Transition In Aqueous Solutions Research Articles

Preparation and thermoresponsive properties of helical polypeptides bearing pyridinium salts

Polypeptides bearing 3-methylpyridinium groups and BF4−prepared by nuleophilic substitution and ion-exchange reaction showed upper critical solution temperature (UCST)-type transitions in aqueous solutions.

遠紫外分光法による水溶液の電子遷移の解析と反応解析への展開

波長200 nm以下の遠紫外波長領域（far-ultraviolet, FUV）の光子エネルギーは，分子の様々な電子遷移に対応する．しかし水分子を含め多くの分子はFUV光を強く吸収するため，凝縮系試料のFUVスペクトル測定は困難であった．著者らは，アルミナプリズムを用いた減衰全反射（attenuated total reflection, ATR）型FUV分光システム（測定波長: 140～300 nm）を開発して，様々な液体，固体試料のFUVスペクトルを測定し，量子化学計算を援用して試料分子の電子遷移を明らかにしてきた．特に，液体水分子の第一電子遷移（Ã←X）を水素結合や溶質への水和の観点から検討してきた．また，FUV波長帯は多くの溶質が吸収を示すことから，紫外可視波長領域では吸収を示さない水溶液試料の分光分析を行ってきた．本論文では著者らがFUV分光法で進めてきた様々な電解質水溶液の電子遷移，水溶液溶存アミノ酸の電子遷移，及び水溶液溶存オゾンの光分解反応の解析に関する成果を紹介する．

Thermoresponsive Self‐Assembly of Nanostructures from a Collagen‐Like Peptide‐Containing Diblock Copolymer

Temperature-triggered formation of nanostructures with distinct biological activity offers opportunities in selective modification of matrices and in drug delivery. Toward these ends, diblock polymers comprising poly(diethylene glycol methyl ether methacrylate) (PDEGMEMA) conjugated to a triple helix-forming collagen-like peptide were produced. Triggered by the collapse of the thermoresponsive domain above its LCST, the conjugate undergoes a reversible transition in aqueous solution to form well-defined nanovesicles with diameters of approximately 100 nm, with a transition temperature of 37 °C. The incorporation of CLP domains in these nanostructures may offer opportunities for the selective targeting of collagen-containing matrices.

Noncanonical Self-Assembly of Highly Asymmetric Genetically Encoded Polypeptide Amphiphiles into Cylindrical Micelles

Elastin-like polypeptides (ELPs) are a class of biopolymers consisting of the pentameric repeat (VPGαG)n based on the sequence of mammalian tropoelastin that display a thermally induced soluble-to-insoluble phase transition in aqueous solution. We have discovered a remarkably simple approach to driving the spontaneous self-assembly of high molecular weight ELPs into nanostructures by genetically fusing a short 1.5 kDa (XGy)z assembly domain to one end of the ELP. Classical theories of self-assembly based on the geometric mass balance of hydrophilic and hydrophobic block copolymers suggest that these highly asymmetric polypeptides should form spherical micelles. Surprisingly, when sufficiently hydrophobic amino acids (X) are presented in a periodic sequence such as (FGG)8 or (YG)8, these highly asymmetric polypeptides self-assemble into cylindrical micelles whose length can be tuned by the sequence of the morphogenic tag. These nanostructures were characterized by light scattering, tunable resistive pulse sensing, fluorescence spectrophotometry, and thermal turbidimetry, as well as by cryogenic transmission electron microscopy (cryo-TEM) and small-angle neutron scattering (SANS). These short assembly domains provide a facile strategy to control the size, shape, and stability of stimuli responsive polypeptide nanostructures.

Behavior of supercooled aqueous solutions stemming from hidden liquid-liquid transition in water.

