Micellar Environment Research Articles

Photometric Study of the Interaction of Zinc (II) Complexes of Schiff Bases with Cetyltrimethyl Ammonium Bromide

AbstractA detailed photophysical behavioral study of zinc (II) complexes of Schiff bases 2,6‐bis((E)‐((2‐(dimethylamino) ethyl)imino)methyl)‐4‐R‐phenol, where R = methyl (1)/isopropyl (2)/tertiary butyl (3)/chloro (4) for ligands 1 to 4 (HL1 to HL4) have been done by utilising the surfactant cetyltrimethyl ammonium bromide as the biomimicking environment. Steady state absorption and emission studies have been studied to investigate the course of deciphering of the photophysical behavior of the complexes. The study reveals modification of the photophysical properties of the complexes based on the effect of polarity of the micellar environment. The studies reported in the present investigation describe the initial reduction of fluorescent intensity of all the four complexes followed by an escalation in intensity. The binding constant values reveal that the Schiff bases bind to the micellar compartment. The course of binding is however found to be dependent on the functional group of the ligand which is studied and reported in the present context.

Fluorescent signal transduction in a self-assembled Hg2+ chemosensor tuned by various interactions in micellar aqueous environment

A new OFF-ON fluorescent chemosensor for ions Hg2+ in water has been designed by associating fluorophore 10-methylacridinium perchlorate (MA+ClO4−) with a selective lipophilic sulfur-containing ligand, such as N,N-bis (2-hydroxyethylthio-1-ethyl)dodecyl amine (DBSO). The communication between the two subunits is only due to their spatial closeness, ensured by sodium dodecyl sulfate (SDS) micellar aggregates. The quenching of MA+ fluorescence is ascribed to a photoinduced electron-transfer from the ligand to the excited fluorophore, still the emission is quickly restored by addition of Hg2+ (as chloride salt) due to the complex formation between ligand and metal. The sensitivity of the system improves by decreasing the ligand concentration, as well as by changing the stoichiometry of the ligand-Hg2+ complex from 1:1 to 2:1. DFT calculations give interesting insights into the structures of the complexes and provide useful information on the sensorial activity experimentally observed. The replacement of MA+ClO4− with 9-cyanoanthracene, a fluorophore differently localized on the micellar aggregate, highlights the effect of Cl− coordination to the complexed Hg2+ on the operating mechanism of the sensor. These results demonstrate as the micellar aggregates provide for a straightforward means to regulate the response of the sensorial system without altering the recognition or reporter unit.

Photophysical Properties of Coumarin 1 in Bile Salt Aggregates: An Insight into the Role of Bile Salt Structure on the Aggregation Behavior.

The photophysical behavior of Coumarin 1 (Cou1), a well-known 7-aminocoumarin derivative, is very sensitive to the microenvironment in which it resides. In the present study, the effect of six bile salt variants on the photophysical behavior of Cou1 has been investigated. Dihydroxy (deoxycholates) as well as trihydroxy (cholates) bile salts with conjugated and unconjugated side chains have been chosen to get insight into the role of bile salt structure on the microenvironment of Cou1. Cou1 photophysics was found to be extremely sensitive to the aggregation process of the bile salt variants. The reduced polarity of the micellar environment stabilizes the planar intramolecular charge transferred state of Cou1, resulting in significant modulation in its photophysics in the bile salt media. The changes in the fluorescence parameters such as fluorescence intensity, emission energy, fluorescence quantum yield, anisotropy, and lifetime of Cou1 reveal that there is a distinct difference in the aggregation behavior of deoxycholates from that of cholates. The deoxycholates form micelles more or less critically similar to those of conventional surfactants, whereas the cholates self-assemble rather noncritically over a wide concentration range, thus signifying the vital role of the extra hydroxyl group in the aggregation pattern of trihydroxy bile salts. The conjugated bile salts are found to provide a relatively more compact, rigid, and hydrophobic microenvironment to Cou1 as compared to their unconjugated counterparts. Considering the significant modulation in the photophysical properties of Cou1, it has been employed as a molecular reporter for monitoring the aggregation process of bile salt variants and important information could be obtained about the effect of bile salt structure on the aggregation pattern and also about the micellar properties.

