The phospholipid bilayer is the basic structural motif of most biological membranes. As such, many biological processes occur within or in the proximity of the cell membrane, and therefore, interest in the properties and behavior of lipids in membranes is considerable. For example, it is found that in nature the lipid distribution across the inner and outer leaflet of cell membranes is asymmetric  and this asymmetry plays a prominent role in processes like cell fusion, activation of the coagulation processes and the recognition and removal of apoptotic cells by macrophages. Therefore there is great interest in studying the factors determining lipid movement across membranes as well as the resulting lipid mapping in the membrane, both of which are far from being understood and characterized.
In the literature it is found that there are big discrepancies in the timescale of the occurrence of lipid flip-flop in model bilayer systems, partly due to the fact that these measurements were based on the indirect observation of the process [2,3]. However, with the sub-nanometer spatial resolution of neutron reflectometry, it is possible to directly obtain lipid composition differences in the leaflets of a bilayer [4,5], and in particularly resolve them for times scales as short as a few tens of seconds.
I will describe how the methodological development and the new possibilities offered by the state-of-the-art neutron reflectometers allowed for the first time the direct measurement of the flip-flop kinetics in an isotopically labelled model bilayer.
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