Determining the In-Plane and Out-of-Plane Structure of Model Membranes; Two Recent Examples

Abstract

With the exception of hydrogen, neutrons are found in all atomic nuclei. Importantly, unlike X-rays, neutrons are able to differentiate between the different isotopes of the same element. In biology, the classic example is the isotopic substitution of hydrogen for deuterium, allowing one to selectively tune the sample's contrast in situ with minimal or no change to its native structure. Biological membranes are believed to exist in a disordered state, a fact that presents unique challenges to elucidating their fine structure. In the case of model membranes, to overcome this difficulty we have developed the Scattering Density Profile (SDP) model, which combines neutron and x-ray scattering data, with molecular dynamics simulations to yield robust structural data, including the much sought after area per lipid needed by simulators to refine their force fields. In addition to one-dimensional structural data along the membrane, we have recently exploited the contrast variation offered by neutron scattering (exchange of hydrogen for deuterium), to study - with unprecedented accuracy - the lateral phase separation (in-plane structure) of so-called “raft” forming mixtures. We hope that in the near future we will apply this knowledge to address the question that has vexed biologists and confounded experimentalists for over 40 years: do membrane domains exist in vivo?