Team leader Paul Butler, Center for Neutron Research, National Institute for Standards and Technology, Gaithersburg, Maryland, USA

Insights Into Lipid Membrane Dynamics: Understanding Thickness Fluctuations

Biological membranes are highly complex, self-assembled, two dimensional structures which separates the living cell from its environment and provides a semi-permeable barrier maintaining a delicate balance of substances within and outside the cell.   These two dimensional environments however are far from static sheets, harboring a host of proteins and peptide molecules and hosting a multitude of chemical and physical processes essential to cellular function. While early studies viewed the lipidic membrane as a simple support matrix and focused on the inclusions, specially the membrane proteins, there is a growing consensus, supported by theory, simulation and experiment that the lipid membrane actively participates in its function, supporting, and even regulating the activity of the embedded proteins and oligomers. On the other hand, the role and importance of membrane dynamics (local motions) as opposed to simple structural features in that active participation is only beginning to be appreciated and explored.

In this work we probe the intermediate range dynamics of thickness fluctuations. Such fluctuations have been predicted by, and their features explored through theoretical and simulation work, but not experimentally observed. They are thought to significantly impact such things as the insertion and functioning of membrane proteins as well as pore formation and cellular exchange. We have used neutron spin echo (NSE) to experimentally observe these fluctuations first in the simplest model systems of pure, fully saturated, phosphocholine unilamellar vesicle (ULV), examining the role of both T_m and chain lengths. We then increase the complexity of our model system to explore the effects of using mixed tail lengths (and mixed T_m).  Interestingly we find that the amplitude and relaxation times are independent of chain length in single lipid systems. However, upon mixing a 50/50 ratio of longer and shorter chains we observe a significant enhancement of the fluctuations with temperature above the T_m of the longer chain lipid.  This mixed T_m system also provides an interesting opportunity to explore the effect of segregation and finite size cutoffs to these dynamics.  If time permits I will also discuss our preliminary findings on the effect of introducing pore forming peptides into the membrane. 

Host: Professor Jan Skov Pedersen, iNANO & Department of Chemistry, Aarhus University

Coffee, tea and bread will be served from 10:00 am in front of the auditorium.