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.

The serial millisecond crystallography instrument at the Australian Synchrotron incorporating the "Lipidico" injector.

A serial millisecond crystallography (SMX) facility has recently been implemented at the macromolecular crystallography beamline, MX2 at the Australian Synchrotron. The setup utilizes a combination of an EIGER X 16M detector system and an in-house developed high-viscosity injector, "Lipidico." Lipidico uses a syringe needle to extrude the microcrystal-containing viscous media and it is compatible with commercially available syringes. The combination of sample delivery via protein crystals suspended in a viscous mixture and a millisecond frame rate detector enables high-throughput serial crystallography at the Australian Synchrotron. A hit-finding algorithm, based on the principles of "robust-statistics," is employed to rapidly process the data. Here we present the first SMX experimental results with a detector frame rate of 100 Hz (10 ms exposures) and the Lipidico injector using a mixture of lysozyme microcrystals embedded in high vacuum silicon grease. Details of the experimental setup, sample injector, and data analysis pipeline are designed and developed as part of the Australian Synchrotron SMX instrument and are reviewed here.

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.

Gastric protein breakdown and pancreatic enzyme activities in response to two different dietary protein sources in newly weaned pigs.

Seventy pigs were weaned at 25 d of age and fed diets based on either skim-milk powder (SMP) or soybean protein concentrate (SOY). At 0, 3, 6, and 10 d after weaning, pigs were anesthetized, their pancreases were removed, and digesta were collected from different sections of the digestive tract. The ratio between trichloroacetic acid (TCA)-precipitable protein and total (crude) protein (pp:cp) in gastric digesta was higher with SOY feed than with SMP feed. In the jejunum, no difference was found; hence, the degree of protein breakdown in jejunal chyme did not differ between protein sources. Trypsin activities in jejunal chyme and in pancreatic tissue increased (P < .01) after weaning. Chymotrypsin activity in pancreatic tissue tended to decrease after weaning and did not reach "weaning levels" for at least 10 d. Pancreatic trypsin developed more rapidly than chymotrypsin after weaning. Chymotrypsin activities in jejunal digesta were higher (P < .05) for the pigs fed SMP than for those fed SOY. Protease activities in the jejunum at d 6 after weaning were clearly affected (P < .05) by feed intake after weaning. The ratio between trypsin and chymotrypsin activity in jejunal chyme was higher (P < .05) for SOY-fed pigs than for SMP-fed pigs. It was concluded that the stomach plays an important role in the digestion of milk protein and that the development of pancreatic proteases after weaning (synthesis, secretion, breakdown) depends on feed intake and on dietary protein source.

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.

Biomechanical effects of environmental and engineered particles on human airway smooth muscle cells.

The past decade has seen significant increases in combustion-generated ambient particles, which contain a nanosized fraction (less than 100 nm), and even greater increases have occurred in engineered nanoparticles (NPs) propelled by the booming nanotechnology industry. Although inhalation of these particulates has become a public health concern, human health effects and mechanisms of action for NPs are not well understood. Focusing on the human airway smooth muscle cell, here we show that the cellular mechanical function is altered by particulate exposure in a manner that is dependent upon particle material, size and dose. We used Alamar Blue assay to measure cell viability and optical magnetic twisting cytometry to measure cell stiffness and agonist-induced contractility. The eight particle species fell into four categories, based on their respective effect on cell viability and on mechanical function. Cell viability was impaired and cell contractility was decreased by (i) zinc oxide (40-100 nm and less than 44 microm) and copper(II) oxide (less than 50 nm); cell contractility was decreased by (ii) fluorescent polystyrene spheres (40 nm), increased by (iii) welding fumes and unchanged by (iv) diesel exhaust particles, titanium dioxide (25 nm) and copper(II) oxide (less than 5 microm), although in none of these cases was cell viability impaired. Treatment with hydrogen peroxide up to 500 microM did not alter viability or cell mechanics, suggesting that the particle effects are unlikely to be mediated by particle-generated reactive oxygen species. Our results highlight the susceptibility of cellular mechanical function to particulate exposures and suggest that direct exposure of the airway smooth muscle cells to particulates may initiate or aggravate respiratory diseases.

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.