions. Competitive probe results show that the main HSA-PFAS binding site is in the HSA structure's subdomain IIA. These findings are also consistent with the findings of molecular docking. Molecular dynamics simulation (MD) analysis further shows that the lowest binding energy (-38.83 kcal/mol) is fund in the HSA-PFNA complex, indicating that PFNA binds more readily with HSA. Energy decomposition analysis also indicates that van der Waals and electrostatic interactions are the main forces for the HSA-PFAS complexes. Correlation analysis reveals that DFT quantum chemical descriptors related to electrostatic distribution and characteristics like ESP and ALIE are more representative in characterizing HSA-PFAS binding. This study sheds light on the interactions between HSA and PFAS. It guides health risk assessments and control strategies against PFAS, serving as a critical starting point for further public health research.

Effect of Caffeine and Flavonoids on the Binding of Tigecycline to Human Serum Albumin: A Spectroscopic Study and Molecular Docking

Human serum albumin (HSA) has a very significant role in the transport of drugs, in their pharmacokinetic and pharmacodynamic properties, as well as the unbound concentration of drugs in circulating plasma. The aim of this study was to look into the competition between tigecycline (TGC) and alkaloid (ALK) (caffeine (CAF)), and flavonoids (FLAVs) (catechin (CAT), quercetin (QUE), and diosmin (DIO)) in binding to HSA in simulated physiological conditions using multiple spectroscopic measurements and docking simulations. Fluorescence analysis was used to find the binding and quenching properties of double HSA-TGC and triple HSA-TGC-CAF/FLAV systems. The conformational change of the HSA was analyzed using synchronous fluorescence spectroscopy, Fourier transform infrared spectroscopy, and circular dichroism. Obtained results of spectroscopic analyses indicate that triple complexes of HSA-TGC-CAF/FLAVs are formed without problems and have higher binding affinities than double HSA-TGC. In addition, TGC does not change the microenvironments around the tryptophan (Trp) and tyrosine (Tyr) residues in the presence of ALK and FLAVs. Ultimately, the binding affinity, competition, and interaction nature were explored by docking modeling. Computational outcomes are in good accordance with experimentally obtained results. Accordingly, concluding remarks may be very useful for potential interactions between common food components and drugs.

Dietary polyphenols inhibit plasma protein arabinosylation: Biomolecular interaction of genistein and ellagic acid with serum albumins

The mode of interaction of polyphenolic compounds like genistein (GTN) and ellagic acid (EGA) with human and bovine serum albumin (HSA and BSA, respectively) was found to differ significantly. Stern-Volmer (SV) analysis of the fluorescence quenching data revealed that the binding strength of EGA (1.9 ± 0.09 × 105 M-1) to HSA is about one order of magnitude higher than GTN (2.24 ± 0.06 × 104 M-1). While the static quenching of HSA fluorescence was found to proceed through simple Stern-Volmer (SV) mechanism, a quenching sphere-of-action model was indispensable for BSA. Temperature dependent fluorescence along with a series of other biophysical experiments and ensemble docking calculation revealed that EGA and GTN bind to the serum proteins primarily through the entropy driven process. The α-helical content and the microenvironment near Trp residue of HSA and BSA did not show any appreciable change due to the binding of either GTN or EGA. Interestingly, both GTN and EGA were found to inhibit the formation of advanced glycated end (AGE) product of serum proteins up to the extent of 70-90% within 12-24 h. Relatively moderate binding propensity along with the anti-glycation ability of the polyphenols confirmed that GTN and EGA can be used either as an alternative or towards development of suitable drugs in the prevention of many diabetic-related complications.

Ligand binding constants for human serum albumin evaluated by ratiometric analysis of DSC thermograms

This paper describes an expanded application of our recently reported method (Eskew et al., Analytical Biochemistry 621,1 2021) utilizing thermogram signals for thermal denaturation measured by differential scanning calorimetry. Characteristic signals were used to quantitatively evaluate ligand binding constants for human serum albumin. In our approach the ensemble of temperature dependent calorimetric responses for various protein-ligand mixtures and native HSA were compared, in a ratiometric manner, to extract binding constants and stoichiometries. Protein/ligand mixtures were prepared at various ligand concentrations and subjected to thermal denaturation analysis by calorimetry. Measurements provided the melting temperature, T_m, and free-energy ΔG_cal(37°C) for melting ligand-bound Albumin as a function of ligand concentration. Concentration dependent behaviors of these parameters derived from protein/ligand mixtures were used to construct dose-response curves. Fitting of dose-response curves yielded quantitative evaluation of the ligand binding constant and semi-quantitative estimates of the binding stoichiometry. Many of the ligands had known binding affinity for Albumin with binding constants reported in the literature. Evaluated binding parameters for the ligands impressively agreed with reported literature values determined using other standard experimental methods. Results are reported for 29 drug ligands binding to Albumin. These validate our calorimetry-based process for applications in pre-clinical drug screening.

