P. Madhusudhana Reddy

Destruction of hydrogen bonds of poly(N-isopropylacrylamide) aqueous solution by trimethylamine N-oxide

Trimethylamine N-oxide (TMAO) is a compatible or protective osmolyte that stabilizes the protein native structure through non-bonding mechanism between TMAO and hydration surface of protein. However, we have shown here first time the direct binding mechanism for naturally occurring osmolyte TMAO with hydration structure of poly(N-isopropylacrylamide) (PNIPAM), an isomer of polyleucine, and subsequent aggregation of PNIPAM. The influence of TMAO on lower critical solution temperature (LCST) of PNIPAM was investigated as a function of TMAO concentration at different temperatures by fluorescence spectroscopy, viscosity (η), multi angle dynamic light scattering, zeta potential, and Fourier transform infrared (FTIR) spectroscopy measurements. To address some of the basis for further analysis of FTIR spectra of PNIPAM, we have also measured FTIR spectra for the monomer of N-isopropylacrylamide (NIPAM) in deuterium oxide (D(2)O) as a function of TMAO concentration. Our experimental results purportedly elucidate that the LCST values decrease with increasing TMAO concentration, which is mainly contributing to the direct hydrogen bonding of TMAO with the water molecules that are bound to the amide (-CONH) functional groups of the PNIPAM. We believed that the present work may act as a ladder to reach the heights of understanding of molecular mechanism between TMAO and macromolecule.

Influence of ionic liquids on the critical micellization temperature of a tri-block co-polymer in aqueous media

To explore the role of additives in controlling the critical micellization temperature (CMT) of a block copolymer in aqueous solution, in the present work, a series of ionic liquids (ILs) containing the same cation, 1-butyl-3-methylimidazolium, bmim(+) and commonly used anions such as SCN(-), BF4(-), I(-), Cl(-), CH3COO(-) and HSO4(-) were chosen for study with a triblock copolymer, poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol), (PEG-PPG-PEG) in aqueous solution. The present results suggest that the ability of ILs for decreasing the polymer CMT is mainly a result of not only from charge and size of anions of the ILs, but also due to the weak ion-ion pair interactions within IL. The present study provides important information that can be helpful to tune the IL- or temperature-sensitive copolymer CMT and micelle shapes which are crucial for understanding the drug delivery mechanisms.

The biological stimuli for governing the phase transition temperature of the "smart" polymer PNIPAM in water.

A lack of sufficient knowledge regarding the behaviour of stimuli-responsive polymers to biological stimuli hinders the potential use of responsive polymers as biomaterials and medical devices. Hence, in this study, we demonstrate the impact of various globular proteins on the phase transition temperature of poly(N-isopropylacrylamide) (PNIPAM) in an aqueous solution through the use of fluorescence spectroscopy, dynamic light scattering (DLS), Fourier transform infrared (FTIR) spectroscopy and field-emission scanning electron microscopy (FESEM). Furthermore, we describe the molecular interaction of PNIPAM with proteins by the MolDock method. Our experimental and docking studies revealed that such proteins as α-chymotrypsin (CT), insulin (In) and haemoglobin (Hb) decreased the lower critical solution temperature (LCST) of the polymer, whereas succinyl-concanavalin A (SCA) increased the LCST of PNIPAM. The LCST changed upon increasing the concentration of protein from 0.5mg/mL to 1mg/mL. The thermoresponsive behaviour of PNIPAM can be significantly altered by the functional groups present in the protein. The findings of the present study can be used in the engineering of bioresponsive smart PNIPAM-based devices.

Polyacrylic acid polymer modulates the UCST-type phase behavior of ionic liquid and water

Upper critical solution temperature (UCST)-type phase behavior was observed in the critical mixture of water (W) and hydrophobic ionic liquid, 1-hexyl-3-methylimidazolium tetrafluoroborate (IL), by refractive index and fluorescence measurements. The phase separation temperature depends on the composition of IL as well as water. Furthermore, for the first time, the behavior of high molecular weight polymer polyacrylic acid, PAA (Mw = 450 000 g mol−1), as an impurity in the UCST mixture was investigated by simultaneous refractive index (n) measurements for both the coexisting phases of ILW. The refractive index in each phase of ILW as well as three different PAA concentrations (C = 0.5, 1.0, and 1.5 mg cm−3) in the near critical composition of ILW have been measured at temperatures below the systems upper critical point. We observed that the polymer significantly affected on the critical region of ILW. The phase-transition region of coexisting phases of ILW significantly shifts down with an increase in the concentrations of PAA. The Tc values decreases linearly from 58.256 (free of PAA) to 55.168 °C (PAA in ILW) with increasing PAA concentrations in ILW. This indicates that the polymer chain entangles with the coexisting phases, thereby the polymer monomers strongly interact with neighbor solvent particles. At temperatures T close enough to Tc, the critical exponent (β) values were obtained from the measured n values of both the coexisting liquid phases and was found to increase from 0.332 to 0.379, when the PAA concentration changes from 0.5 to 1.5 mg cm−3. The obtained 3D Ising values are modified in the presence of PAA and obviously belong to the renormalized class. These values are higher than that of 0.326 ± 0.002 of pure ILW, which is compatible with the 3D Ising value β = 0.325. In addition, we have performed the fluorescence measurement for the determination of Tc for ILW and in the presence of PAA as an impurity. The observed increase in the fluorescence intensities with temperature predicts unambiguously the formation of the solvation structure at Tc for the critical mixture in presence of the polymer.