Study of the Antioxidative Properties of Several Amino Acid‐Type Surfactants and their Synergistic Effect in Mixed Micelle

AbstractCysteine and methionine, two sulfur‐containing amino acids (AA), were introduced in their surfactant forms as potential antioxidants. The antioxidative (AOX) properties of lauroyl methionine (C12‐Met) and lauroyl cysteine (C12‐Cys) was investigated by means of the oxygen radical absorbance capacity assay. Both the surfactants exhibited excellent AOX behavior at the premicellar state and micellar medium. The AOX behavior was found to be comparable for both the surfactants at their premicellar states. However, in micellar medium, C12‐Met showed better AOX property than C12‐Cys. The AOX power of the surfactants was compared with other previously developed AA‐type surfactants. The order of the AOX power was found to be: C12‐tryptophan > C12‐tyrosine ≈ C12‐methionine ≈ C12‐cysteine > C12‐histidine at the premicellar state and C12‐tryptophan > C12‐tyrosine > C12‐methionine > C12‐cysteine > C12‐histidine at the micellar state. C12‐Cys displayed lower AOX property in micellar medium due to its dimer formation tendency. Based on the HPLC and UPLC‐Q‐TOF‐MS analysis, the dimer formation of C12‐Cys was found to be accelerated due to the micellar environment and results into negative synergistic effect on other aromatic AA‐type surfactants. However, the presence of C12‐His in the micellar solution of C12‐Cys resulted no synergistic effect due to stronger H‐bonding between the surfactants and resulting less dimer formation.

Interaction of imidazolium based ionic liquids with aqueous Triton X-100 surfactant: Clouding, fluorescence and NMR studies

The micellar environment of aqueous Triton X-100 is dependent on its concentration and is driven by changes in the surfactant-water interactions. A mutual solubility curve is observed in clouding of Triton X-100 with a lower critical solution temperature at 339.5 K. Thermodynamic analysis indicate that the clouding process is entropy and enthalpy controlled below and above the lower critical solution temperature respectively. Enthalpy-entropy compensation effect has been invoked to discuss the clouding process. Addition of imidazolium based ionic liquids with variable alkyl chain length affect the micellar structure differently. The effect of different concentrations of ionic liquids on clouding of 2% Triton X-100 is studied to gauge their influence on the clouding process. The observed variations in clouding can be attributed to the hydrophobicity of the alkyl chain length of ionic liquids where, the hexyl and octyl alkyl chains results in substantial increase in cloud point for 2% Triton X-100. Fluorescence, 1H NMR and DLS studies are conducted to authenticate the changes occurring in the micellar systems. While fluorescence studies have shown a decrease in polarity of the micellar environment with increase in alkyl chain length, 1H NMR studies confirm the penetration of long alkyl chain ionic liquid in the micelles. DLS measurements show that ionic liquids are adsorbed on the micellar surface.

Structural transition dynamics of biologically active flavins in alkylglucoside surfactants aggregates.

The photophysics and structural transition dynamics of a bio-active flavin lumichrome (LM) entrapped in two sugars based neutral surfactants were reported. Sugar-based surfactants, which were used for research purpose are potential environmentally friendly, green alternative amphiphilic surface active substance compared to other kinds of common surfactants. Here, two alkyl glucoside surfactants n-octyl-β-D-glucopyranoside (OBG) and n-octyl-β-D-thioglucopyranoside (OBTG) were used. This work is carried out by using steady-state absorption and fluorescence emission spectroscopy along with time-resolved fluorescence emission techniques. Photophysics of LM was modulated several folds in these two sugar-based neutral micelles. LM exhibits excitation and emission wavelength dependent fluorescence properties in these two sugars based neutral micelles. LM confined in the micellar environments exhibited structural transition dynamism, i.e. different kinds of conformers are equilibrated. Herein, different conformers of LM are identified and explained with the help of spectroscopic methods. From the fluorescence anisotropy measurement, it was found that the rotational relaxation time of LM in OBG micelle was more compared to that in OBTG micelle, which indicates that the LM molecule faced much more constrained environment in OBG micellar media.

Native Ion Mobility-Mass Spectrometry Reveals the Formation of β-Barrel Shaped Amyloid-β Hexamers in a Membrane-Mimicking Environment.

