Probing The Microstructure Of Biomaterials With Positrons

Abstract

In this work we present a novel and promising tool for characterizing the microstructural changes in biomaterials e.g. lipid bilayers. Positron annihilation lifetime spectroscopy (PALS) is a widely used tool to study atomic scale defects in semiconductors [1] and routinely used to study the voids in polymer materials [2]. Applying PALS to study biomaterials is uncommon and until recent years mostly unheard of, though preliminary studies were performed in early 1980’s [3]. Through the increased understanding of the biomolecular materials, results from PALS experiments can now be compared with simulations and further analysis of the results is possible. In biomolecular material, a thermalized positron forms a meta-stable bound state, Positronium, with an electron from the material. An ortho-Positronium-atom can be applied as a probe, because the Positronium lifetime in the material is strongly affected by the free volume characteristics of the probed material.Our study combines the experimental results achieved with PALS and atomistic MD simulations. Preliminary PALS measurements were performed with DPPC and POPC-lipid bilayers, complemented with MD simulations [4]. The results from both methods are in full agreement with each other and thus showing PALS as a viable tool to study the free volume changes, or the changes in hydrocarbon tail dynamics inside the lipid bilayer.As well as studying manufactured lipids with PALS, also in vivo studies of organic biomaterial are possible, such as studying the changes in internal free volume/dynamics of a mammalian lens and lipid membranes separated from lenses.[1] F. Tuomisto et al, Phys. Rev. Lett. 93 (2004).[2] O. E. Mogensen, Positron annihilation in Chemistry (Springer-Verlag, Heidelberg, 1995).[3] Y.Y. Wang et al, J. Am. Chem. Soc. 105 (1983).[4] P. Sane et al “Probing Biomembranes with Positrons”, submitted to J. Am. Chem. Soc. 15.09.2008.