A popular hypothesis that explains the anomalies of supercooled water is the existence of a metastable liquid-liquid transition hidden below the line of homogeneous nucleation. If this transition exists and if it is terminated by a critical point, the addition of a solute should generate a line of liquid-liquid critical points emanating from the critical point of pure metastable water. We have analyzed thermodynamic consequences of this scenario. In particular, we consider the behavior of two systems, H2O-NaCl and H2O-glycerol. We find the behavior of the heat capacity in supercooled aqueous solutions of NaCl, as reported by Archer and Carter [J. Phys. Chem. B 104, 8563 (2000)], to be consistent with the presence of the metastable liquid-liquid transition. We elucidate the non-conserved nature of the order parameter (extent of "reaction" between two alternative structures of water) and the consequences of its coupling with conserved properties (density and concentration). We also show how the shape of the critical line in a solution controls the difference in concentration of the coexisting liquid phases.

Coil‐to‐Globule Transition of Poly(N‐isopropylacrylamide) in Aqueous Solution: Kinetics in Bulk and Nanopores

Differential scanning calorimetry (DSC) is used to study the kinetics of the coil‐to‐globule transition in aqueous solutions of poly(N‐isopropoylacrylamide) (PNIPAM) prepared in the bulk (3 and 10 wt%) and nanoconfined (10 wt% inside 30 nm silica pores) forms. It is demonstrated that the kinetics can be described in terms of the classical nucleation model. The proposed treatment affords estimating the free‐energy barrier and pre‐exponential factor of the transition. The application of the nucleation model to the DSC data collected for the three systems studied provides physical insights into the effect of increasing the transition temperature due to dilution and nanoconfinement. Dilution appears to raise the free‐energy barrier, whereas nanoconfinement causes a decrease in the pre‐exponential factor. image

Response of Tb(III) and Eu(III) centered luminescence on phase transitions in aqueous solutions of triblock copolymers

The work introduces the temperature induced quenching of Tb(III) and Eu(III) centered luminescence sensitized by ligands 2,2′:6′,2″-terpyridine and difloxacin in aqueous solutions of triblock copolymers, namely (PEO)13(PPO)30(PEO)13 (L64), (PPO)14(PEO)24(PPO)14 (17R4) and (PPO)8(PEO)22(PPO)8 (10R5). The results reveal the temperature induced shifting of the complex formation equilibriums in solutions of the triblock copolymers as the reason of the quenching of Eu(III) and Tb(III) centered luminescence. The correlation between the temperature induced quenching and the aggregation behavior of the triblock copolymers is revealed. Both the nature of the ligand and the architecture of the triblock copolymer affect the Tb(III) and Eu(III) centered luminescence response to the temperature induced phase transitions.

PH controlled size/shape in CTAB micelles with solubilized polar additives: A viscometry, scattering and spectral evaluation

In this manuscript we report pH induced micellar transition in aqueous solution of cetyltrimethylammonium bromide (CTAB) in the presence of three weakly polar aromatic additives viz. p-toluic acid, p-cresol and p-toluidine scrutinized by viscosity, nuclear magnetic resonance (NMR), dynamic light scattering (DLS) and small-angle neutron scattering (SANS) measurements. Interaction between these additives and CTAB micelles changes the size and shape of the micelles. Variation in pH alters the charge on the polar group and leads to protonation/deprotonation of acidic/basic group of the additives. Depending upon the pH of solution additives interact with CTAB micelles and accordingly change the solution behavior/aggregation characteristics. Morphological changes of surfactant aggregates as a function of additive concentration and pH were monitored by SANS measurements. NMR studies reveal a pH dependent location of the additive molecules in the micelles.

Thermoresponsive polymerized gemini dicationic ionic liquid

A new type of gemini poly(ionic liquid), poly[(1,8-octanediyl-bis(tri-n-butylphosphonium) 4-styrene sulfonate], was synthesized via the free radical polymerization of a dicationic ionic liquid monomer in DMF. This poly(ionic liquid) presented a lower critical solution temperature (LCST)-type phase transition in aqueous solution. Unusually this phase transition can be observed even at a concentration as low as 0.1 wt%. Additionally, the type and concentration of external salt can influence the cloud point. Copolymerisation of this gemini dicationic ionic liquid monomer with divinylbenzene crosslinker in water led to the formation of a hydrogel, which exhibited a temperature-triggered volume change in water.