The mechanisms behind the Amyloid-β (Aβ) peptide neurotoxicity in Alzheimer's disease are intensely studied and under debate. One suggested mechanism is that the peptides assemble in biological membranes to form β-barrel shaped oligomeric pores that induce cell leakage. Direct detection of such putative assemblies and their exact oligomeric states is however complicated by a high level of heterogeneity. The theory consequently remains controversial, and the actual formation of pore structures is disputed. We herein overcome the heterogeneity problem by employing a native mass spectrometry approach and demonstrate that Aβ(1-42) peptides form coclusters with membrane mimetic detergent micelles. The coclusters are gently ionized using nanoelectrospray and transferred into the mass spectrometer where the detergent molecules are stripped away using collisional activation. We show that Aβ(1-42) indeed oligomerizes over time in the micellar environment, forming hexamers with collision cross sections in agreement with a general β-barrel structure. We also show that such oligomers are maintained and even stabilized by addition of lipids. Aβ(1-40) on the other hand form significantly lower amounts of oligomers, which are also of lower oligomeric state compared to Aβ(1-42) oligomers. Our results thus support the oligomeric pore hypothesis as one important cell toxicity mechanism in Alzheimer's disease. The presented native mass spectrometry approach is a promising way to study such potentially very neurotoxic species and how they could be stabilized or destabilized by molecules of cellular or therapeutic relevance.

Effect of the reverse micelle and oil content in reverse micelle on nonlinear optical properties of Rhodamine B

The optical nonlinearity of Rhodamine B dye at different concentration (10−3 M and 5 × 10−4 M) within reverse micelle prepared by water-in-decane microemulsion having constant molar ratio of water-to-surfactant molecules of 40 (W = [H2O]/[AOT] = 40) as well as function of mass fraction of nano-droplet (MFD) values of 0.01,0.04, 0.07, and 0.1 has been studied by Z-scan method, Fluorescence, FTIR, and UV–Visible. The optical responses of RhB were characterized by measuring the intensity dependent refractive index (n2) of the micellar environment using Z-scan technique. The open aperture Z-scan trace of the dye in bulk water and in AOT micelle displayed both saturable and reverse saturable absorption depending upon different parameters.The closed apertures Z-scan trace also vary as a function of environment and concentrations of dye. The nonlinearity of absorption comes due to droplets interactions and nonlinearity of refractive index appears due to thermal effect.

Unveiling the interaction between carbon nanodot and IR light emitting fluorescent dyes inside the confined micellar environment

In this contribution, using microwave heating of polyethylene glycol (PEG 200), we have prepared carbon nanodot (CND). It is found that in aqueous medium as well as in the micellar solutions of triblock copolymer, P123 and in anionic surface active ionic liquid (SAIL), 1-butyl-3-methylimidazolium n-octylsulfate ([C4mim][C8SO4]) the fluorescence intensity of CND gets quenched in the presence of cationic cyanine dye HITCP and cationic phenoxazine dye Nile blue (NB). Stern-Volmer constant indicates that the quenching of fluorescence intensity is more in the aqueous medium compared to P123 and [C4mim][C8SO4] i.e., (BmimOs) micelles. Time-correlated single photon counting (TCSPC) measurement reveals that the fluorescence quenching is static in nature and the quenching through Förster resonance energy transfer (FRET) process is not dominant here. Addition of both the cationic dye molecules (HITCP and NB) in CND solution generates new excited state absorption peaks in transient absorption measurement. This observation suggests that a transient species is generated through photoinduced electron transfer (PET) between the donor-acceptor pairs and this PET is responsible for the fluorescence quenching of CNDs by the dye molecules. Moreover, this electron transfer kinetics is relatively faster in bulk water compared to that inside P123 and BmimOs micelle due to the intervention of the surfactant molecules between CND and the dye molecules. Therefore, this study provides a new insight about the PET dynamics between CND and fluorescent dye molecules.

Proteorhodopsin Function Is Primarily Mediated by Oligomerization in Different Micellar Surfactant Solutions.

