Dynamics of solvent and rotational relaxation of Coumarin 153 in a room temperature ionic liquid, 1-butyl-3-methylimidazolium octyl sulfate, forming micellar structure

Exploration of the impact of graphene oxide, acetylenic gemini, and CTAT on the photophysical and aggregation properties of dipolar coumarin 153

Advanced spectroscopic techniques have been utilized to study the interaction between the laser dye coumarin 153 (C153) and graphene oxide (GO) nanoparticles. GO was synthesized using a modified Hummers' method and characterized by UV-vis spectroscopy, Raman laser spectroscopy, FTIR-ATR spectroscopy, FESEM, HR-TEM, and XRD techniques. The GO@C153 composite was formed by mixing two aqueous solutions of GO and C153 due to their strong interaction through stacking and hydrophobic interactions. In this case, GO acts as an effective fluorescence quencher for C153 molecules, which undergo H-type aggregation in the presence of GO. The Stern-Volmer equation and time-dependent fluorescence studies were utilized to analyse the mechanism of fluorescence quenching. According to the findings, both static and dynamic quenching processes are responsible for the reduction in fluorescence intensity. The effect of surfactants (both cetyltrimethylammonium p-toluenesulfonate (CTAT) and synthesized N,N'-dihexadecyl-N,N,N',N'-tetramethyl-N,N'-but-2-ynediyl-di-ammonium chloride (16-4-16)) on the aggregation and photophysical properties of the dye was investigated using surface tensiometry, conductometry, UV-vis absorption spectroscopy, steady-state fluorescence measurements, DLS, and time-dependent fluorescence spectroscopy. Surfactants change the microenvironment of the C153 dye, leading to spectrum shifting and a higher quantum yield, which causes a rapid rise in fluorescence intensity in the micellar medium. It has been noted that in a micellar medium rather than in an aqueous one, the luminous intramolecular charge transfer (ICT) state of C153 stabilises. Lastly, we investigated the photophysical behavior of the GO-C153-micelle ternary system and discovered that, in the presence of a micellar medium, the quenched and blue-shifted (H-type aggregation) fluorescence peak of C153 (in the presence of GO) began to intensify once more. The main goal of this work is to create an effective and fairly cost powerful fluorescence sensor. Additionally, the ternary system (GO-C153-micelle) analytical idea can be employed to identify the onset of micelle formation. In wastewater treatment analysis, the GO-C153-surfactant ternary system concept can also be used to regenerate the adsorbent (in this case, GO) from dye molecules by allowing the dye molecules to exit the adsorbent and enter the micellar medium.

Hydrophobically Driven Morphologically Diverse Self-Assembled Architectures of Deoxycholate and Imidazolium-Based Biamphiphilic Ionic Liquids in Aqueous Medium

Biamphiphilic ionic liquids (BAILs) having amphiphilic cation and anion are thought to exhibit improved surface activity and colloidal stability to be utilized in different applications. For their effective use, a control over synergetic hydrophobic and electrostatic interactions between oppositely charged ions along with the possibility of tuning of hydrophobicity of the core of aggregates is required. Focusing on this, new BAILs comprising a bile salt anion, deoxycholate, [DC]^-, and 1-alkyl-3-methylimidazolium cations, [C _nmim]⁺ ( n = 2, 4, 6, 8, and 12), were synthesized and characterized for their behavior at air-solution interface as well as in bulk. The synthesized BAILs exhibit high surface activity and self-assemble in the form of different architectures ranging from nanosheets (NSs), nanorods, and vesicles with varying hydrophobicities of the formed core of aggregates, depending on the length of alkyl chain of [C _nmim]⁺. Analysis of various parameters obtained from investigated techniques suggested the changing role of [C _nmim]⁺ from a counterion ( n = 2 and 4) to a cosurfactant ( n = 8 and 12) via a borderline case of [C₆mim]⁺. This changeover in the nature of counterion controlled by hydrophobicity of alkyl chain resulted in morphological diversification in self-assembled architectures via varying set of interactions. It is believed that the present work would offer new perspectives in the self-assembly phenomenon of surfactants in general and surface active ionic liquids in particular to devise new strategies for inducing morphology-dependent functionality in self-assembled structures of BAILs.

