Density determination by analytical ultracentrifugation in a rapid dynamical gradient: application to lipid and detergent aggregates containing proteins.

A rapidly developing dynamical gradient can be formed in the analytical centrifuge when a buffer solution prepared in D2O is underlayered under the same buffer solution prepared in H2O in a specially designed double sector cell. In a short time the boundary layer spreads to form the gradient. Heavy particles (S greater than or equal to 10S) will band in the gradient corresponding to their density, which can be determined accurately. To this end, the buffer may contain density adjusting additives such as sucrose. We present results with this technique for lipid vesicles, for a Ca-antagonist bound to vesicles, as well as for a lipoprotein and reconstituted regular membrane protein-lipid arrays.