The diverse functionalities of membrane proteins (MPs) have garnered much interest in leveraging these biomolecules for technological applications. One challenge of studying MPs in artificial micellar surfactant environments is that many factors modulate their structures and functionalities, including the surfactants that interact with the MP or their assembly into oligomers. As oligomerization offers a means by which MPs could selectively interact among the copious environmental factors in biological environments, we hypothesized that MP function is predominantly modified by oligomerization rather than interactions with local surfactants that, by comparison, largely interact with MPs nonspecifically. To test this, we study the light-activated proton pump proteorhodopsin (PR) in micellar surfactant solutions because it is functionally active in monomeric and oligomeric forms, the light-activated functionalities of which can be assessed in detail. The surfactant composition and oligomerization are correlated with PR function, as measured by the protonation behaviors of aspartic acid residue 97, which mediates light-activated proton transport, and the associated photocycle kinetics. The results demonstrate that oligomerization dominantly mediates PR function in different surfactant environments, whereas some surfactants can subtly modulate proton-pumping kinetics. This work underscores the importance of understanding and controlling oligomerization of MPs to study and exploit their function.

Surfactant-mediated enzymatic superactivity in water/ionic liquid mixtures, evaluated on a model hydrolytic reaction catalyzed by α-chymotrypsin

In this work, the influence of the C12-, C14-, and C16-alkyl chain derivatives of 1-alkyl-3-methyl-imidazole tetrafluoroborate over the hydrolisis of p-nitrophenyl trimethylacetate catalyzed by α-chymotrypsin was studied in water and water - ionic liquids mixtures. The ionic liquid used is 1-butyl-3-methyl-imidazole tetrafluoroborate (BMIMBF4).1-alkyl-3-methylimidazole tetrafluoroborate derived surfactants can increase the catalytic efficiency of α-chymotrypsin on the hydrolysis of p-nitrophenyl trimethylacetate. This effect is negatively affected by the decrease on the surfactant's critical micelle concentration (CMC) and the partition of the substrate between the micelles and the external media observed above the critical miscellar concentration (CMC). In water/BMIMBF4 mixtures, the presence of the ionic liquid (IL), render the C14 and C16 surfactants insoluble, and overrides the effect elicited by the C12 alkyl chain surfactant on the activity of α-chymotrypsin. The loss of the surfactants influence on the enzymatic activity due to the presence of BMIMBF4 seems to be a complex process, controlled by a decreased affinity between α-chymotrypsin and the surfactant molecules and, to a larger extent, the appearance of an “acquired resistance” of the enzyme to the influence of the surfactants, related to a more compact and resilient conformation of the protein in the presence of BMIMBF4. The influence of BMIMBF4 is not only limited to the surfactant/enzyme interaction, but also it does modifies the partition of the substrate between the aqueous media and the micellar environment.

Selectivity in the Photo-Fries Rearrangement of Some Aryl Benzoates in Green and Sustainable Media. Preparative and Mechanistic Studies.

Irradiation of a series of p-substituted aryl benzoates under N2 atmosphere in homogeneous and micellar media was investigated by means of steady-state condition and of time-resolved spectroscopy. A notable selectivity in favor of the 2-hydroxybenzophenone derivatives was observed in micellar media. The benzophenone derivatives were the main photoproduct. On the other hand, in homogeneous media (cyclohexane, acetonitrile, and methanol) the observed product distribution was entirely different, viz. substituted 2-hydroxybenzophenones, p-substituted phenols, benzyl and benzoic acid were found. The binding constants in the surfactant were also measured and NOESY experiments showed that the aryl benzoates were located in the hydrophobic core of the micelle. Laser flash photolysis experiments led to the characterization of both p-substituted phenoxy radical and substituted 2-benzoylcyclohexadienone transients in homogeneous and micellar environment.

Effect of Cationic/Anionic Mixed Micelles on Reaction Kinetics of Alkaline Hydrolysis of Crystal Violet

Kinetics of alkaline hydrolysis of crystal violet, a triphenylmethane dye in the micellar environment of cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfonate (SDS) and binary mixtures of these surfactants was studied. The regression parameters, together with rate constants and binding constants were obtained by analyzing the rate surfactant profiles using cooperativity model. It was observed that the reaction was catalyzed by both surfactants. The catalytic factor increased by 10 times in SDS and 38 times in CTAB indicating that binding of crystal violet to the micellar surface is stronger in pure CTAB than SDS but the strength drastically reduced in the mixtures of the surfactants. Reduction of binding constant became more important as the mole fraction of CTAB was improved in the mixture. The kinetic data were investigated using Piszkiewicz model and Raghavan-Srinivasan model. The data obtained from the models were in good agreement with the experimental values.

