ATR-FTIR studies on the effect of strong salting-out salts on the phase separation scenario in aqueous solutions of poly(N-isopropylacrylamide) [PNIPA] (original) (raw)
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Journal of Polymer Science Part B: Polymer Physics, 2004
The effect of two strong salting-out salts (Na 2 SO 4 and K 2 SO 4 ) on the temperature-induced phase-separation process in aqueous solutions of poly(N-isopropylacrylamide) (PNIPA) was examined by attenuated total reflectance/Fourier transform infrared spectroscopy, differential scanning calorimetry, and viscosity measurements. On the basis of these measurements, a detailed scenario of the phase-separation process was deduced. The phase-separation scenario of solutions containing PNIPA and water was altered in the presence of sulfate ions. Here, the sulfate ions induced partial intrachain collapse, manifested by a relatively compact structure well below the lower critical solution temperature. This led to a more gradual, smooth phase transition, with temperature-resolved intrachain collapse and interchain aggregation and a lesser extent of hysteresis. Although at the macrolevel one may not be able to differentiate among various scenarios altering the solvent into a poor solvent, the aforementioned microlevel measurements provided a way to expose the difference between raising the temperature and adding cosolutes. Follow-up studies on the effect of salting-in salts will be presented.
Journal of Molecular Structure, 2010
We have demonstrated an application of ''concatenated" 2D correlation analysis in quantitative or semiquantitative examination of the reversibility of the temperature-induced hydration variation of PNiPMA in water solution. Hydration reversibility of hydrophilic amide group and hydrophobic CH group are compared in different temperature ranges. The appearance of significant cross-peaks in the asynchronous spectrum calculated from the roundtrip data matrix indicates the existence of certain inter-asynchronicity between heating and cooling cycle. The contribution of different factors to the 2D asynchronous spectrum could be separated by selecting the range of the spectral dataset for concatenation. The irreversibility of the temperature-induced hydration variation in PNiPMA is not constant during the whole round trip: it is most obvious in the phase separation process, while in the range far away from that period it is nearly neglectable. The hydration variation of PNiPMA manifests significant irreversibility in the phase separation stage. The asynchronous peaks of hydrophobic CH groups are much weaker than that of hydrophilic amide group during any temperature range in the temperature round trip. The irreversibility of CH groups is not as strong as that of amide group during the round trip. It further supports the conclusion that the hydration variation of hydrophilic amide group is dominated in the temperature-induced phase separation of the polymer in water solution.
The Journal of Physical Chemistry B, 2004
Chain length and site dependencies of amide I local mode frequencies of R-helical polyalanines are theoretically studied by carrying out semiempirical quantum chemistry calculations. A theoretical model that can be used to quantitatively predict both the local amide I mode frequencies and coupling constants between two different local amide I modes is developed. Using this theoretical model and performing molecular dynamics simulation of an R-helical polyalanine in liquid water, we investigate conformational fluctuation and hydrogen-bonding dynamics by monitoring amide I frequency fluctuations. The instantaneous normal-mode analysis method is used to obtain densities of states of the one-and two-exciton bands and to quantitatively investigate the extent of delocalization of the instantaneous amide I normal modes. Also, by introducing a novel concept of the so-called weighted phase-correlation factor, the symmetric natures of the delocalized amide I normal modes are elucidated, and it is also shown that there is no unique way to classify any given amide I normal mode of the R-helical polyalanine in liquid water to be either A-mode-like or E 1 -mode-like. From the ensembleaveraged dipole strength spectrum and density of one-exciton states, the amide I infrared absorption spectrum is numerically calculated and its asymmetric line shape is theoretically described. Considering both transitions from the ground state to one-exciton states and those from one-exciton states to two-exciton states, we calculate the two-dimensional IR pump-probe spectra and directly compare them with recent experimental results. A brief discussion on the cross-peaks previously observed in the two-dimensional difference spectrum is presented.
Journal of Solution Chemistry, 2011
Solvation properties of aliphatic alcohol-water and fluorinated alcohol-water solutions were probed by amide molecules as solutes using infrared (IR) and 1 H and 13 C NMR techniques. These include four alcohols: ethanol (EtOH), 2-propanol (2-PrOH), 2,2,2trifluoroethanol (TFE), and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and three amides: N-methylformamide (NMF), N-methylacetamide (NMA) and N-methylpropionamide (NMP). The hydrogen bonds of the amide carbonyl oxygen with water are gradually weakened as the alcohol content increases. This decreases in the order of HFIP > TFE ≈ 2-PrOH > EtOH. In TFE-and HFIP-water solutions, the hydrogen bond between the amide amino hydrogen and water is also gradually broken with increasing x A. This trend is more notable in the order of NMP > NMA > NMF. The hydrophobic moieties of the amide methyl and ethyl groups are solvated by the fluoroalkyl groups of fluorinated alcohols due to the hydrophobic interaction among them. Thus, the steric hindrance generated by the solvated alkyl group of amides promotes the breaking of the hydrogen bonds between amide and water.