Physicochemical Understanding of Self-Aggregation and Microstructure of a Surface-Active Ionic Liquid [C4mim] [C8OSO3] Mixed with a Reverse Pluronic 10R5 (PO8EO22PO8)

Physicochemical studies on aqueous mixtures of ionic liquids (ILs) and reverse pluronics are limited. Self-aggregation dynamics and microstructure of a surface-active IL (SAIL), 1-butyl-3-methylimidazolium octylsulfate [C₄mim] [C₈OSO₃], in the presence of a reverse pluronic, PO₈EO₂₂PO₈ (known as 10R5), were studied using isothermal titration calorimetry (ITC), high-resolution nuclear magnetic resonance (NMR), and small-angle neutron scattering (SANS) methods. Also, cryo-/freeze-fracture transmission electron microscopy was employed to determine the microstructures of SAIL/10R5 mixtures. The ITC and NMR results revealed facilitation of SAIL aggregation in the presence of 10R5 forming mixed aggregates as well as free SAIL micelles. ²H spin relaxation rate data pointed out the onset of slow dynamics of the aqueous SAIL/10R5 mixture with an increase in either the former or the latter. Globular morphologies of the mixed species as well as their individual components were corroborated from the measurements. The preferential location of interaction of the SAIL with the 10R5 was identified from ¹³C NMR chemical shift findings to be in the interfacial region of the assembled SAIL. The formed species were mixed interacted aggregates but not mixed micelles that arise from mixed surfactants. The physicochemical information acquired herein would enrich the literature on the 10R5/SAIL mixed microheterogeneous systems having importance in the making of useful green drug carrier systems and templates for the synthesis of nanomaterials.

Ionic liquid mediated micelle to vesicle transition of a cationic gemini surfactant: a spectroscopic investigation

In this contribution, we have examined a composition dependent self aggregated structural modification of a catanionic mixture of the surface active ionic liquid (IL) 1-butyl-3-methylimidazolium octyl sulphate and a cationic gemini surfactant (14-5-14) in aqueous medium. We have observed that the hydrodynamic diameter of the aggregates increases with increasing IL concentration and microscopic evidence (HRTEM, FESEM, and LCSM) shows the formation of vesicle like aggregates (Dh ≈ 200 nm) at XIL = 0.5. The steady state fluorescence anisotropy of the membrane binding probe DPH shows a micelle to vesicle transition at this composition. The viscosity of the solution shows a peak at XIL = 0.3, indicating the formation of a worm like micelle as an intermediate of the micelle to vesicle transition. The rotational dynamics shows a stiffer surfactant packing in the vesicles compared to the micelles, whereas, the solvation dynamics measurements indicate a higher abundance of bound type water in the vascular medium compared to that for the micelle. The formed vesicles also show stability towards temperature and biomolecules, which can be used for respective applications.

Spectroscopic and Calorimetric Studies of Molecular Recognitions in a Dendrimer-Surfactant Complex

Molecular recognitions, causing supramolecular complex formation between a hyperbranched polymer molecule (polyamidoamine (PAMAM) dendrimer generation 3) with oppositely charged surfactant sodium dodecyl sulfate (SDS) in aqueous solution, were studied by using various spectroscopic techniques and calorimetric titration of heat change measurements. Spectroscopic measurements were performed using dynamic Stokes shift (DSS), rotational anisotropy decay, and translational diffusion of a fluorescent probe molecule coumarin 153 (C153) noncovalently attached to the dendrimer-surfactant complex. All these studies unanimously confirm that the critical aggregation concentration (CAC) of SDS falls to ∼0.8 mM (from its critical micelle concentration (CMC) ∼ 8 mM) in the presence of ∼0.2 mM dendrimer. Further studies of isothermal titration calorimetry (ITC) measurement show that the CAC of SDS in the presence of dendrimer remains invariant to the dendrimer concentration. Complexation reaction between SDS and dendrimer is highly exothermic in nature. A maximum heat release (ΔH∼ -6.6 kJ/mol of SDS binding) was observed at a SDS-to-dendrimer mole ratio of ∼3-5; where up to 3 to 5 SDS molecules were encapsulated by one dendrimer molecule to form dendrimer-SDS encapsulation complex. When negatively charged SDS was replaced with a positively charged surfactant dodecyl-trimethylammonium-bromide (DTAB), we found that the DTAB hardly interacted with positively charged dendrimer due to the charge-charge repulsions.

