pH-regulated formation of amyloid-like beta-sheet assemblies from polyglutamate grafted polyallylamine

A novel artificial protein with simple primary structure, poly(gamma-methyl-L-glutamate)-grafted polyallylamine, has been prepared and the resultant peptide has shown a unique property of pH-regulated conformation and morphology.     

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles in solution

Temperature-induced reversible morphological changes of polystyrene-block-poly(ethylene oxide) micelles with degrees of polymerization of 962 for the PS and 227 for the PEO blocks (PS962-b-PEO227) in N,N-dimethylformamide (DMF)/water, in which water is a selective solvent for the PEO block, were observed. For a system with 0.2 wt % copolymer concentration and 4.5 wt % water concentration in DMF/water, the micelle morphology observed in transmission electron microscopy changed from vesicles at room temperature to worm-like cylinders and then to spheres with increasing temperature. Mixed morphologies were also formed in the intermediate temperature regions. Cooling the system back to room temperature regenerated the vesicle morphology, indicating that the morphological changes were reversible. No hysteresis was observed in the morphological changes during heating and cooling. Dynamic light scattering revealed that the hydrodynamic radius of the micelles decreased with increasing temperature. Combined static and dynamic light scattering results supported the change in morphology with temperature. The critical micellization temperatures and critical morphological transition temperatures were determined by turbidity measurements and were found to be dependent on the copolymer and water concentrations in the DMF/water system. The morphological changes were only possible if the water concentration in the DMF/water system was low, or else the mobility of the PS blocks would be severely restricted. The driving force for these morphological changes was understood to be mainly a reduction in the free energy of the corona and a minor reduction in the free energy of the interface. Morphological observations at different time periods of isothermal experiments indicated that in the pathway from one equilibrium morphology to another, large compound micelles formed as an intermediate or metastable stage.     

pH-driven conformational switch of "i-motif" DNA for the reversible assembly of gold nanoparticles

Herein we demonstrate that gold nanoparticles conjugated to "i-motif" DNA behave like a pH dependent switch that undergoes reversible aggregations which can be easily visualized by the naked eye.     

Intelligent colloidal hybrids via reversible pH-induced complexation of polyelectrolyte and silica nanoparticles

We present novel intelligent colloidal polymer/silica nanocomposites, in which the complexation of cationic silica nanoparticles and a weak anionic polyelectrolyte can be manipulated simply by pH change through a hydrogen-bonding interaction and ionic complexation caused by hydrogen-transfer interactions between the constituents. Special silica particles which have nanometer size (diameter approximately 3.0 nm) and two independent proton-accepting sites were developed in this study. Both the silica and poly(acrylic acid) form transparent colloidal solutions in water, while a white turbid dispersion was obtained just after mixing the two solutions due to the complexation. The pH-induced association-dissociation behavior was confirmed by the turbidity and potentiometric titration measurements. The assembled structures of the hybrids were visualized by scanning force microscopy.     

pH-induced reversible permeability control of the 4-carboxy acrylanilide-methyl methacrylate copolymer membrane

To develop a polymer membrane whose permeability of water-soluble compounds could be controlled in response to a pH change of the medium, a polyelectrolyte membrane containing an aromatic carboxyl group, i.e., 4-carboxyacrylanilide (CAAn)-methyl methacrylate(MMA) copolymer membrane, was prepared. The water content of the CAAn-MMA copolymer membrane increased with increasing pH owing to ionization change in the CAAn moiety. This increase was particularly remarkable in the pH range from 5 to 7. The change in water content was reversible and could be controlled by the CAAn composition in the copolymer. Permeation of 1,4-bis(2-hydroxyethoxy)benzene(DHEB) through the CAAn-MMA copolymer membrane was investigated in solutions of various pH. Change in the permeability of DHEB in function of the pH was similar to the change in membrane water content. This was explained by an increase in the permeation pathway of DHEB in the membrane, in proportion to that in the water content. Moreover, when a pH-sensitive membrane was placed in a solution in which an enzymatic reaction occurred, substrate-induced permeation control of DHEB through the CAAn-MMA copolymer membrane was possible.     

