Debasis Banik

Vesicles Formed in Aqueous Mixtures of Cholesterol and Imidazolium Surface Active Ionic Liquid: A Comparison with Common Cationic Surfactant by Water Dynamics

The formation of stable unilamellar vesicles which hold great potential for biological as well as biomedical applications has been reported in the aqueous mixed solution of a surface active ionic liquid (SAIL), 1-hexadecyl-3-methylimidazolium chloride ([C16mim]Cl) and cholesterol. To make a comparison we have also shown the formation of such stable vesicles using a common cationic surfactant, benzyldimethylhexadecylammonium chloride (BHDC) which has a similar alkyl chain length but different headgroup region to that of [C16mim]Cl. It has been revealed from dynamic light scattering (DLS), transmission electron microscopy (TEM), nuclear magnetic resonance (NMR), and other optical spectroscopic techniques that the micelles of [C16mim]Cl and BHDC in aqueous solutions transform into stable unilamellar vesicles upon increasing concentration of cholesterol. We find that, as the concentration of cholesterol increases, the solvation and rotational relaxation time of C153 in [C16mim]Cl/cholesterol solution as well as in BHDC/cholesterol solution gradually increases indicating a significant decrease in the hydration behavior around the self-assemblies upon micelle-vesicle transition. However, the extent of increase in solvation and rotational relaxation time is more prominent in the case of [C16mim]Cl/cholesterol solutions than in the BHDC/cholesterol system. This indicates that [C16mim]Cl/cholesterol vesicular membranes are comparatively less hydrated and more rigid than the BHDC/cholesterol vesicular bilayer.

An Investigation into the Effect of the Structure of Bile Salt Aggregates on the Binding Interactions and ESIHT Dynamics of Curcumin: A Photophysical Approach To Probe Bile Salt Aggregates as a Potential Drug Carrier

This work demonstrates the utilization of bile salt aggregates as a potential biological host system for studying the binding interactions and dynamics of the poorly-water-soluble drug curcumin by means of photophysical techniques. We found that the level of degradation of curcumin is greatly suppressed upon encapsulation into the nanocavities of three different bile salt aggregates. However, NaTC aggregates are more effective to suppress the level of degradation of curcumin than NaCh and NaDC aggregates. We also report the modulation of the photophysical and dynamical properties of curcumin into the nanocavities of bile salt aggregates using steady-state and time-resolved fluorescence spectroscopy. The reduced level of interaction of curcumin with water upon incorporation into the different binding sites of bile salt aggregates results in an enhanced fluorescence intensity along with the blue shift in the emission maxima of curcumin. However, the observation of higher fluorescence quantum yield as well as longer fluorescence lifetime in NaTC aggregates compared to that in NaCh and NaDC aggregates clearly indicates a more effective decrease in the excited-state intramolecular hydrogen atom transfer (ESIHT) mediated nonradiative deactivation of curcumin by the interaction with the anionic headgroup of NaTC. The binding and location of curcumin into the bile salt aggregates has been further confirmed from the steady-state fluorescence anisotropy measurements. In addition, we have shown the effect of addition of salt on the photophysical properties of curcumin in the confined environments of bile salt aggregates. Our results indicate that on addition of salt the time scale of ESIHT process of curcumin in bile salt aggregates is markedly increased.

How Does the Surface Charge of Ionic Surfactant and Cholesterol Forming Vesicles Control Rotational and Translational Motion of Rhodamine 6G Perchlorate (R6G ClO4)

The rotational dynamics and translational diffusion of a hydrophilic organic molecule, rhodamine 6G perchlorate (R6G ClO4) in small unilamellar vesicles formed by two different ionic surfactants, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS), with cholesterol have been investigated using fluorescence spectroscopic methods. Moreover, in this article the formation of vesicle using anionic surfactant, SDS at different cholesterol-to-surfactant molar ratio (expressed by Q value (Q = [cholesterol]/[surfactant])) has also been reported. Visual observation, dynamic light scattering (DLS) study, turbidity measurement, steady state fluorescence anisotropy (r0) measurement, and eventually microscopic images reveal the formation of small unilamellar vesicles in aqueous solution. Also, in this study, an attempt has been made to observe whether the cationic probe molecule, rhodamine 6G (R6G) experiences similar or different microenvironment in cholesterol-SDS and cholesterol-CTAB assemblies with increase in cholesterol concentration. The influence of cholesterol on rotational and translational diffusion of R6G molecules has been investigated by monitoring UV-vis absorption, fluorescence, time-resolved fluorescence anisotropy, and finally fluorescence correlation spectroscopy (FCS) measurements. In cholesterol-SDS assemblies, due to the strong electrostatic attractive interaction between the negatively charged surface of vesicle and cationic R6G molecules, the rotational and diffusion motion of R6G becomes slower. However, in cholesterol-CTAB aggregates, the enhanced hydrophobicity and electrostatic repulsion induces the migration of R6G from vesicle bilayer to aqueous phase. The experimental observations suggest that the surface charge of vesicles has a stronger influence than the hydrophobicity of the vesicle bilayer on the rotational and diffusion motion of R6G molecules.

