Neutron-Scattering Determination of the Li+ Environment in an Aqueous Poly(Ethylene Oxide) Solution

Study of PEGylated lipid layers as a model for PEGylated liposome surfaces: molecular dynamics simulation and Langmuir monolayer studies

We have combined Langmuir monolayer film experiments and all-atom molecular dynamics (MD) simulation of a bilayer to study the surface structure of a PEGylated liposome and its interaction with the ionic environment present under physiological conditions. Lipids that form both gel and liquid-crystalline membranes have been used in our study. By varying the salt concentration in the Langmuir film experiment and including salt at the physiological level in the simulation, we have studied the effect of salt ions present in the blood plasma on the structure of the poly(ethylene glycol) (PEG) layer. We have also studied the interaction between the PEG layer and the lipid bilayer in both the liquid-crystalline and gel states. The MD simulation shows two clear results: (a) The Na(+) ions form close interactions with the PEG oxygens, with the PEG chains forming loops around them and (b) PEG penetrates the lipid core of the membrane for the case of a liquid-crystalline membrane but is excluded from the tighter structure of the gel membrane. The Langmuir monolayer results indicate that the salt concentration affects the PEGylated lipid system, and these results can be interpreted in a fashion that is in agreement with the results of our MD simulation. We conclude that the currently accepted picture of the PEG surface layer acting as a generic neutral hydrophilic polymer entirely outside the membrane, with its effect explained through steric interactions, is not sufficient. The phenomena we have observed may affect both the interaction between the liposome and bloodstream proteins and the liquid-crystalline-gel transition and is thus relevant to nanotechnological drug delivery device design.

Molecular crowding stabilizes folded RNA structure by the excluded volume effect

Crowder molecules in solution alter the equilibrium between folded and unfolded states of biological macromolecules. It is therefore critical to account for the influence of these other molecules when describing the folding of RNA inside the cell. Small angle X-ray scattering experiments are reported on a 64 kDa bacterial group I ribozyme in the presence of polyethylene-glycol 1000 (PEG-1000), a molecular crowder with an average molecular weight of 1000 Da. In agreement with expected excluded volume effects, PEG favors more compact RNA structures. First, the transition from the unfolded to the folded (more compact) state occurs at lower MgCl(2) concentrations in PEG. Second, the radius of gyration of the unfolded RNA decreases from 76 to 64 A as the PEG concentration increases from 0 to 20% wt/vol. Changes to water and ion activities were measured experimentally, and theoretical models were used to evaluate the excluded volume. We conclude that the dominant influence of the PEG crowder on the folding process is the excluded volume effect.

Lower consolute boundaries of the nonionic surfactant C8E5 in aqueous alkali halide solutions: An approach to reproduce the effects of alkali halides on the cloud-point temperature

In the temperature-composition phase diagram of the nonionic surfactant n-octyl-hydroxypenta(oxyethylene), C(8)E(5), there are three principal curves; the one for the critical micelle concentration (cmc), the one delineating the existence of the hexagonal phase, and then the lower consolute boundary (lcb). In this work it is clarified how the presence of the alkali halides NaF, LiCl, NaCl, NaBr and NaI in the aqueous solutions, up to high molalities, change the lcb temperature-position and shape. The lcbs are obtained from measurements of cloud-point temperatures. Rather marked anion-controlled shifts are observed in the boundary temperature-position, and the order of the anions is in accordance with the prediction of the Hofmeister series. Also the shape of the boundary is affected in an anion-specific way, so that the largest changes are found with the strongest salting-out agent. The separation point varies in distinctly non-linear manners with the molality of the studied alkali halides. An approach is presented that can reproduce the effects of the alkali halides on the cloud-point temperature of C(8)E(5) and a poly(ethylene oxide) polymer, at low amounts of the macroentities. In this approach use is made of the known behaviour of the electrolytes at the air/water surface and the virial expansion, to account for the initial salting-out/-in effect and the variation of the effect with electrolyte molality.

Structural and Thermodynamic Aspects of Ionic Solvation in Concentrated Aqueous Poly(ethylene glycol)

Solvation of ions in concentrated aqueous poly(ethylene glycol) (PEG) has been studied from thermodynamic and structural viewpoints using ion-transfer voltammetry at the interface between aqueous and nitrobenzene phases and X-ray absorption fine structure (XAFS). Systematic changes in the ion-transfer potential from water to aqueous PEG have been confirmed for several ions relative to the corresponding potential of tetraethylammonium ion (Et4N+), which is almost independent of PEG concentration. The results obtained for alkali cations strongly suggest the involvement of their complexation with PEG even in relatively diluted PEG solutions. It has been implied that the solvation circumstances of Br- and ClO4- are drastically altered when the PEG concentration becomes higher than particular critical values (e.g., 30-50% PEG200), where free water molecules are diminished because of the hydration of PEG. XAFS measurements have also been performed for K+ and Br- to get direct evidence for these findings. Although the spectra at the K K-edge clearly indicate the presence of a PEG complex of K+ in relatively diluted PEG solutions ( approximately 33% PEG200), an obvious increase in its ion-transfer potential has been detected at lower PEG concentrations, indicating that complexes formed at the interface rather than in bulk solution are transferred into an organic phase. Br- is fully hydrated in 0-50% PEG solutions, whereas some water molecules are replaced by PEG when the PEG concentration increases. Increasing the PEG concentration causes decreases in the coordination number from 6 in water to 2-3 in neat PEG. Thus, the present approach not only has elucidated the structural and thermodynamic aspects of ionic solvation in aqueous PEG but also has provided the information of the hydration of PEG.

Solvation Structure of Li+ in Concentrated LiPF6−Propylene Carbonate Solutions

Time-of-flight neutron diffraction measurements were carried out for 6Li/7Li isotopically substituted 10 mol % LiPF6-propylene carbonate-d6 (PC-d6) solutions, in order to obtain structural information on the first solvation shell of Li+. Structural parameters concerning the nearest neighbor Li+...PC and Li+...PF6- interactions were determined through least-squares fitting analysis of the observed difference function, DeltaLi(Q). It has been revealed that the first solvation shell of Li+ consists in average of 4.5(1) PC molecules with an intermolecular Li+...O(PC) distance of 2.04(1) A. The angle Li+...O=C bond angle has been determined to be 138(2) degrees.