ANOMALOUS X-RAY SCATTERING ON LIPID BICELLES

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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. Lipid bicelles are flat disk-like micelles with a radius around several tens of nanometers formed by binary and tertiary mixtures of long- and short-chained phospholipids in water. One of the interesting properties of these bicelles is their ability to partially orient under an external magnetic field in the presence of lanthanide ions such as ytterbium. Taking advantage of the significantly improved beam characteristics provided by the SPEAR3 upgrade, in particular improved beam brightness and stability, the feasibility of anomalous solution scattering experiments at BL4-2 was recently tested successfully on horse spleen ferritin. Encouraged by this result and using energy-resolving drift diode detector to measure absorption spectra accurately in fluorescence mode, we made an attempt to measure the anomalous scattering signal from Ytterbium ions in a suspension of lipid bicelles. The majority of the ions are thought to be closely associated with the headgroup region of the bicelles but whether they are predominantly in the flat bilayer region of the disk or on the rim is not known. To gain some insight into the ion distribution around the bicelles we conducted anomalous x-ray scattering experiments near the L3 absorption edge (8944eV) of ytterbium. We detected a difference between the signal below and at the absorption edge due to the anomalous scattering, which was not present in the sample without the ions. The accurate analysis though is complicated by the fact that not all the ions can be assumed to be directly associated with the bicelles. We are currently working on the detailed modeling and interpretation of the data.