Molecular Mechanism of Long-Range Diffusion in Phospholipid Membranes Studied by Quasielastic Neutron Scattering

Abstract

The motion of phospholipids has previously been studied on many time scales due to the significance for living cells and technological applications. The motions on a pico- to nanosecond time scale were determined by quasielastic neutron scattering (QENS) to be much faster than the ones on the microsecond scale covered by fluorescence recovery after photobleaching (FRAP). This was explained by assuming that the molecules rattle fast in a cage of neighbors (observed with QENS) from which they escape once in a while; this escape was then the primary step of the slower diffusion measured by FRAP. However, nanosecond MD simulation studies could not observe any escape events; recent findings even suggested that the long-range motion in phospholipid membranes on short time scales is not diffusive but has flow-like characteristics. To check this novel view, we have repeated the QENS experiments with today's significantly improved instrumentation. By using the advantage of QENS that allows tuning of the observation time in the pico- to nanosecond range, it was possible to study the evolution of motions in this time frame. Localized motions, e.g., of the head and tail groups, appear separated from the long-range motion and do not obfuscate the analysis as they do in a mean squared displacement plot. The results for the long-range motion are indeed compatible with flow patterns, whereas the localized motions can account for the fast motions interpreted as motions in a cage before. Hereby, we give experimental evidence for a completely different mechanism of long-range motion on short time scales in phospholipid membranes.

Interplay between intramolecular and intermolecular structures of 1,1,2,2-tetrachloro-1,2-difluoroethane

“…60 As can be seen in the inset of Fig. 4, the H free model is preferred to the H 2/3 model because the χ 2 PDF has its peak at a smaller value, therefore disfavoring the E g = E t scenario.…”

“…Parameter The proposed algorithm to find the structure describing the data within their errors is implemented in the free and open, publicly available program FABADA, 59 and it has already successfully been used to describe the dynamics from quasielastic neutron-scattering experiments 60,61 and from dielectric spectroscopy. 62 …”

Incoherent Neutron Spin-Echo Spectroscopy as an Option to Study Long-Range Lipid Diffusion

“…And ballistic lipid motion in fluid membranes has indeed been recently reported from quasielastic neutron scattering (QENS) using a neutron backscattering spectrometer [8]. Furthermore, it has been reported that lipids move coherently in loosely bound clusters, rather than as independent molecules [9][10][11]. A "hopping" diffusion of lipids into nearest neighbour sites was observed in single supported bilayers [12], and, recently, it has also been suggested that there is a flow-like component to the motion of the lipid molecules over long length scales [11].…”

“…Furthermore, it has been reported that lipids move coherently in loosely bound clusters, rather than as independent molecules [9][10][11]. A "hopping" diffusion of lipids into nearest neighbour sites was observed in single supported bilayers [12], and, recently, it has also been suggested that there is a flow-like component to the motion of the lipid molecules over long length scales [11]. In biology, diffusion often occurs in crowded media, which was found to lead to anomalous diffusion [13,14].…”

Resolving the kinetics of lipid, protein and peptide diffusion in membranes

“…Studies of multilamellar liposomal membranes by neutron scattering (Busch et al, 2010) and simulation (Falck et al, 2008) have found evidence for flow-like behavior, in which lipid molecules move collectively.…”