Self-assembly of a short-chain ionic liquid within deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) are receiving increased attention from both academic and industrial research due to their immense application potential. These designer solvents are environmentally friendly in nature with tunable physicochemical properties. In the present investigation, we have studied the aggregation behavior of a short-chain IL 1-butyl-3-methylimidazolium octylsulphate [Bmim][OS] within aqueous DESs using fluorescence, UV-vis, dynamic light scattering (DLS) and FT-IR spectroscopic techniques. We have prepared two DESs, ChCl–urea and ChCl–Gly, which are obtained by heating a mixture of an ammonium salt choline chloride with hydrogen bond donor urea or glycerol, respectively, in 1 : 2 molar ratios. The local microenvironment and size of the aggregates are obtained from steady state fluorescence (using pyrene and pyrene-1-carboxaldehyde as polarity probes) and DLS measurements, respectively. DLS results shows that IL [Bmim][OS] forms relatively larger micelles within the aqueous solution of DES ChCl–urea (avg. hydrodynamic radii = 209 nm) than compared to ChCl–Gly (avg. hydrodynamic radii = 135 nm). A significant decrease in the critical micelle concentration and increase in the aggregation number (N_agg) are observed within DES solutions as compared to that in water, thus indicating that the micellization process of the IL [Bmim][OS] is much favored in the DES solutions. Molecular interactions of [Bmim][OS] in DESs are revealed from FT-IR spectroscopic investigation. Furthermore, these systems were applied to study the IL-drug binding of the antidepressant drug promazine hydrochloride (PH).

Self-assembly of short-chain imidazolium-based ILs within DESs have been investigated by fluorescence, UV-Vis, DLS and FT-IR spectroscopy. Further, these micellar systems [Bmim][OS]-DESs are utilized to study the IL-drug binding of an antidepressant drug (PH).

Colloidal systems of surface active ionic liquids and sodium carboxymethyl cellulose: physicochemical investigations and preparation of magnetic nano-composites

Carboxymethyl cellulose-surface active ionic liquid colloidal formulations for preparation of magnetic nano-composites.

Self-assembly of a short-chain ionic liquid within deep eutectic solvents

Self-assembly of short-chain imidazolium-based ILs within DESs have been investigated by fluorescence, UV-Vis, DLS and FT-IR spectroscopy. Further, these micellar systems [Bmim][OS]-DESs are utilized to study the IL-drug binding of an antidepressant drug (PH).

Probing the Interaction between a DNA Nucleotide (Adenosine-5'-Monophosphate Disodium) and Surface Active Ionic Liquids by Rotational Relaxation Measurement and Fluorescence Correlation Spectroscopy

This article demonstrates the interaction of a deoxyribonucleic acid (DNA) nucleotide, adenosine-5'-monophosphate disodium (AMP) with a cationic surface active ionic liquid (SAIL) 1-dodecyl-3-methylimidazoium chloride (C₁₂mimCl), and an anionic SAIL, 1-butyl-3-methylimidazolium n-octylsulfate ([C₄mim][C₈SO₄]). Dynamic light scattering (DLS) measurements and ¹H NMR (nuclear magnetic resonance) studies indicate that substantial interaction is taking place among the DNA nucleotide (AMP) and the SAILs. Moreover, cryogenic transmission electron microscopy (cryo-TEM) suggests that SAILs containing micellar assemblies are transformed into larger micellar assemblies in the presence of DNA nucleotides. Additionally, the rotational motion of two oppositely charged molecules, rhodamine 6G perchlorate (R6G) and fluorescein sodium salt (Fl-Na), have been monitored in these aggregates. The rotational motion of R6G and Fl-Na differs significantly between SAILs micelles and SAILs-AMP containing larger micellar aggregates. The effect of negatively charged DNA nucleotide (AMP) addition into the cationic and anionic SAILs is more prominent for the cationic charged molecule R6G than that of anionic probe Fl-Na due to the favorable electrostatic interaction between the AMP and cationic R6G. Moreover, the influence of the anionic DNA nucleotide on the cationic and anionic SAIL micelles is monitored through the variation of the lateral diffusion motion of oppositely charged probe molecules (R6G and Fl-Na) inside these aggregates. This variation in diffusion coefficient values also suggests that the interaction pattern of these oppositely charged probes are different within the SAILs-nucleotide containing aggregates. Therefore, both rotational and translational diffusion measurements confirm that the DNA nucleotide (AMP) renders more rigid microenvironment within the micellar solution of SAILs.

