Sedimentation Boundary Analysis of Interacting Systems: Use of the Apparent Sedimentation Coefficient Distribution Function
New methods for the analysis of sedimentation velocity data have extended the sensitivity of the UV scanning and Rayleigh interferometric optical systems of the analytical ultracentrifuge by 2-3 orders of magnitude. Boundaries with concentrations lower than 10 μ/ml can be visualized readily with the Rayleigh optical system allowing the thermodynamic analysis of interacting systems in a concentration range previously inaccessible to the analytical ultracentrifuge. The increase in sensitivity has been achieved by a combination of analytical (Stafford, 1992; Stafford, 1992; Stafford, 1994) techniques that use the time derivative of the concentration profile and of instrumental techniques (Liu and Stafford, 1992; Yphantis et al., 1994) that employ a rapid acquisition video- based Rayleigh optical system. Use of the time derivative achieves an automatic optical background correction, and the video system allows signal averaging of the sedimentation patterns, resulting in a considerable increase in the signal-to- noise ratio. Sedimenting boundaries are represented as apparent sedimentation coefficient distribution functions, g(s*) vs. s*, where s* is the apparent sedimentation coefficient defined as s*=ln(r/rm)/ω 2t and g(s*) has units proportional to concentration per svedberg. A plot of g(s*) vs. s* is geometrically similar to the corresponding plot of dc/dr vs. r obtained with the schlieren optical system. This chapter will describe methods for the analysis of both self-associating and hetero-associating systems using the apparent distribution function.
KeywordsInteract System Sedimentation Velocity Standard Free Energy Sedimentation Coefficient Analytical Ultracentrifuge
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