Sulfonamide drugs: structure, antibacterial property, toxicity, and biophysical interactions

Sulfonamide (or sulphonamide) functional group chemistry (SN) forms the basis of several groups of drug. In vivo sulfonamides exhibit a range of pharmacological activities, such as anti-carbonic anhydrase and anti-t dihydropteroate synthetase allowing them to play a role in treating a diverse range of disease states such as diuresis, hypoglycemia, thyroiditis, inflammation, and glaucoma. Sulfamethazine (SMZ) is a commonly used sulphonamide drug in veterinary medicine that acts as an antibacterial compound to treat livestock diseases such as gastrointestinal and respiratory tract infections. Sulfadiazine (SDZ) is another frequently employed sulphonamide drug that is used in combination with the anti-malarial drug pyrimethamine to treat toxoplasmosis in warm-blooded animals. This study explores the research findings and the work behaviours of SN (SMZ and SDZ) drugs. The areas covered include SN drug structure, SN drug antibacterial activity, SN drug toxicity, and SN environmental toxicity.

Sulfonylurea Class of Antidiabetic Drugs Inhibit Acetylcholinesterase Activity: Unexplored Auxiliary Pharmacological Benefit toward Alzheimer's Disease

Contemporary literature documents extensive research on common causative mechanisms, pathogenic pathways and dual effective remedies for Alzheimer's disease (AD) and Type 2 diabetes mellitus (T2DM). Tolbutamide (TBM), chlorpropamide (CPM), and glyburide (GLY) are three sulfonylurea antidiabetic drugs of different generations. All these drugs were found to exhibit moderate to strong inhibitory efficiency on the neurotransmitter degrading enzyme acetylcholinesterase (AChE) with GLY (IC50 = 0.74 ± 0.02 μM) being the most potent, followed by CPM (IC50 = 5.72 ± 0.24 μM) and TBM (IC50 = 28.9 ± 1.60 μM). Notably, the inhibition efficiency of GLY is even comparable with the FDA approved AD drug, donepezil (DON). The larger size of GLY spans almost the full gorge of AChE ranging from catalytic active site (CAS) to the peripheral active site (PAS) with relatively strong binding affinity (6.0 × 105 M-1) and acts as a competitive inhibitor for AChE. On the other hand, while they show relatively weak binding ((2-6) × 104 M-1), both CPM and TBM act as noncompetitive binders. While these two drugs can bind to PAS, MD simulation results predict an alternative noncompetitive inhibition mechanism for CPM. These results open the possibility of repurposing the antidiabetic drugs, particularly GLY, in the treatment of AD. The consequential side effect of excess acetylcholine production, due to the administration of these drugs to AD-unaffected patients, can be rectified by using colloidal gold and silver nanofluids as potential AChE activity boosters.

Combined computational and experimental studies on cysteine-sulfadiazine adduct formation

The electrochemical characterization of sulfadiazine-cysteine (SD-CYS) adduct formation was performed in phosphate buffer (pH 7) on the basis of voltammetric current and peak potential measurements. Due to the association of cysteine with sulfadiazine, the reduction peak currents of mercuric and mercurous cysteine thiolates decreased and their peak potentials simultaneously shifted to less negative potentials. By using the current changes of mercurous cysteine thiolate, it was determined that cysteine and sulfadiazine are associated with a 1:1 stoichiometry with a conditional association constant of 1.99 ×104 M-1 . In addition to experimental studies, a computational approach was carried out to study the geometrical parameters, electron densities, and UV-Vis absorption spectra of sulfadiazine and SDCYS adduct in water. Calculated (B3LYP/6-311++G(d,p) level) and experimental UV-Vis absorption spectra of the compounds were found to be in good agreement in water. The computational study suggests that cysteine bound to the C(5) on the pyrimidine ring via SH-group nucleophilic attack. Computational results reveal that sulfadiazine and its derivatives effectively bind cysteine and may lead to new molecules/drugs to target cysteine.