Surfactants induced release of a red emitting dye from the nanocavity of a molecular container: A spectroscopic and calorimetric study

Supramolecular interaction of a red emitting dye Nile blue A (NBA) with Cucurbit[7]uril (CB7) in aqueous solution was studied and the release of the dye from the hydrophobic cavity of CB7 was reported. To investigate the supramolecular host-guest complex formation and release of dye, we have used the steady state absorption, fluorescence and time resolved fluorescence emission spectroscopy, (1)H NMR spectroscopy and isothermal titration calorimetry (ITC). The spectral properties of NBA were changed in the presence of CB7. The change in spectral features of NBA in presence of CB7 indicates the formation of supramolecular host-guest complexes. By using the SED equation the diameter of the complex was estimated. The complex formation further affirmed by the (1)H NMR study. Upfield and downfield shifts of the protons of NBA was observed in both the aliphatic and aromatic region. From the ITC measurement, we have drawn up the forces involved for the complexation of NBA with CB7. We have studied the release of NBA from the hydrophobic cavity of CB7 by using ionic, neutral surfactants and ionic liquid with the help of spectroscopic and calorimetric techniques. It is observed that on addition of SDS and ionic liquid (<cmc) ion-pair formation takes place between NBA and surfactant monomer whereas, it was not observed for neutral and cationic surfactant. Above cmc of the surfactants, complex is formed between NBA and micelle.

Effect of alkyl chain functionalization of ionic liquid surfactants on the complexation and self-assembling behavior of polyampholyte gelatin in aqueous medium

The interactional behavior of ILSs towards gelatin forming structurally different ILS mediated self-assemblies depending on the nature of the ILS and counterion binding is shown.

Aggregation of 1-alkyl-3-methylimidazolium octylsulphate ionic liquids and their interaction with Triton X-100 micelles

Halogen-free surface active and biamphiphilic ionic liquids 1-alkyl-3-methylimidazolium octylsulphates (Cnmim C₈SO₄, n = 4, 6, 8, 10) were synthesized and their aqueous solution behaviour was studied using NMR and scattering techniques.

Micellization behavior of morpholinium-based amide-functionalized ionic liquids in aqueous media

Morpholinium-based amide-functionalized ionic liquids (ILs) [C(n)AMorph][Br], where n = 8, 12, and 16, have been synthesized and characterized for their micellization behavior in aqueous medium using a variety of state of the art techniques. The adsorption and micellization behavior of [CnAMorph][Br] ILs at the air-solution interface and in the bulk, respectively, has been found to be much better compared to that observed for nonfunctionalized homologous ILs and conventional cationic surfactants, as shown by the comparatively higher adsorption efficiency, lower surface tension at the critical micelle concentraiton (γ(cmc)), and much lower critical micelle concentration (cmc) for [C(n)AMorph][Br] ILs. Conductivity measurements have been performed to obtain the cmc, degree of counterion binding (β), and standard free energy of micellization (ΔG(m)°). Isothermal titration calorimetry has provided information specifically about the thermodynamics of micellization, whereas steady-state fluorescence has been used to obtain the cmc, micropolarity of the cybotactic region, and aggregation number (N(agg)) of the micelles. Both dynamic light scattering and atomic force microscopy have provided insights into the size and shape of the micelles. 2D (1)H-(1)H nuclear Overhauser effect spectroscopy experiments have provided insights into the structure of the micelle, where [C16AMorph][Br] has shown distinct micellization behavior as compared to [C8AMorph][Br] and [C12AMorph][Br] in corroboration with observations made from other techniques.