Effect of cyclodextrins on pH-induced conformational transition of poly(methacrylic acid)

A pH-induced conformational transition of atactic poly(2-methylprop-2-enoic acid) (poly(methacrylic acid), PMMA) from the contracted to expanded conformation was investigated by viscometry, potentiometric titration, and anthracene solubilisation in the presence of low-molecular-mass non-ionogenic co-solutes-glucose, α-cyclodextrin (αCD), and γ-cyclodextrin (γCD), respectively. No effect of glucose and αCD on the conformational transition was observed with either of the methods used. On the other hand, the characteristic features of the conformational transition were absent in the presence of γCD. The different effects of the co-solutes indicate that the interaction between PMAA and γCD corresponds to the partial inclusion of the PMAA chain into the γCD cavity. The viscometry and anthracene solubilisation imply that γCD promotes the expanded conformation of PMAA at low pH. The potentiometric titration does not support this conclusion. Even though there is no break on the Henderson-Hasselbalch plot, a characteristic of the conformational transition, the potentiometric behaviour corresponds to that of the contracted PMMA conformation. Thus the results suggest the hierarchical picture of the PMAA conformation at low pH in which the local arrangement of the PMAA chain is a prerequisite for clustering on a larger scale.     

pH-controlled reversible assembly of peptide-functionalized gold nanoparticles

The assembly/disassembly process of carboxylated peptide-functionalized gold nanoparticles (peptide-GNPs) was studied within the pH interval of 2.5 to 10. The assembly process was not well controlled at pH 2.5, leading to the formation of 3D structures of GNPs, whereas at pH 4 we observed controlled assembly with the formation of only a network of 1D chains. In the pH range of 2.5 to 4, the assembly proceeded with the formation of a combination of two extremes (i.e., having both 1D and 2D nanostructures). The assembly process was reversed on changing the pH of the medium to 10. The assembly/disassembly process was monitored using UV-vis spectroscopy and finally confirmed by TEM analysis. This assembly resulted from the intermolecular H-bonding between two carboxylic acid groups of peptides bound to the two adjacent GNPs and were confirmed by FTIR spectroscopy.     

Correlation of chemical reactivity of Nudaurelia capensis omega virus with a pH-induced conformational change

Nudaurelia capensis omega virus, which undergoes one of the largest known structural changes of icosahedral viruses in response to its environment, exhibits chemical reactivity which depends on its conformational state.     

Acetanilide mediated reversible assembly and disassembly of Au nanoparticles

Herein we report the generation of Au nanoparticles (NPs) by sparingly soluble acetanilide in water. We also report the formation of linear chain-like superstructures of self-assembled Au NPs, in the presence of excess acetanilide. This was achieved in two different ways. In the first method, acetanilide was added, with increasing concentration, into aqueous HAuCl(4) to produce Au NPs as well as for the formation of assembly, which varied according to the concentration of acetanilide. The other route involved formation of spherical Au NPs at the lowest concentration of acetanilide, which was followed by the formation of assembly of various lengths upon further addition of variable amount of acetanilide. The assemblies were stable in aqueous solution for days with characteristic UV-vis absorption spectra consisting of two peaks. While the wavelength of the first peak remained the same, the position of the second peak changed to longer wavelength with increasing acetanilide concentration. Interestingly, the linear chain-like arrays could be broken into individual particles by first dilution of the solution concentration followed by treatment with ultrasonic waves. The individual Au NPs again formed linear chain-like arrays upon addition of excess acetanilide.     

Model for reversible nanoparticle assembly in a polymer matrix

The clustering of nanoparticles (NPs) in solutions and polymer melts depends sensitively on the strength and directionality of the NP interactions involved, as well as the molecular geometry and interactions of the dispersing fluids. Since clustering can strongly influence the properties of polymer-NP materials, we aim to better elucidate the mechanism of reversible self-assembly of highly symmetric NPs into clusters under equilibrium conditions. Our results are based on molecular dynamics simulations of icosahedral NP with a long-ranged interaction intended to mimic the polymer-mediated interactions of a polymer-melt matrix. To distinguish effects of polymer-mediated interactions from bare NP interactions, we compare the NP assembly in our coarse-grained model to the case where the NP interactions are purely short ranged. For the "control" case of NPs with short-ranged interactions and no polymer matrix, we find that the particles exhibit ordinary phase separation. By incorporating physically plausible long-ranged interactions, we suppress phase separation and qualitatively reproduce the thermally reversible cluster formation found previously in computations for NPs with short-ranged interactions in an explicit polymer-melt matrix. We further characterize the assembly process by evaluating the cluster properties and the location of the self-assembly transition. Our findings are consistent with a theoretical model for equilibrium clustering when the particle association is subject to a constraint. In particular, the density dependence of the average cluster mass exhibits a linear concentration dependence, in contrast to the square root dependence found in freely associating systems. The coarse-grained model we use to simulate NP in a polymer matrix shares many features of potentials used to model colloidal systems. The model should be practically valuable for exploring factors that control the dispersion of NP in polymer matrices where explicit simulation of the polymer matrix is too time consuming.     

