Supratik Sen Mojumdar

An FCS study of unfolding and refolding of CPM-labeled human serum albumin: role of ionic liquid.

The effect of a room temperature ionic liquid (RTIL) on the conformational dynamics of a protein, human serum albumin (HSA), is studied by fluorescence correlation spectroscopy (FCS). For this, the protein was covalently labeled by a fluorophore, 7-dimethylamino-3-(4-maleimidophenyl)-4-methylcoumarin (CPM). On addition of a RTIL ([pmim][Br]) to the native protein, the diffusion coefficient (D(t)) decreases and the hydrodynamic radius (R(h)) increases. This suggests that the RTIL ([pmim][Br]) acts as a denaturant when the protein is in the native state. However, addition of [pmim][Br] to a protein denatured by GdnHCl causes an increases in D(t) and decrease in R(h). This suggests that in the presence of GdnHCl addition of RTIL helps the protein to refold. In the native state, the conformational dynamics of protein is described by three distinct time constants: ~3.6 ± 0.7, ~29 ± 4.5, and 133 ± 23 μs. The faster components (~3.6 ± 0.7 and ~29 ± 4.5 μs) are ascribed to chain dynamics of the protein, while the slowest component (133 μs) is responsible for interchain interaction or concerted motion. On addition of [pmim][Br], the conformational dynamics of HSA becomes slower (~5.1 ± 1, ~43.5 ± 2.8, and ~311 ± 2.3 μs in the presence of 1.5 M [pmim][Br]). The time constants for the protein denatured by 6 M GdnHCl are 3.2 ± 0.4, 34 ± 6, and 207 ± 38 μs. When 1.5 M [pmim][Br] is added to the denatured protein (in 6 M GdnHCl), the time constants become ~5 ± 1, ~41 ± 10, and ~230 ± 45 μs. The lifetime histogram shows that, on addition of GdnHCl to HSA, the contribution of the shorter lifetime component decreases and vanishes at 6 M GdnHCl. The shorter lifetime component immediately reappears after addition of RTIL to unfolded HSA. This suggests recoiling of the unfolded protein by RTIL.

Role of ionic liquid on the conformational dynamics in the native, molten globule, and unfolded states of cytochrome c: a fluorescence correlation spectroscopy study.

The role of a room temperature ionic liquid (RTIL, [pmim][Br]) on the size and conformational dynamics of a protein, horse heart cytochrome c (Cyt C) in its native, molten globule (MG-I and II), and unfolded states is studied using fluorescence correlation spectroscopy (FCS). For this purpose, the protein was covalently labeled by a fluorescent dye, Alexa Fluor 488. It is observed that the addition of the RTIL leads to an increase in the hydrodynamic radius (r(H)) of the protein, Cyt C in the native or MG-I state. In contrast, the addition of RTIL causes a decrease in the size (hydrodynamic radius, r(H)) of Cyt C unfolded by GdnHCl or MG-II state. The decrease in size indicates the formation of a relatively compact structure. We detected two types of conformational relaxation of the protein. The shorter relaxation time component (~3-5.5 μs) corresponds to the protein folding or intrachain contact formation, while the relatively longer time component (~63-122 μs) may be assigned to the motion of the protein side chains or concerted chain dynamics. The burst integrated fluorescence lifetime histograms indicate that the increase in size of the protein is accompanied by an increase in the contribution of the shorter component (~0.3-0.4 ns) with a concomitant decrease of the contribution of the longer component (~2.8-3.6 ns). An opposite trend is observed during the decrease in size of the protein.

Ultrafast and ultraslow proton transfer of pyranine in an ionic liquid microemulsion.

Effect of a room temperature ionic liquid (RTIL) and water on the ultrafast excited state proton transfer (ESPT) of pyranine (8-hydroxypyrene-1,3,6-trisulfonate, HPTS) inside a microemulsion is studied by femtosecond up-conversion. The microemulsion consists of the surfactant, triton X-100 (TX-100) in benzene (bz) and contains the RTIL, 1-pentyl-3-methyl-imidazolium tetrafluoroborate ([pmim] [BF(4)]) as the polar phase. In the absence of water, HPTS undergoes ultrafast ESPT inside the RTIL microemulsion (RTIL/TX-100/bz) and the deprotonated form (RO(-)) exhibits three rise components of 0.3, 14, and 375 ps. It is proposed that in the RTIL microemulsion, HPTS binds to the TX-100 at the interface region and participates in ultrafast ESPT to the oxygen atoms of TX-100. On addition of water an additional slow rise of 2150 ps is observed. Similar long rise component is also observed in water/TX-100/benzene reverse micelle (in the absence of [pmim] [BF(4)]). It is suggested that the added water molecules preferentially concentrate (trapped) around the palisade layer of the RTIL microemulsion. The trapped water molecules remain far from the HPTS both in the presence and absence of ionic liquid and gives rise to the slow component (2150 ps) of ESPT. Replacement of H(2)O by D(2)O causes an increase in the time constant of the ultraslow rise to 2350 ps.