Anisotropic dynamics of guest molecules in aerosol OT lamellar structures

The present work is focused on developing a description of an anisotropic microheterogeneous medium, exploiting the dynamics of a guest molecule. The medium in question is the lamellar structures formed in the aqueous layer of ternary mixtures containing aerosol OT (AOT), water and n-heptane. The guest used in this study is the fluorescent probe, coumarin 153 (C153). The dynamics of this molecule, within the lamellar structure, have been studied using a combination of steady state and time resolved fluorescence, as well as fluorescence correlation spectroscopy (FCS). The fluorophore is strongly solvatochromatic and so, the wavelength of excitation can be tuned so as to selectively excite fluorescent molecules residing in different regions of the microheterogeneous media, even if the spatial separation between these regions is below the diffraction limit. The excitation wavelength in the present experiments is chosen so as to exclusively excite those C153 molecules that reside in the hydrophobic region of the lamellar structures. This triggers two different modes of diffusion, one along and the other perpendicular to the bilayers of the AOT. Thus, the dynamics of the fluorescent probe provide an elegant manifestation of the anisotropy of the host medium.

Spontaneous transition of micelle-vesicle-micelle in a mixture of cationic surfactant and anionic surfactant-like ionic liquid: a pure nonlipid small unilamellar vesicular template used for solvent and rotational relaxation study

The micelle-vesicle-micelle transition in aqueous mixtures of the cationic surfactant cetyl trimethyl ammonium bromide (CTAB) and the anionic surfactant-like ionic liquid 1-butyl-3-methylimidazolium octyl sulfate, [C4mim][C8SO4] has been investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), surface tension, conductivity, and fluorescence anisotropy at different volume fractions of surfactant. The surface tension value decreases sharply with increasing CTAB concentration up to ∼0.38 volume fraction and again increases up to ∼0.75 volume fraction of CTAB. Depending upon their relative amount, these surfactants either mixed together to form vesicles and/or micelles, or both of these structures were in equilibrium. Fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in this system at different composition of surfactant indicates the formation of micelle and vesicle structures. The apparent hydrodynamic diameter of these large multilamellar vesicles is about ∼200 nm-300 nm obtained by DLS measurement and finally confirmed by TEM micrographs. The large multilamellar vesicles are transformed into small unilamellar ones by sonication using a Lab-line instruments probe sonicator with a diameter of ∼90-125 nm. To investigate the heterogeneity, solvent, and rotational relaxation of coumarin-153 (C-153) have been investigated in these unilamellar vesicles by using picosecond time-resolved fluorescence spectroscopic technique. The solvation dynamics of C-153 in these vesicles is found to be biexponential with average time constant ∼580 ps. This indicates the slow relaxation of water molecules in the surfactant bilayer. In accordance with solvation dynamics, fluorescence anisotropy analysis of C-153 in unilamellar vesicles also indicates hindered rotation compared to bulk water.

Rotational diffusion of coumarin 153 in nanoscopic micellar environments of n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol mixtures

The microstructure of mixed micelles containing n-dodecyl-β-D-maltoside and n-dodecyl-hexaethylene-glycol, two nonionic surfactants belonging to the alkyl polyglucoside and polyoxyethyelene alkyl ether families, respectively, has been investigated. With the aim of understanding how the micellar composition affects the microenvironmental properties of micelles, we have examined the photophysics and dynamics of the neutral probe coumarin 153 in the binary mixtures of the surfactants across the entire composition range. We present data on the steady-state absorption and emission spectra of the probe, as well as fluorescence lifetimes and both steady-state and time-resolved fluorescence anisotropies. These data indicate that the participation of the ethoxylated surfactant in the mixed micelle induces an increasing hydration in the palisade layer of the micelle, which forces the probe to migrate toward the inner micellar region, where it senses a slightly less polar environment. The time-resolved fluorescence anisotropy data were analyzed on the basis of the two-step and wobbling-in-cone model. The average reorientation time of the probe molecule was found to decrease with the presence of the ethoxylated surfactant, in good agreement with steady-state fluorescence anisotropy data, suggesting a reduction of the microviscosity in the solubilization site of the probe. The behavior of all diffusion reorientation parameters was analyzed on the basis of two factors: the micellar hydration and the headgroup flexibility of both surfactants. It was concluded that the increasing participation of the ethoxylated surfactant induces a greater hydration in the micellar palisade layer, producing the formation of a less compact microenvironment where the probe experiences a faster rotational reorientation.