Study on the interaction of ertugliflozin with human serum albumin in vitro by multispectroscopic methods, molecular docking, and molecular dynamics simulation

Ertugliflozin is a potent and selective inhibitor of sodium-dependent glucose cotransporters 2 (SGLT2) and used as a monotherapy to improve glycemic control in adult patients with type 2 diabetes. In this study, ertugliflozin binding to human serum albumin (HSA) was investigated by multispectroscopic and computer simulations. The fluorescence spectra demonstrated that the quenching mechanism of ertugliflozin and HSA was static quenching. Thermodynamic parameters indicated that hydrogen bonding and van der Waals forces played a key role in the binding. Fluorescence competition experiments and molecular docking revealed that ertugliflozin bound to HSA sites II. In three-dimensional fluorescence, circular dichroism spectroscopy, and molecular dynamics simulation, ertugliflozin did not affect the basic skeleton structure of HSA but slightly increased the α-helical structure content and changed the microenvironment around amino acid residues. Results provide valuable information on the basis of the interaction of ertugliflozin with HSA.

Therapeutic potency of substituted chromones as Alzheimer's drug: Elucidation of acetylcholinesterase inhibitory activity through spectroscopic and molecular modelling investigation

Introduction: Documentation on the potency of chromones as acetylcholinesterase (AChE) antagonists has paved the way for the design and usage of new chromone analogues as inhibitors of AChE modelled on the hypothesis based on cholinergic pathway of Alzheimer's disease (AD). Here, 2 minimally substituted chromones, namely 3-cyanochromone (CyC) and 7-amino-3- methylchromone (AMC), were checked for their AChE inhibition efficacies and plasma protein modulation. Methods: Colorimetric enzymatic assay as well as fluorescence measurements were performed for obtaining the experimental results, which were further corroborated by molecular docking and simulation studies. Results: The investigated systems exhibited strong inhibition activities against AChE, with CyC (IC50= 85.12 ± 6.70 nM) acting as better inhibitor than AMC (IC50 = 103.09 ± 11.90 nM) and both having IC50 values in the range of FDA approved cholinergic drug Donepezil (IC50 = 74.13 ± 8.30 nM). Non-competitive inhibition was observed in both the cases with the inhibitors binding near the peripheral anionic site (PAS) of the enzyme. Having one planar nitrile group in CyC as compared to sp3 hybridised substituents in AMC facilitated stacking interactions in the former, accounting for its higher inhibitory efficacy. A significant decrease in the inhibition potency of CyC (~32%) was noted in comparison with AMC (~5%) when the experiments were performed in presence of human serum albumin (HSA) instead of pure aqueous buffer. Conclusion: This comparative study affirms the importance of meticulous substitution in the chromone scaffold to promote maximum inhibition potency, while considering their usage as AD drugs.

Photophysics and luminescence quenching of carbon dots derived from lemon juice and glycerol

During the past decade, carbon dots have emerged as a fascinating class of luminescent nanomaterials with versatile application potentials in bioimaging, labeling, photocatalysis and optoelectronics. Currently, intensive research is concentrated on understanding the intriguing optical properties of these promising materials and their utility as luminescence sensors. In this article, we describe the photoluminescence of carbon dots obtained from a bioresource (lemon juice) and from a small molecule precursor (glycerol), especially the quenching of their emission by nitrobenzene and Hg²⁺ ions, as representative cases. Stern-Volmer analysis using steady-state and time-resolved emission measurements, suggests the involvement of both transient quenching and dynamic quenching mechanisms in the interaction of the carbon dots with nitrobenzene. The radius of the quenching sphere is estimated to be slightly greater than the contact distances between the respective carbon dots and nitrobenzene, which is in reasonable agreement with the "sphere of action" model for transient quenching. In the interaction with Hg²⁺ ions, electrostatic attraction plays a major role, and the quenching mechanism involves predominantly static and dynamic quenching. The static quenching constant matches well with the binding constant of the carbon dots with the metal ion.

