X-RAY CONTRAST VARIATION IN LIPID SUSPENSIONS USING SUCROSE SOLUTION

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This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. In x-ray solution scattering, the means to vary the electron density of the solvent in order to change the x-ray contrast are rather limited mainly due to the chemical influence of the contrast agent on the system. As a result the method has not gained as much popularity as its counterpart in neutron scattering. Yet, applied to aqueous solutions of membrane proteins reconstituted in lipid vesicles or bicelles the method has the potential to distinguish between the lipid and the protein scattering intensities. With this ability the technique of small angle solution scattering to determine the shape of soluble proteins could be extended to certain membrane proteins. A project was started on BL4-2 to explore the possibilities of this approach. Sucrose was chosen as a contrast agent as it is expected to have only a weak effect on the bilayer structure of the lipid. Starting with pure lipid bicelles we have shown that only small changes in lateral size of the bicelles can be observed up to a sucrose concentration of about 30% (w/w) in the aqueous phase. Thus one can assume that a protein embedded in the lipid bicelle will not experience significant changes in its environment due to the addition of the sucrose. We will continue to study the effects of contrast agents in the next series of experiments.