Surface adsorption and micelle formation of imidazolium-based zwitterionic surface active ionic liquids in aqueous solution

A novel class of zwitterionic surface active ionic liquids (SAILs), N-alkyl-N'-carboxymethyl imidazolium inner salts ([N-C(n), N'-CO(2)-Im], n=10, 12, 14), was synthesized. Their aggregation behavior in aqueous solution was investigated by surface tension, isothermal titration calorimetry, and steady-state fluorescence. Compared with the reported imidazolium-based cationic SAILs, 1-alkyl-3-methylimidazolium bromide ([C(n)mim]Br) and zwitterionic betaine surfactants, (C(n)H(2)(n)(+1)N(CH(3))(2)CH(2)COO(-)), [N-C(n), N'-CO(2)-Im] exhibits significantly lower critical micelle concentration (cmc) and surface tension at cmc (γ(cmc)) values. It is attributed to the incorporation of a deprotonated carboxylic group into the head group, which weakens the electrostatic repulsion between head groups and favors micellization. The micellar aggregation number of [N-C(n), N'-CO(2)-Im] is larger than that of [C(n)mim]Br, while less than that of C(n)H(2)(n)(+1)N(CH(3))(2)CH(2)COO(-). Similar to the traditional zwitterionic surfactants, the surface activity and adsorption properties of [N-C(12), N'-CO(2)-Im] at air/water interface have a slight variation with temperature, pH, and ionic strength. This indicates that the present zwitterionic SAILs display the aggregation behavior much similar to zwitterionic surfactants, distinctly different from imidazolium-based cationic SAILs. Sets in low sensitivity to the environmental conditions, superior surface activity and unique physicochemical properties of ionic liquids, [N-C(n), N'-CO(2)-Im] can be exploited for utilizing as a potential substitute for conventional surfactants in certain fields.

Influence of counterions on micellization of tetramethylammonium perfluorononanoic carboxylate in 1-butyl-3-methylimidazolium ionic liquid

The influence of counterions on micellization of perfluorononanoic carboxylate ammonium salts in water and 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim]BF(4)) solutions was investigated by surface tension and (19)F NMR measurements and freeze-fracture transmission electron microscopy (FF-TEM) observations. Changes in the counterions of the fluorocarbon surfactants have different effects on the two solvents. With the increase of counterion volume, the critical micelle concentration (cmc) value of relevant fluorinated surfactant decreases in aqueous solutions. This is because the counterions with larger size, such as (+)N(CH(3))(4), can be little hydrated, which can screen the electrostatic repulsion of the headgroups of the fluorocarbon surfactant and thus facilitate micelle formation. However, the fluorocarbon surfactants can dissolve and form micelles in [bmim]BF(4) only when they provide with largest counterion such as (+)N(CH(3))(4). This is because the counterion, (+)N(CH(3))(4), disperses the charge of the cations, which could weaken the electrostatic interaction between the ion pair of the surfactant, leading to a higher degree of counterion binding. The thermodynamic parameters estimated from the temperature dependence of the cmc values tell us that the micelle formation for tetramethylammonium perfluorononanoic carboxylate (C(8)F(17)COON(CH(3))(4), PFNT) in ionic liquids (ILs) is an entropy-driven process at low temperature but an enthalpy-driven process at high temperature. The driving force of the micellization for fluorocarbon surfactants in [bmim]BF(4) is the solvophobic effect, due to the hydrophobic and oleophobic properties of fluorocarbon chains.

