[RASMB] SAUCE - x-ray optics for the AUC

[Borries Demeler]

> 
> Just a general thought, colleagues - which maybe has been considered
> already. But am I correct in thinking that to get multiple species resolved
> into a form sufficiently 'pure' for SAXS analysis you would be using 'zonal'
> separation i.e. band-forming cells?
> 
> Although few folks seem to actually use this approach in a conventional AUC,
> tit does in fact work well, as does the SEDFIT software for analysis of
> same. You need to avoid working with low MW solutes, of course. Do I guess
> that the Spin Analytical CFA instrument could handle it all OK?
> 
Arthur,

band sedimentation is also very useful for the new multiwavelength
optics developed by Helmut Coelfen's lab. All of the HPC optimization
methods implemented in UltraScan (2-dimensional spectrum analysis,
genetic algorithm analysis, molecular weight constrained method, and the
new multiwavelength analysis methods) support band sedimentation and
Vinograd cells. You can also mix standard 2 channel and band-forming
cell experiments for global fits over multiple velocity experiments
with different cell types for added hydrodynamic resolution
(see Brookes E, Cao W, Demeler B.  A two-dimensional spectrum
analysis for sedimentation velocity experiments of mixtures with
heterogeneity in molecular weight and shape. Eur Biophys J. 2009, Epub:
http://www.ncbi.nlm.nih.gov/pubmed/19247646?ordinalpos=1&itool=EntrezSystem2.PEntrez.Pubmed.Pubmed_ResultsPanel.Pubmed_DefaultReportPanel.Pubmed_RVDocSum).

For multiwavelength analysis the advantages of band sedimentation (especially
for higher MW species, as you say) is that the spectra for different components
do not overlap and you can separate species not just hydrodynamically, but
also spectrally. The same principle should apply in the SAXS experiments.

Another method that may prove helpful is analytical buoyant density
gradient sedimentation that will separate species based on densities and
provide static peaks that can be analyzed spectrally. We have used this
successfully for analyzing virus packaging, where different stages of
packaged viruses separate because they contain different amounts of DNA,
and hence have significantly different densities. The spectral patterns
of the peaks differ because of different contributions of DNA and protein,
which can be deconvoluted with UltraScan to identify relative amounts.

I am not sure how long SAXS exposures would take in the AUC, but perhaps
having this static peak separated out may be useful, compared to a rapidly
moving boundary.

-Borries