NMR Study of Thermoresponsive Hyperbranched Polymer in Aqueous Solution with Implication on the Phase Transition

High-resolution 1H NMR has been used on the thermoresponsive hyperbranched polyethylenimines (HPEIs) modified with isobutyramide (IBAm) groups (HPEI-IBAm), to study the structure and dynamics of the macromolecules in aqueous solution before and after the phase transition. It shows that the HPEI-IBAm macromolecule having a high IBAm substitution degree has a clear phase transition in aqueous solution, whereas the HPEI-IBAm macromolecule having a low IBAm substitution degree does not. The different phase transition behaviors have been attributed to the content as well as the distribution of the IBAm groups in the macromolecules. In order to deepen the understanding of the phase transition, the hydrophobic–hydrophobic interaction inside the HPEI-IBAm macromolecules was investigated by monitoring the 1H–1H NOEs between the different hydrophobic groups. An enhanced hydrophobic–hydrophobic interaction was observed in the HPEI-IBAm macromolecule having a high IBAm substitution degree after the phase transition, ...

General nature of liquid–liquid transition in aqueous organic solutions

The presence or absence of a liquid-liquid transition in water is one of the hot topics in liquid science, and while a liquid-liquid transition in water/glycerol mixtures is known, its generality in aqueous solutions has remained elusive. Here we reveal that 14 aqueous solutions of sugar and polyol molecules, which have an ability to form hydrogen bonding with water molecules, exhibit liquid-liquid transitions. We find evidence that both melting of ice and liquid-liquid transitions in all these aqueous solutions are controlled solely by water activity, which is related to the difference in the chemical potential between an aqueous solution and pure water at the same temperature and pressure. Our theory shows that water activity is determined by the degree of local tetrahedral ordering, indicating that both phenomena are driven by structural ordering towards ice-like local structures. This has a significant implication on our understanding of the low-temperature behaviour of water.

Simulation study on dynamics of A- to B-form transition in aqueous DNA solution: Effect of alkali metal counterions

DNA and its conformational transition can be used to design nanometer-scale structures, nano-tweezers and nanomechanical devices. Experiments and molecular simulations have been used to study the concentration effect on the A-DNA→B-DNA conformational transition, but a systematical investigation on counterion effect on the dynamics of this transition has not been reported up to now. In present work, restrained and unrestrained molecular dynamics (MD) simulations have been performed to characterize the stability of DNA conformations and the dynamics of A-DNA→B-DNA transitions in aqueous solutions with different alkali metal counterions. The DNA duplex d(CGCGAATTCGCG)2, coion Cl− and counterions Li+, Na+, K+, Rb+ and Cs+ as well as water molecule were considered using the PARM99 force field in the AMBER8 package. It was found that B-form DNA is more stable than A-form DNA in aqueous electrolyte solutions with different alkali metal counterions. Increasing KCl concentration in solution hinders the A-DNA→B-DNA transition and the transition times for different alkali metal counterions conform to neither the simple sequence related to naked ion size nor to hydrated diameter, but an apparently abnormal sequence of K+ < Rb+ < Cs+ < Na+ < Li+. This abnormal sequence can be well understood in terms of an electrostatic model based on the effective cation diameters and the modified mean-spherical approximation (MMSA). The present results provide valuable information for the design of DNA-based nanomaterials and nanodevices.

Cationic Poly(ionic liquid) with Tunable Lower Critical Solution Temperature-Type Phase Transition.

A cationic polyelectrolyte based on the styrenic ionic liquid tributyl-4-vinylbenzylphosphonium pentanesulfonate was found to undergo a lower critical solution temperature (LCST)-type phase transition in aqueous solutions. This phase transition occurs in a wide temperature range in terms of polymer concentration as well as type and concentration of externally added salts. Anion exchange and salting out effects are responsible for the flexible phase transition temperature.

