title: Probing biomembranes with positrons.
abstract: Free volume pockets inside a cell membrane play a prominent role in a variety of dynamic processes such as the permeability of small molecules across membranes and the diffusion of, e.g., lipids, drugs, and electron carriers in the plane of the membrane. Nonetheless, by now the chances for characterizing free volume voids in a nonperturbative manner through experiments have been very limited. Here we use lipid membranes as an example to show how positron annihilation spectroscopy (PALS) together with atomistic simulations can be employed to gauge changes in free volume pockets in biological macromolecular complexes. The measurements show that PALS is a viable technique to probe free volume in biomolecular systems. As examples, we consider the gel-to-fluid transition and the role of increasing cholesterol concentration in a lipid membrane. Further applications proposed in this work for PALS are likely to provide a great deal of insight into the understanding of the role of free volume in the dynamics of biomolecular complexes.
authors.affiliation: Department of Applied Physics, Helsinki University of Technology, Finland.
authors.forename: Petri
authors.initials: P
authors.lastname: Sane
authors.forename: Emppu
authors.initials: E
authors.lastname: Salonen
authors.forename: Emma
authors.initials: E
authors.lastname: Falck
authors.forename: Jarmila
authors.initials: J
authors.lastname: Repakova
authors.forename: Filip
authors.initials: F
authors.lastname: Tuomisto
authors.forename: Juha M
authors.initials: JM
authors.lastname: Holopainen
authors.forename: Ilpo
authors.initials: I
authors.lastname: Vattulainen
mesh_terms: D002784:Cholesterol / Q000737:chemistry; D003198:Computer Simulation; D004583:Electrons; D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D013057:Spectrum Analysis
chemical_list: D008051:Lipid Bilayers; D002784:Cholesterol