Diffusion of Organic Dyes in Immobilized and Free Catanionic Vesicles

Fluorescence correlation spectroscopy (FCS) has been used to study the motion of fluorescent dyes in a giant (diameter 20 000 nm = 20 μm) catanionic vesicle comprised of the surfactant sodium dodecyl sulfate (SDS) and dodecyltrimethyl ammonium bromide (DTAB). The diffusion in the anion (SDS) rich catanionic vesicle was studied both in bulk water and in an immobilized vesicle attached to a positively charged glass surface. In the case of the immobilized vesicle, the diffusion coefficients (D(t)) of R6G (rhodamine 6G), DCM (4-dicyanomethylene-2-methyl-6-p-dimethyl aminostyryl-4H-pyran), and C343 (coumarin 343) are found to be 1.5, 2.5, and 10 μm(2)/s, respectively, which are 280, 120, and 55 times slower compared to those for the same dyes in bulk water. The magnitude of D(t) is found to vary for different vesicles. This was attributed to the difference in size and shape of the immobilized vesicles. In bulk, R6G binds completely to the vesicle and exhibits extremely slow diffusion with D(t) = 0.5 ± 0.1 μm(2)/s (∼850 and 3 times slower compared to that of R6G in bulk water and within the immobilized vesicle). This is attributed to very slow overall diffusion of the very large size vesicles (20 μm = 20 000 nm). Both of the dye molecules (DCM and C343) show two different diffusion coefficients for the vesicles in bulk. In this case, the small D(t) (0.5 ± 0.1 μm(2)/s) corresponds to the diffusion of the vesicle as a whole and the large D(t) value (300 and 550 μm(2)/s for DCM and C343, respectively) corresponds to the free dye molecules in bulk water.

Marcus-like inversion in electron transfer in neat ionic liquid and ionic liquid-mixed micelles.

Ultrafast photoinduced electron transfer (PET) from N,N-dimethylaniline (DMA) to coumarin dyes in a room-temperature ionic liquid (RTIL, [pmim][BF(4)]) and in a mixed micelle containing the RTIL and a triblock copolymer, (PEO)(20)-(PPO)(70)-(PEO)(20), (Pluronic P123) is studied using femtosecond upconversion. A Marcus-like inversion in the rate of PET is observed in neat RTIL. This is attributed to high viscosity and nanostructuring of the RTIL. Diffusion and the rate of PET in the neat RTIL are slower than those in the RTIL-P123 mixed micelle. The coumarin dyes exhibit faster electron transfer and translational diffusion (anisotropy decay) in the RTIL-P123 mixed micelle compared to that in the P123 micelle.

Deuterium Isotope Effect on Femtosecond Solvation Dynamics in an Ionic Liquid Microemulsion: An Excitation Wavelength Dependence Study

The deuterium isotope effect on the solvation dynamics and the anisotropy decay of coumarin 480 (C480) in a room temperature ionic liquid (RTIL) microemulsion is studied by femtosecond up-conversion. The microemulsion consists of the RTIL 1-pentyl-3-methyl-imidazolium tetra-fluoroborate ([pmim][BF(4)]) in triton X-100 (TX-100)/benzene. Replacement of H(2)O by D(2)O in the microemulsion causes retardation of solvation dynamics. The average solvation time of C480 (tau(s)) in RTIL microemulsion with 5 wt % D(2)O is approximately 1.5-1.7 times slower compared to that in the H(2)O containing RTIL microemulsion. This suggests that the main species in the microemulsion responsible for solvation is the water molecules. In both D(2)O and H(2)O containing RTIL microemulsion, the solvation dynamics exhibits marked dependence on the excitation wavelength (lambda(ex)) and becomes about 15 times faster as lambda(ex) increases from 375 to 435 nm. This is ascribed to the structural heterogeneity in the RTIL microemulsion. For lambda(ex) = 375 nm, the region near the TX-100 surfactant is probed where bound water molecules cause slow solvation dynamics. At 435 nm, the RTIL pool is selected where the water molecules are more mobile and hence gives rise to faster solvation. The average time constant of anisotropy decay shows opposite dependence on lambda(ex) and increases about 2.5-fold from 180 ps at lambda(ex) = 375 nm to 500 ps at lambda(ex) = 435 nm for D(2)O containing RTIL microemulsion. The slower anisotropy decay at lambda(ex) = 435 nm is ascribed to the higher viscosity of RTIL which causes greater friction at the core.

Study of γ-cyclodextrin host-guest complex and nanotube aggregate by fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) has been used to study the formation of large nanotube aggregates involving γ-cyclodextrin (γ-CD) and coumarin 153 (C153). It is observed that the length of a γ-CD:C153 nanotube aggregate is ∼770 nm. This is ∼480 times larger than the length of a 1:1 γ-CD:C480 complex (∼1.6 nm) and ∼950 times that of a γ-CD. This implies that 950 γ-CD units are noncovalently attached in the γ-CD:C153 aggregate. Binding constants (K(b)) of both the dyes to γ-CD were obtained from the fluctuation in fluorescence intensity. The rate of association and dissociation are obtained from the inverse of τ(off) and τ(on), respectively. The binding constant for the 1:1 γ-CD:C480 complex is ∼1000 M(-1). The burst integrated fluorescence lifetime (BIFL) histogram reveals presence of three distinct lifetime 1.8 ns (18%), 2.8 ns (69%), 3.2 ns (13%). These three lifetimes correspond to C153 present in bulk water and at the end and middle of the γ-CD:C153 nanotube aggregate, respectively. The lifetime of C480 in the 1:1 γ-CD:C480 complex is found to be 3.7 ns.