Binding of organic dyes with human serum albumin: a single-molecule study

Kinetics of binding of dyes at different sites of human serum albumin (HSA) has been studied by single-molecule spectroscopy. The protein was immobilized on a glass surface. To probe different binding sites (hydrophobic and hydrophilic) two dyes, coumarin 153 (C153, neutral) and rhodamine 6G (R6G, cationic) were chosen. For both the dyes, a major (ca. 96-98%) and minor (ca. 3%) binding site were detected. Rate constants of association and dissociation were simultaneously determined from directly measuring fluctuations in fluorescence intensity (τ(off) and τ(on)) and from this the equilibrium (binding) constants were calculated. Fluorescence lifetimes at individual sites were obtained from burst-integrated lifetime analysis. Distributions of lifetime histograms for both the probes (C153 and R6G) exhibit two maxima, which indicates the presence of two binding domains in the protein. Unfolding of the protein has been studied by adding guanidinium hydrochloride (GdnHCl) to the solution. It is observed that addition of GdnHCl affects the dissociation and association kinetics and hence, binding equilibrium of the association of C153. However, the effect of binding of R6G is not affected much. It is proposed that GdnHCl affects the hydrophobic binding sites more than the hydrophilic site.

Binding characteristics and molecular mechanism of interaction between ionic liquid and DNA

The binding characteristics and molecular mechanism of the interaction between a typical ionic liquid (IL), 1-butyl-3-methylimidazolium chloride ([bmim]Cl), as a green solvent and DNA were investigated for the first time by conductivity measurements, fluorescence spectroscopy, dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryo-TEM), circular dichroism spectroscopy, (31)P nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared spectroscopy, isothermal titration calorimetry (ITC), and quantum chemical calculations. It was found that the critical aggregation concentration of [bmim]Cl is decreased in the presence of DNA, and the addition of [bmim]Cl induced a continuous fluorescence quenching of the intercalated probe ethidium bromide (EtBr), indicating that the interaction between the ionic liquid and DNA is sufficiently strong to exclude EtBr from DNA. DLS results show that [bmim]Cl can induce a coil-to-globule transition of DNA at a low IL concentration, which was confirmed by the cryo-TEM images of DNA-IL complexes. With [bmim]Cl added, the resulting globular DNA structures and the extended DNA coils are first compacted, and then grow in size. During the binding process, DNA maintains the B-form, but the base packing and helical structure of DNA are altered to a certain extent. The (31)P NMR and IR spectra indicate that the cationic headgroups of bmim(+) groups interact with the phosphate groups of DNA through electrostatic attraction, and the hydrocarbon chains of bmim(+) groups interact with the bases through strong hydrophobic association. ITC results reveal the interaction enthalpy between [bmim]Cl and DNA and show that the hydrophobic interaction between the hydrocarbon chains of [bmim]Cl and the bases of DNA provides the dominant driving force in the binding. On the basis of quantum chemical calculations, it can be inferred that at a low IL concentration, the cationic headgroups of [bmim]Cl would be localized within several angstroms of the DNA phosphates, whereas the hydrophobic chains would be arranged parallel to the DNA surface. When the IL concentration is above 0.06 mol/L, the cationic headgroups are near DNA phosphates, and the hydrocarbon chains are perpendicularly attached to the DNA surface.

Interaction of bovine serum albumin with dipolar molecules: fluorescence and molecular docking studies

Interaction of bovine serum albumin (BSA) with two series of dipolar molecules having both rigid and flexible structures has been studied by monitoring the spectral and temporal behavior of the intramolecular charge transfer fluorescence of the systems. The binding sites of the molecular systems in BSA have been located with the help of docking studies. Three different sites of varying hydrophobicity have been identified where these molecules are located. Binding in the hydrophobic domains of BSA leads to a blue shift of the fluorescence spectra and an enhancement of fluorescence intensity and lifetime. This enhancement is found to be the largest for flexible systems in which internal motion serves as a nonradiative decay route. In the BSA-bound condition, some of the dipolar molecules exhibit not-so-common "dip-rise-dip" time-resolved fluorescence anisotropy profiles. It is shown that a large difference of the fluorescence lifetimes of the protein-bound and unbound molecules is one of the factors that contributes to this kind of anisotropy profiles. As internal motion is often responsible for the short fluorescence lifetime of the flexible dipolar molecules, a large increase in the fluorescence lifetime of these systems occurs if binding to BSA leads to disruption/prevention of this motion. It thus appears that it might be possible to obtain information on the prevention/disruption of nonradiative pathway on protein binding from the anisotropy profiles of the kind discussed above. However, since the present study reveals cases where a large change in fluorescence lifetime also occurs due to other reasons, one needs to be careful prior to making any conclusion.