Spectroscopy and Fluorescence Lifetime Imaging Microscopy To Probe the Interaction of Bovine Serum Albumin with Graphene Oxide

The interaction of graphene oxide (GO) with bovine serum albumin (BSA) in aqueous buffer solution has been investigated with various spectroscopic and imaging techniques. At single molecular resolution this interaction has been performed using fluorescence correlation spectroscopy (FCS) and fluorescence lifetime imaging microscopy (FLIM) techniques. The conformational dynamics of BSA on GOs influence have been explored by FCS and circular dichroism (CD) spectroscopy. For the FCS studies BSA was labeled covalently by a fluorophore, Alexa Fluor 488. On the addition of GO in phosphate buffer of 10 mM at pH 7.4 the diffusion time (τD) and the hydrodynamic radius (Rh) of BSA increase due to adsorption of BSA. Conformational relaxation time components of native BSA drastically vary with the addition of GO, signifying the change of conformational dynamics of BSA after addition of GO. The adsorption isotherm also indicates significant adsorption of BSA on the GO surface. Adsorption of BSA on the GO surface has shown in direct images of atomic force microscopy (AFM) and FLIM. Fluorescence quenching study of BSA with addition of GO also indicates that there is strong interaction between BSA and GO.

A Novel Ionic Liquid-in-Oil Microemulsion Composed of Biologically Acceptable Components: An Excitation Wavelength Dependent Fluorescence Resonance Energy Transfer Study

In this work we have reported the formulation of a novel ionic liquid-in-oil (IL/O) microemulsion where the polar core of the ionic liquid, 1-ethyl-3-methylimidazolium n-butylsulfate ([C2mim][C4SO4]), is stabilized by a mixture of two nontoxic nonionic surfactants, polyoxyethylene sorbitan monooleate (Tween-80) and sorbitan laurate (Span-20), in a biological oil phase of isopropyl myristate (IPM). The formation of the microemulsion droplets has been confirmed from the dynamic light scattering (DLS) and phase behavior study. To assess the dynamic heterogeneity of this tween-based IL/O microemulsion, we have performed an excitation wavelength dependent fluorescence resonance energy transfer (FRET) from coumarin 480 (C480) to rhodamine 6G (R6G). The multiple donor-acceptor (D-A) distances, ∼15, 30, and 45 Å, obtained from the rise times of the acceptor emission in the presence of a donor can be rationalized from the varying distribution of the donor, C480, in the different regions of the microemulsion system. With increasing the excitation wavelength from 375 to 408 nm, the contribution of the rise component of ∼240 ps which results the D-A distance of ∼30 Å increases significantly due to the enhanced contribution of the C480 probe molecules closer to the acceptor in the ionic liquid pool of the microemulsion.

Picosecond solvation dynamics--a potential viewer of DMSO-water binary mixtures.

In this work, we have investigated the composition dependent anomalous behavior of dimethyl sulfoxide (DMSO)-water binary mixture by collecting the ultrafast solvent relaxation response around a well known solvation probe Coumarin 480 (C480) by using a femtosecond fluorescence up-conversion spectrometer. Recent molecular dynamics simulations have predicted two anomalous regions of DMSO-water binary mixture. Particularly, these studies encourage us to investigate the anomalies from experimental background. DMSO-water binary mixture has repeatedly given evidences of its dual anomalous nature in front of our systematic investigation through steady-state and time-resolved measurements. We have calculated average solvation times of C480 by two individual well-known methods, among them first one is spectral-reconstruction method and another one is single-wavelength measurement method. The results of both the methods roughly indicate that solvation time of C480 reaches maxima in the mole fraction of DMSO XD = 0.12-0.17 and XD = 0.27-0.35, respectively. Among them, the second region (XD = 0.27-0.35) is very common as most of the thermodynamic properties exhibit deviation in this range. Most probably, the anomalous solvation trend in this region is fully guided by the shear viscosity of the medium. However, the first region is the most interesting one. In this region due to formation of strongly hydrogen bonded 1DMSO:2H2O complexes, hydration around the probe C480 decreases, as a result of which solvation time increases.