A Fluorescence Correlation Spectroscopy Study of the Diffusion of an Organic Dye in the Gel Phase and Fluid Phase of a Single Lipid Vesicle

The mobility of the organic dye DCM (4-dicyanomethylene-2-methyl-6-p-dimethyl aminostyryl-4H-pyran) in the gel and fluid phases of a lipid vesicle is studied by fluorescence correlation spectroscopy (FCS). Using FCS, translational diffusion of DCM is determined in the gel phase and fluid phase of a single lipid vesicle adhered to a glass surface. The size of a lipid vesicle (average diameter approximately 100 nm) is smaller than the diffraction limited spot size (approximately 250 nm) of the microscope. Thus, the vesicle is confined within the laser focus. Three lipid vesicles (1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)) having different gel transition temperatures (-1, 23, and 41 degrees C, respectively) were studied. The diffusion coefficient of the dye DCM in bulk water is approximately 300 microm(2)/s. In the lipid vesicle, the average D(t) decreases markedly to approximately 5 microm(2)/s (approximately 60 times) in the gel phase (for DPPC at 20 degrees C) and 40 microm(2)/s ( approximately 8 times) in the fluid phase (for DLPC at 20 degrees C). This clearly demonstrates higher mobility in the fluid phase compared with the gel phase of a lipid. It is observed that the D(t) values vary from lipid to lipid and there is a distribution of D(t) values. The diffusion of the hydrophobic dye DCM (D(t) approximately 5 microm(2)/s) in the DPPC vesicle is found to be 8 times smaller than that of a hydrophilic anioinic dye C343 (D(t) approximately 40 microm(2)/s). This is attributed to different locations of the hydrophobic (DCM) and hydrophilic (C343) dyes.

Solvation Dynamics under a Microscope: Single Giant Lipid Vesicle

Picosecond spectroscopy under a confocal microscope is employed to study solvation dynamics of coumarin 153 (C153) inside a single giant lipid vesicle (1,2-dilauroyl-sn-glycero-3-phosphocholine, DLPC) of diameter 20 μm. Fluorescence correlation spectroscopy (FCS) indicates that the diffusion coefficient (D(t)) of the probe (coumarin153, C153) in the immobilized vesicle displays a wide distribution from ~3 to 21 μm(2) s(-1). The distribution of D(t) suggests that the microenvironment of the probe (C153) is highly heterogeneous and the local friction is different for probe molecules in different regions. The values of D(t) is significantly smaller than that for the same dye in bulk water (550 μm(2) s(-1)). This suggests that the probe is located in the interface or membrane region rather than in the water pool of the vesicle. The solvation time of C153 in different regions of the lipid vesicle varies between 750 to 1200 ps. This result clearly shows that a confocal microscope is able to resolve the spatial heterogeneity in local friction (i.e., D(t)) and solvation dynamics within a lipid vesicle.

Effect of ionic liquid on the native and denatured state of a protein covalently attached to a probe: solvation dynamics study.

Effect of a room temperature ionic liquid (RTIL, [pmim][Br]) on the solvation dynamics of a probe covalently attached to a protein (human serum albumin (HSA)) has been studied using femtosecond up-conversion. For this study, a solvation probe, 7-diethylamino-3-(4-maleimidophenyl)-4-methylcoumarin (CPM) has been covalently attached to the lone cysteine group (cys-34) of the protein HSA. Addition of 1.5 M RTIL or 6 M GdnHCl causes a red shift of the emission maxima of CPM bound to HSA by 3 nm and 12 nm, respectively. The average solvation time 〈τ(s)〉 decreases from 650 ps (in native HSA) to 260 ps (~2.5 times) in the presence of 1.5 M RTIL and to 60 ps (~11 times) in the presence of 6 M GdnHCl. This is ascribed to unfolding of the protein by RTIL or GdnHCl and therefore making the probe CPM more exposed. When 1.5 M RTIL is added to the protein denatured by 6 M GdnHCl in advance, a further ~5 nm red shift along with further ~2 fold faster solvent relaxation ( ~30 ps) is observed. Our previous fluorescence correlation spectroscopy study [D. K. Sasmal, T. Mondal, S. Sen Mojumdar, A. Choudhury, R. Banerjee, and K. Bhattacharyya, J. Phys. Chem. B 115, 13075 (2011)] suggests that addition of RTIL to the protein denatured by 6 M GdnHCl causes a reduction in hydrodynamic radius (r(h)). It is demonstrated that in the presence of RTIL and GdnHCl, though the protein is structurally more compact, the local environment of CPM is very different from that in the native state.