Synthesis, characterization, and micellization of pH-responsive poly(4-vinylpyridine)-block-poly(methacrylic acid) four-armed star-shaped block copolymers

pH-sensitive poly(4-vinylpyridine)-block-poly(methacrylic acid) (P4VP-b-PMAA) four-armed star-shaped block copolymers were synthesized by two-step atom transfer radical polymerization (ATRP), followed by hydrolysis of P4VP-b-poly(tert-butyl methacrylate) (P4VP-b-PtBMA). The chemical structure and molecular weight of the as-synthesized block copolymers were characterized by Fourier transform infrared spectrometry (FTIR), nuclear magnetic resonance (1H and 13C NMR), and gel permeation chromatography (GPC) determinations. The solution behavior was investigated by surface tension technique, ultraviolet visible (UV-vis) transmittance, transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potentials measurements. The experimental results indicated that the copolymers can spontaneously assemble into spherical-shaped core-shell micelle aggregates, with a critical micelle concentration (CMC) about 200 mg L−1, hydrodynamic diameters from 90 to 210 nm, depending on the environmental pH values and compositional ratios. The transmittance measurements revealed that the block copolymers produce evident phase transition in aqueous solution at pH from 6.5 to 7.0. Zeta potential data revealed high micelle stability. The as-synthesized block copolymers are anticipated to find their applications in the realms of specific drug release, metal loading and heterogeneous catalysis. Open image in new window

The effect of temperature induced phase transitions in aqueous solutions of triblock copolymers and Triton X-100 on the EPR, magnetic relaxation and luminescent characteristics of Gd(III) and Eu(III) ions

The present report is inspired by the idea to develop novel spectroscopy based techniques to detect the phase transitions in solutions of triblock copolymers with the use of Gd(III) and Eu(III) ions as probes. The choice of (PEO)13(PPO)30(PEO)13 (L64), (PPO)14(PEO)24(PPO)14 (17R4), (PPO)8(PEO)22(PPO)8 (10R5) and Triton X-100 is conditioned by their phase transitions occurring in mild conditions. The cloud point extraction (CPE) data reveal the conditions (pH and chelating agent) where the binding of lanthanides with surfactant rich phase of triblock copolymers and Triton X-100 occurs. The effect of phase transitions in solutions of triblock copolymers on the temperature resolved line widths in EPR spectra of Gd(III) aqua ions and transverse relaxation rates of Gd(III) inner-sphere water protons depends on the architecture of triblock copolymers and pH conditions. The steady state luminescence of Eu(III) complexes with phosphine oxide in aqueous solutions of triblock copolymers and TX-100 is influenced by the temperature induced aggregation and phase separation in solutions of reverse triblock copolymers 17R4 and 10R5, while the effect of the aggregation is insignificant for L64.

Rapid pH-dependent phase transition and elasticity of stimuli-responsive cationic poly(N,N-dimethylaminoethyl methacrylate) hydrogels prepared with a dimethacrylate crosslinker

Stimuli-responsive hydrogels prepared from poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) and its copolymers have attracted much interest to serve in biomedical and pharmaceutical applications. To investigate pH-dependent swelling and elasticity, a series of cationic hydrogels based on N,N-dimethylaminoethyl methacrylate were prepared by free radical crosslinking copolymerization at 60 °C in the presence of tetraethylene glycol dimethacrylate as the crosslinker. The equilibrium swelling and the mechanical properties of the PDMAEMA hydrogels were investigated as a function of the gel preparation concentration. To explain the effect of pH on the equilibrium swelling of the hydrogels, pH-dependent swelling studies were carried out in solutions of pH ranging from 2.1 to 10.7. It was found that the PDMAEMA hydrogels exhibit a rapid pH-dependent phase transition in aqueous solutions; that is, the gels first remain in the swollen state at acidic pH then collapse in a very narrow range of pH. The results showed that the volume of PDMAEMA hydrogels in acidic conditions is about 10- to 40-fold larger than that in the basic pH region. By using the Flory–Rehner theory, the characteristic network parameters of the PDMAEMA hydrogels were calculated and good agreement obtained between the swelling equilibria of hydrogels and their mechanical properties over the whole range of gel preparation concentration. © 2012 Society of Chemical Industry

NMR and Thermodynamic Study of Phase Transition in Aqueous Solutions of Thermoresponsive Amphiphilic Polymer

Abstract 1H NMR techniques combined with differential scanning calorimetry (DSC) experiments are employed to investigate the temperature-induced phase transition in aqueous poly(vinyl methyl ether) solutions. NMR data were used to construct van’t Hoff plots, and changes in the enthalpy and entropy characterizing the phase transition were determined. It follows from a comparison of NMR and DSC thermodynamic parameters (ΔH values) for solutions with c = 1–30 wt % that the size of the cooperative units (domains) undergoing the transition corresponds to the whole polymer chain.