Exogenous delivery of a pyrazole based bioactive probe to natural DNA through non-ionic TX-165 micellar carrier

In this article, we have reported the exogenous delivery of a newly synthesized biologically relevant fluorophore, namely, (E)-1,5-diphenyl-3-styryl-4,5 -dihydro-1H-pyrazole (DSDP) from a non-ionic Triton X-165 micellar nanocarrier to the biomolecular target DNA, exploiting an external stimulant β-cyclodextrin. Vivid steady state and time resolved spectroscopic studies reveal that in the presence of both the micelle and the ctDNA, the probe remains exclusively within the micellar environment because of the higher binding affinity of the probe with the former. However, upon addition of β-CD, the probe is displaced from the micellar environment to get bound to the DNA grooves. Inclusion complexation property of the stimulant with the hydrophobic chain of the constituents of the micelle is assigned to be responsible for the transfer of the probe from the micellar carrier to the DNA. Non-toxic biocompatible micellar carrier and cyclodextrin of proper dimension may be exploited to execute the present strategy of exogenous activation for effective and targeted delivery of drugs.

Submerging a Biomimetic Metallo-Receptor in Water for Molecular Recognition: Micellar Incorporation or Water Solubilization? A Case Study.

Molecular recognition in water is an important topic, but a challenging task due to the very competitive nature of the medium. The focus of this study is the comparison of two different strategies for the water solubilization of a biomimetic metallo-receptor based on a poly(imidazole) resorcinarene core. The first relies on a new synthetic path for the introduction of hydrophilic substituents on the receptor, at a remote distance from the coordination site. The second involves the incorporation of the organosoluble metallo-receptor into dodecylphosphocholine (DPC) micelles, which mimic the proteic surrounding of the active site of metallo-enzymes. The resorcinarene ligand can be transferred into water through both strategies, in which it binds ZnII over a wide pH window. Quite surprisingly, very similar metal ion affinities, pH responses, and recognition properties were observed with both strategies. The systems behave as remarkable receptors for small organic anions in water at near-physiological pH. These results show that, provided the biomimetic site is well structured and presents a recognition pocket, the micellar environment has very little impact on either metal ion binding or guest hosting. Hence, micellar incorporation represents an easy alternative to difficult synthetic work, even for the binding of charged species (metal cations or anions), which opens new perspectives for molecular recognition in water, whether for sensing, transport, or catalysis.

Kinetic Model of Two‐Phase Epoxidation with Ionic Liquids as Micellar Catalysts

AbstractThe biphasic catalytic epoxidation of cyclooctene using the ionic liquid (IL) 1,2‐dimethyl‐3‐octyl‐imidazolium perrhenate ([OMMIM]ReO4) as micellar catalyst and H2O2 as oxidant was investigated. Kinetic experiments were carried out in the intrinsic kinetic regime as proved by variation of stirring rate and temperature. Variation of catalyst concentration allowed for determination of the critical micellar concentration (CMC) of the catalytic IL. The effect of substrate concentrations on the reaction rate was also assessed. Based on the experiments, a kinetic model adapted from enzyme catalysis was proposed to account for the micellar reaction environment. The model takes into account the onset of micelle formation at the CMC. The application of the kinetic model illustrated the good agreement with the experimental data. The model will be applied to other micellar epoxidation reactions and for the design of an appropriate reaction setup in the future.