The Journal of Physical Chemistry B, 2002
A series of dipeptides AX and XA (X ) G, K, L, S, and V) were investigated by polarized visible Raman and FTIR-spectroscopy to examine the conformational determinants of the amide III band. A spectral decomposition combined with density functional calculations revealed that the amide III band has a multicomponent structure in that three different modes contribute to amide III vibrations. One of them (amide III 2 ) dominates the Raman spectra particularly of the cationic species. Its normal mode displays an in-phase combination of NH and C R1 H in plane bending vibrations, which makes it sensitive to changes of the dihedral angle ψ. Indeed, our Raman data show that amide III 2 varies with ψ but remains practically unaffected by variation of φ in the region between -95°and -75°, which is sampled by the investigated AX peptides. Our data support the Lord hypothesis that amide III depends solely on ψ (Lord, R. Appl. Spectrosc. 1977, 31, 187) but specifies to which of the amide III modes this statement applies. Our data further reveal that all amide III modes can interact with side chains vibrations. For some residues this causes a mode delocalization which yields a reduction of the Raman cross section. Amide S, which is a structure sensitive band resonance enhanced with UV-excitation, disappears for ψ-values outside of the -sheet region due to changes of the normal mode compositions of several modes between 1300 and 1420 cm -1 . This explains its absence in the UV-Raman spectra of R-helical structures. Our data suggest that all AX peptides exhibit ψ angles around 150°.
Journal of Molecular Liquids
Interactions with water are one of the key factors which determine protein stability and activity in aqueous solutions. However, the protein hydration is still insufficiently understood. N-methylacetamide (NMA) is regarded as a minimal part of the peptide backbone and the relative simplicity of its structure makes it a good model for studies on protein-water interactions. In this paper, the influence of NMA and N,N-dimethylacetamide (DMA) on surrounding water molecules in a range of temperature (25-75 o C) is studied by means of the FTIR spectroscopy. The results of the difference HDO spectra method are compared with the results of theoretical DFT calculations of NMA and DMA aqueous complexes. Both NMA and DMA can be regarded as "structure-makers", yet their hydration spheres are different. These molecules exhibit a mixed and mutually dependent types of hydration: hydrophilic and hydrophobic. In the case of a NMA molecule that has one methyl group less than DMA, the type of
1H MAS NMR studies of the phase separation of poly (N-isopropylacrylamide) gel in binary solvents
Preferential interactions of solvents with poly(N-isopropylacrylamide) (PNIPAM) gel networks in binary water/ alcohol (water/methanol and water/ethanol) mixtures have been investigated using variable-temperature high-resolution 1 H MAS NMR. NMR results for PNIPAM gel in the binary solvents reveal the existence of two distinct types of water/alcohol mixtures above the LCST: confined binary solvents bound inside the gel, and free binary solvents expelled from the gel. It is interesting to find that the alcohol concentration in confined solution is significantly higher than that in free solution. Moreover, of the two alcohols, ethanol is more significantly concentrated in the confined solution. These results demonstrate that the polymer preferentially interacts with alcohol molecules over water and that the alcohol with higher hydrophobicity exhibits higher preferential absorption on PNIPAM. Our results also show that 1 H NMR measurements made on two distinct types of solution provide a convenient, direct means of characterizing the preferential adsorption of solvent on polymer.
The Journal of Physical Chemistry, 1996
This paper reports two-dimensional Fourier-transform infrared (2D FT IR) correlation spectroscopy study of temperature-dependent spectral variations of N-methylacetamide (NMA) in the pure liquid. The 2D correlation spectroscopic analysis has demonstrated that the amide I band of NMA consists of at least four distinct bands at 1685, 1665, 1650, and 1635 cm -1 . The signs of asynchronous cross peaks corresponding to these four bands have shown that the sequence of spectral intensity change in the ascending order of temperature is given by 1635 cm -1 < 1650 cm -1 < 1665 cm -1 < 1685 cm -1 . These bands at 1635, 1650, 1665, and 1685 cm -1 may be due to the amide I modes of chain oligomers of various sizes and dimer of NMA. The longer the chain, the lower the frequency. Similarly, it has been found that the amide A, II, and III bands consist of at least two bands. For example, the amide II is split into two bands at 1570 and 1545 cm -1 . The 2D correlation analysis between different spectral regions has enabled to make correlations among each band of the amide A, I, II, and III. The amide I band at 1635 cm -1 is correlated with the amide A at 3275 cm -1 , amide II at 1570 cm -1 , and amide III at 1300 cm -1 , while the amide I band at 1665 cm -1 is correlated with the amide A at 3335 cm -1 , amide II at 1545 cm -1 , and amide III at 1285 cm -1 . The asynchronous correlation peaks have provided new information for bands due to asymmetric and symmetric bending and rocking modes of the methyl groups. The intensities of the bands at 1455 and 1159 cm -1 due to the asymmetric methyl bending mode and methyl (-N) rocking mode, respectively, decrease at a lower temperature than those of other bands assignable to amide modes. The result indicates that the conformational changes of the methyl groups is a precursor of the full dissociation of amide hydrogen bondings.