pH-induced protonation of lysine in aqueous solution causes chemical shifts in X-ray photoelectron spectroscopy

We demonstrate the applicability of X-ray photoelectron spectroscopy to obtain charge- and site-specific electronic structural information of biomolecules in aqueous solution. Changing the pH of an aqueous solution of lysine from basic to acidic results in nitrogen 1s and carbon 1s chemical shifts to higher binding energies. These shifts are associated with the sequential protonation of the two amino groups, which affects both charge state and hydrogen bonding to the surrounding water molecules. The N1s chemical shift is 2.2 eV, and for carbon atoms directly neighboring a nitrogen the shift for C1s is approximately 0.4 eV. The experimental binding energies agree reasonably with our calculated energies of lysine(aq) for different pH values.     

Time-dependent conformational changes of polyelectrolyte complexes in solution

Changes in the conformation of the polyelectrolytes when contacted with oppositely charged polyelectrolytes or when subjected to shifts in solution conditions (pH), have been studied in this work along with reversibility of the changes using fluorescence spectroscopy. While changes due to both of the above are marked, they are measurably different from each other. Thus, the extent of coiling of the complexes formed between the anionic polyelectrolyte, maleicacid anhydride–propene and the cationic polyelectrolytes was much higher than that achieved by the change in pH alone. Also, while the changes due to pH shifts were fast and reversible, that due to complexation between oppositely charged ones involved first a rapid uncoiling followed by slow recoiling to a new structure. Interestingly, shifting the coiled conformation to an even more coiled one resulted in a new reversible state, but shifting to a stretched state by complexation led to a somewhat irreversible structure. Also maximum interaction obtained between the anionic and one cationic polyelectrolyte was markedly higher than that between the former and another cationic polymer. These observations using fluorescence spectroscopy was consistent with that obtained by the potentiometric titration. The study clearly shows the importance of the manner in which the polyelectrolytes are equilibrated to desired solution conditions. These results are interpreted here in terms of deprotonation/protonation of the polyelectroytes upon pH change and complexation with oppositely charged ones resulting in screening of charges as well as stiffening.     

The conformational features of palytoxin in aqueous solution

Conformational features of palytoxin and acetylated palytoxin were investigated by detailed analyses of NOESY spectra. The conformational differences between palytoxin and acetylated palytoxin may account for the difference in the assembly state of palytoxin, which exists as an associated dimer, and the acetylated derivative, which exists as a monomer in aqueous solution. Two palytoxin units in the dimer may come in contact with each other at the hydrophobic region (C21-40) and the region around two conjugated double bonds (C60-84). The amino group of palytoxin is important for biological activities via Na/K ATPase, but it was not found to be involved in the contact faces of the two palytoxin units. This information should aid in revealing how palytoxin interacts with Na/K ATPase.     

Dithiosalicylides and trithiosalicylides. Their conformational behaviour in solution

The trithiosalicylide derivatives ($\text{11}$) - ($\text{14}$) have been synthesised and shown to exist in solution as ring inverting ($\text{15}$?$\text{15}$^*) enantiomeric helical conformations.     

Theoretical conformational analysis of substituted nitroethenes in solution

Theoretical conformational analysis of 1-nitro- and 1-bromo-1-nitro-2-(trichloromethyl)ethenes dissolved in methylene chloride and benzene was carried out by the B3LYP/6-31G* method. The calculated structures of these compounds were found to nicely fit experiment: Both in the gas phase and in solution, 1-nitro-(2-trichloromethyl)ethene is an E isomer, while its bromine-containing analog is a Z isomer.