Concurrent ground and excited state proton transfer of (E)-2-((napthalen-2-ylimino)-methyl)phenol: Modulation in micellar media

The phenomena of ground and excited state proton transfer (GSPT and ESPT) of the synthesized Schiff base, (E)-2-((napthalen-2-ylimino)-methyl)phenol (NMP) have been unraveled in aqueous medium using various spectroscopic techniques. Changes in the absorption and emission spectra of an aqueous solution of the probe upon addition of an external base are ascribed to the proton transfer in both the ground and the excited states. Existence of equilibria between the neutral and the deprotonated species in the two electronic states is established from the observation of the isosbestic and isoemissive points in the absorption and emission spectra. Acid dissociation constants in the ground state (pKa) and photoexcited state (pKa*) of NMP in aqueous medium have been derived from the steady-state experiments using Weller’s method. The time resolved experiments clearly indicate the reversibility of the proton transfer reaction in the photo-excited state. The pKa* value determined from the kinetic measurements corroborates that obtained from the steady state methods. The higher value of pKa* than pKa is rationalized from the higher electron density on oxygen in the photoexcited state relative to the ground state, as obtained from the Mulliken charge densities through quantum chemical calculations. Extending our studies to the micellar environments, it is established that the deprotonation process is favored in cationic micelles while the same is disfavored in anionic or non-ionic micelles. This has been rationalized in terms of the surface charge characteristics of the individual micelles.

Probing the reverse micelle environment with a cationic dye by varying oil and water content of micelles.

In the present work, spectroscopic properties of crystal violet (CV) within water-in-hexane sodium base (2-ethyl hexyl) sulfosuccinate (AOT) reverse micellar environment was investigated as a function of water contents (W = [H2O]/[AOT] = 6,10) in AOT and also function of mass fraction of nanodroplet (MFD = 0.01, 0.04, 0.07, and 0.1) by using UV/visible, and fluorescence techniques. A deviation in the absorption spectra of crystal violet at the high CV concentration (0.002 M) was observed in the AOT reversed micelles. The Quenching in the emission intensity of crystal violet at high CV concentration and blue shift in λmax of fluorescence of dye was observed as a function of MFD in AOT RMs. At high concentration of dye, molecules of CV can be reside interfacial region but near core of AOT. At low dye concentration, the CV molecules also can be in interfacial region but far from core. The Stokes shift of CV at high concentration decreased with mass fraction of nano-droplet (MFD), whereas at the low CV concentration, its variation often increased. The ratio of ground state to the excited state dipole moment of crystal violet dye also affected by the change in W values and MFD values. The change in dipole moment and stokes shift indicates the different environment around CV for different W and MFD values.

An ESIPT Probe for the Ratiometric Imaging of Peroxynitrite Facilitated by Binding to Aβ-Aggregates.

A series of 3-hydroxyflavone (3-HF) ESIPT (excited-state intramolecular proton transfer) boronate-based fluorescent probes have been developed for the detection of peroxynitrite (ONOO-). The dyes are environmentally sensitive, and each probe exhibited a ratiometric response toward ONOO- in a micellar environment. The probes were used to image different aggregation states of amyloid-β (Aβ) in the presence of ONOO-. The 3-HF-OMe probe was found to produce a ratiometric response toward ONOO- when bound to Aβ aggregates, resulting in a novel host-guest ensemble, which adds insight into the development of other ESIPT-based probes for the simultaneous sensing of fibrous proteins/peptides and environmental ROS/RNS.

Encapsulation and modulation of protolytic equilibrium of β-carboline-based norharmane drug by cucurbit[7]uril and micellar environments for enhanced cellular uptake.

The effect of supramolecular nanocavity on photophysical and acid-dissociation properties of Norharmane (NHM), a physiologically important, anxiety control and memory-enhancing β-carboline-based drug, has been investigated using steady-state absorption and fluorescence spectroscopy. Self-assembled organization derived from surfactants and rigid water-soluble macrocyclic host Cucurbit[7]uril (CB7) have been selected for this investigation. The confined-space offered by the supramolecular assemblies modulates the pKa value of NHM (up to 3 units) as it can exist in two protolytic forms at near neutral pH. Therefore, the pH-dependent binding properties, modulation of pKa value and its consequences on the photophysical, chemical and solubility properties are investigated in detail. This investigation shows a large shift in the protolytic equilibrium which in turn causes ca. 15 times solubility-enhancement at near neutral pH. Moreover, the effect of enhanced solubility has been further investigated by the augmentation in the cellular uptake of NHM entrapped inside CB7. Thus, the modulation of the acid-base properties and solubility of β-carboline-based drugs will have immense potential for their formulation, cellular uptake and bioavailability.