Organic Additive, 5-Methylsalicylic Acid Induces Spontaneous Structural Transformation of Aqueous Pluronic Triblock Copolymer Solution: A Spectroscopic Investigation of Interaction of Curcumin with Pluronic Micellar and Vesicular Aggregates

This article presents the interaction of curcumin in the microenvironments provided by aggregation of pluronic triblock copolymer P123 into micellar and vesicular assemblies. The formation of vesicles using triblock copolymer P123 and 5-methylsalicylic acid (5 mS) has been successfully characterized by optical spectroscopy, light scattering measurement, and eventually microscopic techniques. Besides, to make a comparative study between the polymeric micelles, we have also investigated the photophysical changes of curcumin in F127 triblock copolymer micelles having variation in poly(ethylene oxide) (PPO) and poly(propylene oxide) (PEO) unit of polymer chain to that of P123. Time-dependent UV-vis measurement suggests that these polymer micelles are able to stabilize poorly water-soluble curcumin by suppressing the degradation rate in micellar nanocavity. However, experimental observations suggest that P123 micelles are more efficient than F127 to perturb excited state intramolecular proton transfer (ESIPT)-related nonradiative decay of curcumin. We also observed that rigid and confined microenvironment of P123/5 mS vesicles enhances emission intensity and lifetime of curcumin more compared to P123 micelles. All the observations suggest that modulation of photophysics of curcumin is responsible due to its interaction with poly(ethylene oxide) or poly(propylene oxide) unit of triblock copolymer.

Investigation of Fibril Forming Mechanisms of l-Phenylalanine and l-Tyrosine: Microscopic Insight toward Phenylketonuria and Tyrosinemia Type II

Phenylketonuria and tyrosinemia type II, the two metabolic disorders, are originated due to the complications in metabolism of phenylalanine (Phe) and tyrosine (Tyr), respectively. Several neurological injuries, involving microcephaly, mental retardation, epilepsy, motor disease, and skin problems etc., are the symptoms of these two diseases. It has been reported that toxic amyloid fibrils are formed at high concentrations of Phe and Tyr. Our study indicates that the fibril forming mechanisms of Phe and Tyr are completely different. In the case of Phe, -NH3+ and -COO- groups of neighboring molecules interact via hydrogen bonding and polar interactions. On the other hand, there is no role of - NH3+ group in the fibril forming mechanism of Tyr. In Tyr fibril, the two hydrogen bonding partners are -OH and -COO- groups. In addition, we have also investigated the effect of three lanthanide cations on the fibrillar assemblies of Phe. It has been observed that the efficiencies of three lanthanides to inhibit the fibrillar assemblies of Phe follow the order Tb3+< Sm3+< Eu3+.

Effect of encapsulation of curcumin in polymeric nanoparticles: how efficient to control ESIPT process?

This paper demonstrates the photophysics of curcumin inside polymeric nanoparticles (NPs), which are being recently used as targeted drug delivery vehicles. For this purpose, we have prepared three polymeric NPs by ultrasonication method from three well-defined water-insoluble random copolymers. These copolymers having various degrees of hydrophobicity were synthesized via reversible addition-fragmentation transfer (RAFT) method using styrene and three different functional monomers, namely, 2-hydroxyethyl acrylate, 4-formylphenyl acrylate, and 4-vinylbenzyl chloride. The photophysics of the curcumin molecules inside the polymeric NPs have been monitored by applying tools like steady state and time-resolved fluorescence spectroscopy. An increase in fluorescence intensity along with an increase in the lifetime values indicated a perturbation of the excited state intramolecular proton transfer (ESIPT) process of curcumin inside the polymeric NPs.

Ultrafast FRET to Study Spontaneous Micelle-to-Vesicle Transitions in an Aqueous Mixed Surface-Active Ionic-Liquid System†

The spontaneous micelle-to-vesicle transition in an aqueous mixture of two surface-active ionic liquids (SAILs), namely, 1-butyl-3-methylimidazolium n-octylsulfate ([C4mim][C8SO4]) and 1-dodecyl-3-methylimidazoium chloride ([C12mim]Cl) is described. In addition to detailed structural characterization obtained by using dynamic light scattering, transmission electron microscopy (TEM), and cryogenic TEM techniques, ultrafast fluorescence resonance energy transfer (FRET) from coumarin 153 (C153) as a donor (D) to rhodamine 6G (R6G) as an acceptor (A) is also used to study micelle-vesicle transitions in the present system. Structural transitions of SAIL micelles ([C4mim][C8SO4] or [C12mim]Cl micelles) to mixed SAIL vesicles resulted in significantly increased D-A distances, and therefore, increased timescale of FRET. In [C4mim][C8SO4] micelles, FRET between C153 and R6G occurs on an ultrafast timescale of 3.3 ps, which corresponds to a D-A distance of about 15 Å. As [C4mim][C8SO4] micelles are transformed into mixed micelles upon the addition of a 0.25 molar fraction of [C12mim]Cl, the timescale of FRET increases to 300 ps, which suggests an increase in the D-A distance to 31 Å. At a 0.5 molar fraction of [C12mim]Cl, unilamellar vesicles are formed in which FRET occurs on multiple timescales of about 250 and 2100 ps, which correspond to D-A distances of 33 and 47 Å. Although in micelles and mixed micelles the obtained D-A distances are well correlated with their radius, in vesicles the obtained D-A distance is within the range of the bilayer thickness.