Quenching of Luminol Fluorescence at Nano-Bio Interface: Towards the Development of an Efficient Energy Transfer System

Surface modified colloidal gold (Au) and silver (Ag) nanoparticles (NPs) were used as efficient quenchers of luminol (LH₂) fluorescence either in homogeneous aqueous medium or its noncovalent assembly with bovine serum albumin (BSA). The mechanism as well as the extent of fluorescence quenching was found to be strongly dependent on the nature of the nanoparticles. While simple static type fluorescence quenching mechanism was perceived with AuNP, a more complex protocol involving quenching sphere model was envisaged for AgNP quenching. Nevertheless, the magnitude of Stern-Volmer (SV) quenching constant (K_SV ~ 10⁸-10¹⁰ M^-1) was calculated to be ca. 10⁴ times more for surface quoted NPs in comparison with BSA-NP bioconjugates system. On the other hand, a highly efficient (E ≈ 95%) energy transfer (ET) process was predicted for LH₂ captured in the hydrophobic assembly with BSA in presence of AgNP as an acceptor. The ET efficiency is critically dependent on the concentration of BSA and nicely correlated with the extent of NP surface coverage. However, fluorescence quenching on AuNP surface is relatively less responsive towards protein concentration, primarily due to the difference in surface activity as well as the mode of interaction of the protein with NPs. Graphical Abstract Energy transfer from excited luminol to metal nanoparticles is strongly modulated in presence of serum albumins.

Interactions between tetrahydroisoindoline-1,3-dione derivatives and human serum albumin via multiple spectroscopy techniques

Some tetrahydroisoindoline-1,3-dione derivatives (TDDs) possess potent herbicidal activity. To assess possible impacts of TDDs on humans, the interactions between TDDs and human serum albumin (HSA) were evaluated with steady-state and time-resolved fluorescence spectroscopy, synchronous fluorescence spectroscopy, Fourier transform-infrared spectroscopy, and circular dichroism spectroscopy. The thermodynamic data obtained at temperatures of 298, 307, and 316 K indicate that TDDs spontaneously bind to HSA and thus form a TDD-HSA complex. The conformation and secondary structure of HSA are changed, and the intrinsic fluorescence of HSA is statically quenched by TDDs. Moreover, the TDD-HSA complex is formed primarily through electrostatic interactions and has only one binding site on HSA. A competitive ligand-binding assay revealed that site II (subdomain IIIA) displays the greatest affinity for TDDs. In addition, an acute toxicity bioassay showed no zebrafish mortality upon exposure to 4000 μg L^-1 of TDDs. This work is helpful for understanding interactions between TDDs and HSA.

Interactions of hemin with bovine serum albumin and human hemoglobin: A fluorescence quenching study

The binding interactions between hemin (Hmi) and bovine serum albumin (BSA) or human hemoglobin (HHb), respectively, have been examined in aqueous solution at pH=7.4, applying UV-vis absorption, as well as steady-state, synchronous and three-dimensional fluorescence spectra techniques. Representative results received for both BSA and HHb intrinsic fluorescence proceeding from the interactions with hemin suggest the formation of stacking non-covalent and non-fluorescent complexes in both the Hmi-BSA and Hmi-HHb systems, with highly possible concurrent formation of a coordinate bond between a group on the protein surface and the metal in Hmi molecule. All the values of calculated parameters, the binding, fluorescence quenching and bimolecular quenching rate constants point to the involvement of static quenching in both the systems studied. The blue shift in the synchronous fluorescence spectra imply the participation of both tryptophan and tyrosine residues in quenching of BSA and HHb intrinsic fluorescence. Depicted outcomes suggest that hemin is supposedly able to influence the physiological functions of BSA and HHb, the most important blood proteins, particularly in case of its overuse.

Tandem copper and gold nanoclusters for two-color ratiometric explosives detection

We report a sensory platform for the determination of common explosive species (e.g., TNT, PETN, RDX) based on the differential response from two different luminescent metal nanoclusters. In particular, whereas the red emission from bovine serum albumin-protected gold nanoclusters was strongly quenched by nitro-, nitrate-, and nitroamine-containing explosive organic molecules, blue-emitting glutathione-capped copper nanoclusters proved inert to quenching by these same analytes, instead showing evidence for aggregation-induced emission enhancement (AIEE). As a result, this discrete gold/copper nanocluster pairing provides a dual-probe, ratiometric (red-to-blue) system signaling the presence of TNT and other common explosives. This strategy opens up new potential for nanocluster-based analyte signaling, with implications to fluorescence resonance energy transfer (FRET) strategies as well.