Effect of structural variations in cations of ionic liquids on the coexistence curve of isobutyric acid and water

Interestingly, various behaviors of ionic liquids (ILs) having different cation chains with a common anion is observed in the coexistence curve of isobutyric acid–water (IBW). The critical region of IBW was found to increase in the presence of aliphatic chain 1-hexyl-3-methyl-imidazolium tetrafluoroborate [C6mim][BF4], (IL-1) whereas the critical region decreased in the presence of aromatic chain 1-benzyl-3-methylimidazolium tetrafluoroborate [Bnmim][BF4], (IL-2). Adding 0.5–2.0 mg ml−1 of IL-1 or IL-2 to IBW caused the critical temperature (Tc) to decrease by 0.553–2.898 K (for IL-1) and to increase by 0.502–1.664 K (for IL-2). The critical exponent (β) increases with increasing the IL concentrations, which are fully renormalized critical exponents. Further, our dynamic light scattering (DLS) results for the binary critical solution of IBW indicate that there exists a fluctuation in the local density that contributes significantly to the fluctuations in the scattered intensity at Tc. The variations in fluctuations are found to be pronounced in the presence of IL as an impurity. The reason for this behavior may be the formation of large water clusters with the neighboring molecules of ILs at Tc. This indicates that the IL entangles with the both coexisting phases, thereby, the IL strongly interacts with the neighboring solvent molecules. In addition, we report the fluorescence spectroscopy data for the critical mixture of IBW and its mixture with ILs as impurities using Nile Red as a fluorescent probe. Noticeable transitions of the two phases were detected at Tc through fluorescence technique. The observed variations in the fluorescence intensities with temperature predict unambiguously the formation of the solvation structure at Tc for the pure critical mixture as well as in presence of ILs.

Overview of the effect of monomers and green solvents on thermoresponsive copolymers: phase transition temperature and surface properties

The present study explores the effect of the monomer substitution pattern of different copolymers on their volume phase transition temperature (VPTT) and surface wetting properties (SWPs) with the aid of differential scanning calorimetry (DSC), contact angle (CA), scanning electron microscopy (SEM) and Fourier transform-infrared spectroscopy (FT-IR) measurements. The present experimental results unveil that the VPTT and SWPs were greatly governed by the ability of the neighboring side chains to form intramolecular hydrogen bonds and the hydrophobic–hydrophilic balance of the polymer. The substitution of some of the acrylic acid (AA) monomers in the copolymer hydrogel P(NIPAM-co-AA) by the polyethyleneglycolmethacrylate (PEGMA) monomer resulted in the copolymer hydrogel, P(NIPAM-co-PEGMA-co-AA). The PEGMA in P(NIPAM-co-PEGMA-co-AA) can promote the intermolecular hydrogen bonds of the copolymer with the water molecules and thus hinders the formation of the intramolecular hydrogen bonds. Consequently, the VPTT of the P(NIPAM-co-PEGMA-co-AA) occurs at higher temperatures when compared to the P(NIPAM-co-AA) and P(NIPAM-co-PEGMA) systems. Further, in view of the potential applications of ionic liquids (IL) in polymer science, the effect of IL on the VPTT and SWP of the copolymers was investigated. The knowledge from this study can pave the way to engineer stimuli responsive polymers for a wide range of applications in the modern era.

Quantifying the co-solvent effects on trypsin from the digestive system of carp Catla catla by biophysical techniques and molecular dynamics simulations

Here, circular dichroism (CD) spectroscopy, fluorescence spectroscopy, UV-Vis spectroscopy, SDS-PAGE, substrate SDS-PAGE, and molecular dynamics (MD) simulations techniques have been employed to understand the structural behavioral changes of trypsin (MW: 19.72 kDa, source: digestive system of adult Indian major carp, Catla C. catla) in the presence of various chemical environments. The stability of the trypsin can be increased by stabilizers, including trimethylamine N-oxide (TMAO), proline, and betaine, without affecting its native structure. Trypsin has shown unusual high thermal stability in the presence of betaine. Further, these experimental results were confirmed by means of MD simulations. The present results explicitly elucidated that the behavior of a co-solvent may vary depending upon the type of the protein.