Interplay between hydration water and headgroup dynamics in lipid bilayers.

In this study, the interplay between water and lipid dynamics has been investigated by broadband dielectric spectroscopy and modulated differential scanning calorimetry (MDSC). The multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been studied over a broad temperature range at three different water contents: about 3, 6, and 9 water molecules per lipid molecule. The results from the dielectric relaxation measurements show that at temperatures <250 K the lipid headgroup rotation is described by a super-Arrhenius temperature dependence at the lowest hydration level and by the Arrhenius law at the highest hydration level. This difference in the temperature dependence of the lipid headgroup rotation can be explained by the increasing interaction between the headgroups with decreasing water content, which causes their rotational motion to be more cooperative in character. The main water relaxation shows an anomalous dependence on the water content in the supercooled and glassy regime. In contrast to the general behavior of interfacial water, the water dynamics is fastest in the driest sample and its temperature dependence is best described by a super-Arrhenius temperature dependence. The best explanation for this anomalous behavior is that the water relaxation becomes more determined by fast local lipid motions than by the intrinsic water dynamics at low water contents. In support for this interpretation is the finding that the relaxation time of the main water process is faster than that in most other host systems at temperatures below 180 K. Thus, the dielectric relaxation data show clearly the strong interplay between water and lipid dynamics; the water influences the lipid dynamics and vice versa. In the MDSC data, we observe a weak enthalpy relaxation at 203 K for the driest sample and at 179 K for the most hydrated sample, attributed to the freezing-in of the lipid headgroup rotation observed in the dielectric data, since this motion reaches a time scale of about 100 s at about the same temperatures.

Structural relaxations of phospholipids and water in planar membranes.

We have used dielectric spectroscopy and temperature modulated differential scanning calorimetry (TMDSC) to investigate the structural relaxation processes and phase transitions of water and lipids in multilamellar, planar phospholipids. At low hydration levels we observe the main structural relaxation related to the glass transition of the phospholipids. With increasing water content a more pronounced pretransition, attributed to a gel to ripple phase transition, is observed in the TMDSC data. In the proximity of this pretransition, a distinct change in the temperature dependence or alternatively a bifurcation into two processes is observed in the dielectric data. Around this temperature a crossover in the long-range ionic conductivity across the membranes is also observed, which is one of the key parameters for biological membranes. Thus, the major dynamical changes do not occur at the main, i.e., the gel to liquid structural phase transition, but at a pretransition that occurs roughly 20 K below the main transition.

Solvent and lipid dynamics of hydrated lipid bilayers by incoherent quasielastic neutron scattering.

The microscopic dynamics of the planar, multilamellar lipid bilayer system 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) has been investigated using quasielastic neutron scattering. The DMPC was hydrated to a level corresponding to approximately nine water molecules per lipid molecule. Selective deuteration has been used to separately extract the dynamics of the water, the acyl chains, and the polar head groups from the strong incoherent scattering of the remaining hydrogen atoms. Furthermore, the motions parallel and perpendicular to the bilayers were probed by using two different sample orientations relative to the incident neutron beam. For both sample orientations, the results showed an onset of water motions at 260 K on the experimental time scale of about 100 ps. From lack of wave-vector dependence of the onset temperature for water motions, it is evident that the observed water dynamics is of mainly rotational character at such low temperatures. At 290 K, i.e., slightly below the gel-to-liquid transition around 295 K, the nature of the water dynamics had changed to a more translational character, well described by a jump-diffusion model. On the limited experimental time and length (about 10 A) scales, this jump-diffusion process was isotropic, despite the very anisotropic system. The acyl chains exhibited a weak onset of anharmonic motions already at 120 K, probably due to conformational changes (trans-gauche and/or syn-anti) in the plane of the lipid bilayers. Other anharmonic motions were not observed on the experimental time scale until temperature had been reached above the gel-to-liquid transition around 295 K, where the acyl chains start to show more substantial motions.

New class of dynamics in concentrated polymer gels.

We show that concentrated poly(methyl methacrylate) solution exhibits a new class of coupled dynamics, which can be regarded as an intermediate between the collective diffusion of solutions and the structural relaxations of glasses. This class of dynamics have a relaxation rate that is directly proportional to the wave vector. The transition from diffusive to coupled collective dynamics occurs at smaller length scales with increasing polymer concentration and decreasing temperature. The experimental observations can be understood by considering the contributions from physical cross-links interconnected by stiff polymer segments.

Comment on "Merging of and slow relaxation in supercooled liquids".