1H NMR study of temperature-induced phase transitions in aqueous solutions of poly(N-isopropylmethacrylamide)/poly(N-vinylcaprolactam) mixtures

1H NMR spectroscopy was used to investigate the temperature-induced phase transitions in aqueous solutions of poly(N-isopropylmethacrylamide)/poly(N-vinylcaprolactam) (PIPMAm/PVCL) mixtures to find out if the phase transition of the given component (PIPMAm or PVCL) is affected by the presence of the second component. Our results that PVCL and PIPMAm transitions are in polymer mixtures shifted by ~2 K towards higher temperatures in comparison with neat polymers and depend on polymer concentration show that such effect exists. Spin–spin relaxation times of water (HDO) indicate that in solutions with c ≥ 1 wt% a portion of water is predominantly bound in PVCL mesoglobules even at temperatures above the LCST transition of PIPMAm component. Water is with time released from these mesoglobules without any induction period so indicating that it is mostly indirectly bound water. We assume that there is a direct connection between character of the bound water and the transition temperatures.

NMR and DSC study of temperature-induced phase transition in aqueous solutions of poly(N-isopropylmethacrylamide-co-acrylamide) copolymers

Combination of 1H NMR spectroscopy and differential scanning calorimetry (DSC) was used to investigate temperature-induced phase transition in D2O solutions of poly(N-isopropylmethacrylamide-co-acrylamide) random copolymers. Both the NMR and DSC data showed dependence on the acrylamide (AAm) content in the copolymer; with increasing AAm content, the phase transition is shifted to higher temperatures, and both phase-separated fractions determined by NMR and change of the enthalpy determined by DSC decrease faster than the content of thermosensitive N-isopropylmethacrylamide (NIPMAm) units in the copolymer. NMR data were used to construct van't Hoff plots, and changes of the enthalpy ΔH and entropy ΔS, characterizing the phase transition, were determined. As it follows from comparison of NMR and DSC thermodynamical parameters (ΔH values), the size of the cooperative units (domains), undergoing the transition as a whole, decreases with increasing AAm content in the copolymer since the NIPMAm collapsed domains are separated by regions with hydrated AAm and surrounding NIPMAm sequences.

Aggregate Transitions in Aqueous Solutions of Sodium Dodecylsulfate with a “Gemini-Type” Organic Salt

Effects of a "gemini-type" organic salt 1,2-bis(2-benzylammoniumethoxy) ethane dichloride (BEO) on the aggregation behavior of sodium dodecylsulfate (SDS) have been investigated by turbidity, surface tension, isothermal titration microcalorimetry, dynamic light scattering, cryogenic transmission electron microscopy, (1)H NMR spectroscopy, and differential scanning microcalorimetry. The aggregation behavior of the SDS/BEO mixed aqueous solution shows strong concentration and ratio dependence. For the SDS/BEO solution with a molar ratio of 5:1, large loose irregular aggregates, vesicles, and long thread-like micelles are formed in succession with the increase of the total SDS and BEO concentration. Because BEO has two positive charges, the SDS/BEO solution may consist of the (SDS)(2)-BEO gemini-type complex, the SDS-BEO complex and extra SDS. The aggregation ability and surface activity of the SDS/BEO mixture exhibit the characteristics of gemini-type surfactants. Along with the results of DSC and (1)H NMR, the (SDS)(2)-BEO gemini-type structure is confirmed to exist in the system. This work provides an approach to construct the surfactant systems with the characteristics of gemini surfactants through intermolecular interaction between a two-charged organic salt and oppositely charged single-chain surfactants.