Temperature Driven Annealing of Perforations in Bicellar Model Membranes

Abstract

Bicellar model membranes composed of 1,2-dimyristoylphosphatidylcholine (DMPC) and 1,2-dihexanoylphosphatidylcholine (DHPC), with a DMPC/DHPC molar ratio of 5, and doped with the negatively charged lipid 1,2-dimyristoylphosphatidylglycerol (DMPG), at DMPG/DMPC molar ratios of 0.02 or 0.1, were examined using small angle neutron scattering (SANS), 31 P NMR, and 1 H pulsed field gradient (PFG) diffusion NMR with the goal of understanding temperature effects on the DHPC-dependent perforations in these self-assembled membrane mimetics. Over the temperature range studied via SANS (300À330 K), these bicellar lipid mixtures exhibited a well-ordered lamellar phase. The interlamellar spacing d increased with increasing temperature, in direct contrast to the decrease in d observed upon increasing temperature with otherwise identical lipid mixtures lacking DHPC. 31 P NMR measurements on magnetically aligned bicellar mixtures of identical composition indicated a progressive migration of DHPC from regions of high curvature into planar regions with increasing temperature, and in accord with the "mixed bicelle model" (Triba, M. N.; Warschawski, D. E.; Devaux, P. E. Biophys. J. 2005, 88, 1887À1901). Parallel PFG diffusion NMR measurements of transbilayer water diffusion, where the observed diffusion is dependent on the fractional surface area of lamellar perforations, showed that transbilayer water diffusion decreased with increasing temperature. A model is proposed consistent with the SANS, 31 P NMR, and PFG diffusion NMR data, wherein increasing temperature drives the progressive migration of DHPC out of highcurvature regions, consequently decreasing the fractional volume of lamellar perforations, so that water occupying these perforations redistributes into the interlamellar volume, thereby increasing the interlamellar spacing.

Temperature-Resistant Bicelles for Structural Studies by Solid-State NMR Spectroscopy

“…29 Magnetic-Alignment of Conventional DMPC:DHexPC Bicelles by 31 P NMR. The characteristics and morphological structures of conventional DMPC:DHexPC bicelles have been comprehensively investigated using various biophysical techniques, such as NMR, 15,30−32 electron microscopy, 31,33 smallangle neutron scattering, 32,34 and small-angle X-ray scattering. 35 Out of these techniques, 31 P NMR spectroscopy is considered to be a convenient and reliable method to monitor the magnetic alignment and the morphological phases of bicelles over a range of temperatures, as reported previously.…”

“…In the magnetically aligned phase of DDPC:DHexPC bicelles (Figure 3), increasing temperature shifts the DHexPC peak toward the high-field region whereas the DDPC peak slightly shifts toward the low-field region to decrease the chemical shift difference between the two peaks due to the progressive miscibility of DHexPC from the rim regions with the planar DDPC bilayer region as explained below. 32,36 Based on the "mixed bicelle model", the experimentally observed 31 P chemical shift of short-chain detergents can be described by the population of detergents in the planar bilayer regions. In this model, the long-chain lipids and the short-chain detergents are mixed in each domain of bicelles through a rapid exchange of detergents between the edge and the planar bilayer regions of bicelles.…”

Kinetics of lipid mixing between bicelles and nanolipoprotein particles

“…In the fusion pore mechanism, this might be an indication that a particular statistically rare configuration of DHPC guests within the bicelle’s DMPC bilayer domain is required for hemifusion to occur. Since the bilayer concentration of DHPC is thought to increase in bicelles at higher temperatures [42], a rate dependence on NLP concentration might appear at higher temperatures, where more fusion pore sites would be predicted to form. This predicted temperature effect will be investigated in future experiments.…”

When detergent meets bilayer: Birth and coming of age of lipid bicelles

“…Above a q of 7.5, morphology similar to multilamellar vesicles is present, where the detergent gathers in nanopore defects [136]. At higher temperatures, annealing of the nanopore defects is observed [137]. 31 P- and 2 H NMR are the methods of choice to study the structure and dynamics of lipid phases and to quickly map out phase diagrams [138–140].…”