Investigation of the binding affinity in vitamin B12-Bovine serum albumin system using various spectroscopic methods

The binding affinity between vitamin B12 (VitB12) and bovine serum albumin (BSA) has been investigated in aqueous solution at pH=7.4, employing UV-vis absorption and steady-state, synchronous and three-dimensional fluorescence spectra techniques. Representative effects noted for BSA intrinsic fluorescence resulting from the interactions with VitB12 confirm the formation of π-π stacked non-covalent and non-fluorescent complexes in the system VitB12-BSA. All the determined parameters, the binding, fluorescence quenching and bimolecular quenching rate constants (of the order of 10⁴Lmol^-1, 10³Lmol^-1 and 10¹¹Lmol^-1s^-1, respectively), as well as Förster resonance energy transfer parameters validate the mechanism of static quenching. The interaction with VitB12 induces folding of the polypeptide chains around Trp residues of BSA, resulting in a more hydrophobic surrounding. Presented outcomes suggest that the addition of VitB12 can lead to the more organized BSA conformation and its more folded tertiary structure, what could influence the physiological functions of bovine serum albumin, notably in case of its overuse or abnormal metabolism.

Domain-specific interactions between MLN8237 and human serum albumin estimated by STD and WaterLOGSY NMR, ITC, spectroscopic, and docking techniques

Alisertib (MLN8237) is an orally administered inhibitor of Aurora A kinase. This small-molecule inhibitor is under clinical or pre-clinical phase for the treatment of advanced malignancies. The present study provides a detailed characterization of the interaction of MLN8237 with a drug transport protein called human serum albumin (HSA). STD and WaterLOGSY nuclear magnetic resonance (NMR)-binding studies were conducted first to confirm the binding of MLN8237 to HSA. In the ligand orientation assay, the binding sites of MLN8237 were validated through two site-specific spy molecules (warfarin sodium and ibuprofen, which are two known site-selective probes) by using STD and WaterLOGSY NMR competition techniques. These competition experiments demonstrate that both spy molecules do not compete with MLN8237 for the specific binding site. The AutoDock-based blind docking study recognizes the hydrophobic subdomain IB of the protein as the probable binding site for MLN8237. Thermodynamic investigations by isothermal titration calorimetry (ITC) reveal that the non-covalent interaction between MLN8237 and HSA (binding constant was approximately 10⁵ M⁻¹) is driven mainly by favorable entropy and unfavorable enthalpy. In addition, synchronous fluorescence, circular dichroism (CD), and 3D fluorescence spectroscopy suggest that MLN8237 may induce conformational changes in HSA.

Protein-ligand binding affinity determination by the waterLOGSY method: An optimised approach considering ligand rebinding

WaterLOGSY is a popular ligand-observed NMR technique to screen for protein-ligand interactions, yet when applied to measure dissociation constants (K_D) through ligand titration, the results were found to be strongly dependent on sample conditions. Herein, we show that accurate K_Ds can be obtained by waterLOGSY with optimised experimental setup.

Complexation induced aggregation and deaggregation of acridine orange with sulfobutylether-β-cyclodextrin

The present study illustrates intriguing switching of multi-mode binding interactions of acridine orange dye with a sulfobutylether-β-cyclodextrin host.

Unraveling multiple binding modes of acridine orange to DNA using a multispectroscopic approach

The interaction of acridine orange (AOH(+)) with calf thymus DNA (ct-DNA) under different dye-DNA conditions has been investigated in detail using multispectroscopic techniques, unraveling a number of hitherto unexplored intricacies of dye-DNA binding. The observed results intriguingly show contrasting binding features when low (2.4 μM) and significantly high (23 μM) dye concentrations are used. It is conclusively inferred from absorption, steady-state fluorescence, circular dichroism, fluorescence decay and anisotropy decay studies that at low [DNA] to [dye] ratio, especially with higher dye concentration, dimeric AOH(+) predominantly binds externally to DNA surfaces through electrostatic interactions. At sufficiently high [DNA] to [dye] ratios, however, the interaction intriguingly changes to monomeric AOH(+) bound to DNA, predominantly in the intercalative mode between DNA base pairs, with partly an electrostatic binding on DNA surfaces. With very low initial dye concentration, monomeric (AOH(+)) mostly binds to DNA through intercalative and electrostatic modes for most DNA to dye ratios. The present study demonstrates a systematic correlation of the striking changes in the photophysical properties of the dye upon multimode binding with DNA. The observed results are of great significance in understanding the fundamental insights of dye/drug binding to DNA hosts, of use in the design of effective therapeutic agents.