Fujima [T. Fujima, H. Frusawa, and K. Ito, Phys. Rev. E 66, 031503 (2002)] report on broadband dielectric relaxation measurements on two glass-formers. They find that the relaxation times of the beta relaxation follow different temperature dependences above and below the glass-transition temperature, Tg; i.e., there appears to be a crossover at Tg where the activation energy of the beta relaxation change. In this Comment we show that the observed behavior can be explained by analyzing the merging of the alpha and beta relaxations using an approach proposed by Williams. This analysis clearly shows that the low temperature (below Tg) behaviors of the alpha and beta relaxations can be used to describe also the high-temperature behavior (above Tg). The apparent change in activation energy is thus not to be identified with a change in relaxation mechanism.

Diffusive solvent dynamics in a polymer gel electrolyte studied by quasielastic neutron scattering.

A quasielastic neutron scattering study has been performed on a polymer gel electrolyte consisting of lithium perchlorate dissolved in ethylene carbonate/propylene carbonate and stabilized with poly(methyl methacrylate). The dynamics of the solvent, which is crucial for the ion conduction in this system, was probed using the hydrogen/deuterium contrast variation method with nondeuterated solvent and a deuterated polymer matrix. Two relaxation processes of the solvent were studied in the 10-400 microeV range at different temperatures. From analysis of the momentum transfer dependence of the processes we conclude that the faster process ( approximately 100 microeV) is related to rotational diffusion of the solvent and the slower process ( approximately 10 microeV) to translational diffusion of the solvent. The translational diffusion is found to be similar to the diffusion in the corresponding liquid electrolyte at short distances, but geometrically constrained by the polymer matrix at distances beyond approximately 5 A. The study indicates that the hindered diffusion of the solvent on a length scale of the polymer network interchain distance ( approximately 5-20 A) is sufficient to explain the reduced macroscopic diffusivity and ion conductivity of the gel electrolyte compared to the liquid electrolyte.

Dental caries experience in children with congenital heart disease: a case-control study.

OBJECTIVES: To compare the dental health of a group of children with complex congenital heart disease with that of age and gender matched healthy controls. DESIGN: Case-control study. SETTING: Faculty of Medicine and Odontology/Pediatric cardiology and Pedodontics, Umeå University, Sweden. SAMPLE AND METHODS: All the cases and their controls lived in the county of Västerbotten in northern Sweden. Each group comprised 41 children with a mean age of 6.5 years. Data were collected from medical and dental records while all bitewing radiographs were read separately by one of the authors. RESULTS: Children with congenital heart disease had significantly more caries in their primary teeth than the control group. The mean dmfs-value was 5.2 +/- 7.0 in the cardiac group compared to 2.2 +/- 3.5 in the control group (P < 0.05). Twenty-six of the children had all four 6-year-molars, and their mean DMFS-values were 0.9 +/- 1.9 in the cardiac group compared to 0.3 +/- 0.6 in the control group (P > 0.05). The children with congenital heart disease had received more caries prevention based on the use of fluorides than the control group. There was a significant correlation between the number of fluoride varnish treatments and the dmfs value of the child (r = 0.411, P < 0.01). Fifty-two per cent of the children in the cardiac group had been prescribed fluoride tablets on one or more occasions compared to 17% in the control group (P < 0.01). Number of months on digoxin medication and the dmfs-value had a significant correlation (r = 0.368, P < 0.05). Ten of the children had been on digoxin medication between 6 and 87 months; this subgroup had a mean dmfs-value of 10.1 +/- 8.5. CONCLUSION: Swedish children with complex congenital heart disease have poorer dental health than healthy age and gender matched controls in spite of intensive preventive efforts. In many cases, intervention had been given when caries were present. A closer cooperation between paediatric cardiology and paediatric dentistry is needed.

Power laws in polymer solution dynamics.

The dynamical screening length xi(h) in semidilute to highly concentrated polymer solutions of poly(methyl methacrylate) in propylene carbonate has been examined using photon correlation spectroscopy and pulsed field gradient nuclear magnetic resonance. A crossover between different concentration dependent regimes, xi(h) approximately phi(-alpha), where alpha is found to be approximately 0.5, approximately 1, and approximately 2, is observed when the local viscosity is taken into account. Here phi is the volume fraction of polymer in the solution. Well-defined crossovers between alpha=0.5 and alpha=1 corresponding to a transition from a marginal solvent to a theta solvent behavior have been predicted to occur due to the reduction of excluded-volume effects between the spatially correlated polymer segments with increasing polymer volume fraction. However, a clear experimental validation of the crossover has never been presented before. The third regime (alpha approximately 2) is observed in the highly concentrated region where the static screening length is comparable to the persistence length of the polymer. The observation indicates that the rigid rod model previously used to describe concentrated solutions is an oversimplification valid only in the very high concentration limit. The obtained results at high concentrations are discussed in the frame of a simple physical model where segments at the persistence length scale are treated as flexible rodlike segments.