Invited Paper NT-TuM9
Neutron Reflectivity and Scattering for Studying Biomolecules at Interfaces

Tuesday, October 16, 2007, 10:40 am, Room 618

Neutrons are relatively more harmless to matter than X-rays and therefore they are particularly usefull to study biological material. Hydrogen scatters neutron very well and further more the scattering from hydrogen and deuterium is very different. Selective deuteration of one component and contrast matching of the aqueous solvent makes it possible to study one component at a time as the other can be made invisible. The potential of neutron scattering/reflectivity measurements will be discussed for two systems, lipid self-assembly and DNA compaction. There is an increasing demand for methods to study processes at the lipid-aqueous interface as the importance of lipids and lipid self-assembly structures as regulators for biological activity as well as for vehicles for drug delivery. Spreading of vesicles on surfaces are quite an established method to forming bilayers on hydrophilic surface, and spreading can be affected by the surface properties as well as properties that affect the stability of the vesicles. The full potential of non-lamellar liquid crystalline nano particles (LCPN) has only recently been realized, as improved methods and new lipid combinations have been introduced for producing biocompatible and structurally well-defined nano-particle dispersions of different phase structures. These include bicontinuous cubic, reverse hexagonal, and reverse cubic phases. Directly relevant for drug delivery systems is the fusion of these LCPN with a (model) membrane. Again numerous studies has been conducted on vesicle fusion, but very few on the interaction between non-lamellar LCNP and membranes. The compositional changes in the lipid bilayer will be monitored by neutron reflectivity using selective deuteration of the lipids and proper contrast matching. This has given us information on structural changes when the LCNP interacts with a surface and a bilayer. Our results show that interaction between the LCPN lipid components with the acyl chains of the bilayer, consistent with the formation of a mixed bilayer. As we observed a diffraction peak at high q after a certain time, we concluded that the interaction eventually lead to formation of multilayers on the surface. We are today fascinated of the supra-molecular structures formed by DNA packing in the cell nucleus and we are trying to understand to what extent packing regulates transcription/translation. The long-term objective of our work is to design a module for DNA packaging, where the packing and thereby the transcription can be switched on and off. This module would be a transcription competent synthetic analogue of a real cell nucleus. We are therefore study the relation between structure of the aggregate (using dynamic light scattering, small angle x-ray and neutron scattering, cryo-TEM), the degree of compaction (using fluorescence) and transcription/translation (using biochemical assays, including RNAse and luciferase production). It has been shown that DNA can be compacted by cationic lipids both in bulk and at interfaces. By using neutron scattering of DNA coated particles we have shown that the aggregate formed by CTAB when interacting with DNA is elongated and of the same structure on the surface as on the bulk.