Dynamics of the Gel to Fluid Phase Transformation in Unilamellar DPPC Vesicles

Abstract

The dynamics of the gel to fluid phase transformation in 100 nm large unilamellar vesicles (LUV) of 1,2-dipalmitoyl(d62)-sn-glycero-3-phosphocholine (d62-DPPC), has been studied by laser-induced temperature-jump initiation coupled with time-resolved infrared spectroscopy and by MD simulations. The infrared transients that characterize the temperature dependent phase transformation are complex, extending from the nanosecond to the millisecond time scales. An initial fast (submicrosecond) component can be modeled by partial melting of the gel domains, initiated at pre-existing defects at the edges of the faceted structure of the gel phase. Molecular dynamics simulations support the model of fast melting from edge defects. The extent of melting during the fast phase is limited by the area expansion on melting, which leads to a surface pressure that raises the effective melting temperature. Subsequent melting is observed to follow highly stretched exponential kinetics, consistent with collective relaxation of the surface pressure through a hierarchy of surface undulations with different relaxation times. The slowest step is water diffusion through the bilayer to allow the vesicle volume to grow along with its expanded surface area. The results demonstrate that the dominant relaxation in the gel to fluid phase transformation in response to a large T-jump perturbation (compared to the transition width) is fast (submicrosecond), which has important practical and fundamental consequences.

Time-resolved measurements of an ion channel conformational change driven by a membrane phase transition

“…The presence of gD in the sample increases the rate of the phase transition; at 1°C from the T m , the rate of the transition is ∼3.5-fold faster. Previous transient studies have also reported timescales consistent with critical behavior (6,24). At the high temperatures (T f = 30°C and 34°C), τ relax increases ( Fig.…”

“…2 A and B shows the kinetics for peak B of the ester groups for DMPC vesicles with and without gD over a 9°C range above T m : T f = T m + 1,6,10°C. Previous experiments on lipid vesicles, using a variety of probes, have found evidence of multiple relaxation timescales, and a dependence on the particular probe used (6,(22)(23)(24)(25). To address this uncertainty about the dynamics present, how they may depend on vibrational frequency, and which functional form is appropriate to describe the kinetics, we first used a maximum entropy (MaxEnt) method to determine the inverse Laplace transform (ILT) of the kinetic traces and obtain the underlying distribution of rates as a function, shown in Fig.…”

“…We report on the use of infrared (IR) spectroscopy in combination with a rapid temperature jump (T-jump) to monitor membrane protein conformational changes induced by rapid membrane thinning. Time-resolved IR spectroscopy is a labelfree reporter of the structure and dynamics of membranes and proteins using vibrations native to the system (3,6). However, picosecond vibrational lifetimes limit the observable time window in these experiments.…”

Long-timescale motions in glycerol-monopalmitate lipid bilayers investigated using molecular dynamics simulation

“…Most of these studies have been carried out in the context of biologically relevant lipids, typically diglycerides with functionalized headgroups such as dipalmitoylphosphatidylcholine (DPPC) [31,[33][34][35]37,38,41,42,45,47]. However, these lipids remain relatively challenging to simulate, owing to difficulties in the force-field design [50,51] and treatment of electrostatic interactions [50,[52][53][54][55][56][57], and to the slow convergence of system properties with respect to both system size [18,[49][50][51][58][59][60] and simulation timescale [18,58,[61][62][63].…”

Computationally efficient prediction of area per lipid

“…At a given temperature, a lipid bilayer exists in either liquid or solid phase. 24,25,[29][30][31][32][33] The solid phase is commonly referred as a gel phase. The characteristic temperature unique for each lipid defines the transition from the gel to the liquid phase.…”