High pressure effect on the main transition from the ripple gel P′β phase to the liquid crystal (Lα) phase in dipalmitoylphosphatidylcholine. Microcalorimetric study

Interrogating the Structural Dynamics and Energetics of Biomolecular Systems with Pressure Modulation

High hydrostatic pressure affects the structure, dynamics, and stability of biomolecular systems and is a key parameter in the context of the exploration of the origin and the physical limits of life. This review lays out the conceptual framework for exploring the conformational fluctuations, dynamical properties, and activity of biomolecular systems using pressure perturbation. Complementary pressure-jump relaxation studies are useful tools to study the kinetics and mechanisms of biomolecular phase transitions and structural transformations, such as membrane fusion or protein and nucleic acid folding. Finally, the advantages of using pressure to explore biomolecular assemblies and modulate enzymatic reactions are discussed.

Effect of C60 on the phase transition behavior of a lipid bilayer under high pressure

Interactions between fullerenes and cells and effects on the main transition of lipid bilayers have attracted much attention in biophysics in recent years. By employing coarse-grained molecular dynamics simulations, we obtained the temperature–pressure phase diagrams of a dipalmitoylphosphatidylcholine bilayer, which exhibits a gel phase and a fluid phase, with variation of the C₆₀versus lipid ratios. The simulation results show that the critical area per lipid at the fluid–gel main phase transition boundary increases with the increasing ratios of C₆₀. A critical area per lipid of 0.594 ± 0.002 nm² is obtained when the ratio of C₆₀ reaches 6.4% while that of the pure bilayer case is 0.572 ± 0.002 nm². The main transition temperature, T_m, remains almost unchanged with the addition of C₆₀ below a ratio of 4.7%, while a 2 K decrease of T_m is observed at a ratio of 6.4% under various pressures. Consequently, the presence of C₆₀ in the bilayer, with the ratio of C₆₀ less than 4.7%, will not influence the main transition behavior of the bilayer even under pressure as high as 1500 bar. The radial distribution function analyses suggest that the presence of C₆₀ produces no impact on the radial distribution of the lipids in the bilayers. The lateral density profiles show that the incorporation of C₆₀ with relatively high ratios stabilizes the thickness of the bilayer.

By employing coarse-grained molecular dynamics simulations, we obtained the temperature–pressure phase diagrams of a dipalmitoylphosphatidylcholine bilayer, which exhibits a gel phase and a fluid phase, with variation of the C₆₀versus lipid ratios.

MAS 1H NMR Probes Freezing Point Depression of Water and Liquid-Gel Phase Transitions in Liposomes

The lipid bilayer typical of hydrated biological membranes is characterized by a liquid-crystalline, highly dynamic state. Upon cooling or dehydration, these membranes undergo a cooperative transition to a rigidified, more-ordered, gel phase. This characteristic phase transition is of significant biological and biophysical interest, for instance in studies of freezing-tolerant organisms. Magic-angle-spinning (MAS) solid-state NMR (ssNMR) spectroscopy allows for the detection and characterization of the phase transitions over a wide temperature range. In this study we employ MAS ¹H NMR to probe the phase transitions of both solvent molecules and different hydrated phospholipids, including tetraoleoyl cardiolipin (TOCL) and several phosphatidylcholine lipid species. The employed MAS NMR sample conditions cause a previously noted substantial reduction in the freezing point of the solvent phase. The effect on the solvent is caused by confinement of the aqueous solvent in the small and densely packed MAS NMR samples. In this study we report and examine how the freezing point depression also impacts the lipid phase transition, causing a ssNMR-observed reduction in the lipids' melting temperature (T_m). The molecular underpinnings of this phenomenon are discussed and compared with previous studies of membrane-associated water phases and the impact of membrane-protective cryoprotectants.

High-pressure differential scanning microcalorimeter

A differential scanning microcalorimeter for studying thermotropic conformational transitions of biopolymers at high pressure has been designed. The calorimeter allows taking measurements of partial heat capacity of biopolymer solutions vs. temperature at pressures up to 3000 atm. The principles of operation of the device, methods of its calibration, as well as possible applications are discussed.

Pressure Effects on Artificial and Cellular Membranes

We review the combined effect of temperature and pressure on the structure, dynamics and phase behavior of lipid bilayers, differing in chain length, headgroup structure and composition as revealed by thermodynamic, spectroscopic and scattering experiments. The effect of additives, such as ions, cholesterol, and anaesthetics is discussed as well. Our data include also reports on the effect of pressure on the lateral organization of heterogeneous lipid membranes and lipid extracts from cellular membranes, as well as the influence of peptide and protein incorporation on the pressure-dependent structure and phase behavior of lipid membranes. Moreover, the effects of pressure on membrane protein function are summarized. Finally, we introduce pressure as a kinetic variable for studying the kinetics of various lipid phase transformations.