From: jack correia <jcorreia@fiona.umsmed.edu>
  To  : David J. Scott <djs17@york.ac.uk>
  Date: Wed, 22 Apr 1998 11:20:54 -0500

Re: g(s) question.....

	An addendum to Walter's comments - if the equilibrium constant is large
and you are working at 2 orders of magnitude higher concentration, such
that the fraction of complex/aggregate in the boundary is > 99.9%, then the
area under the curve (if you are using absorbance correct this to moles
with an extinction coefficient) is the concentration of complex.  If you
are looking at a mixed system, like the antibody-antigen system, which has
a large equilibrium constant, and you use different stoichiometries, then
the area under the complex boundary is all complex and the area under the
trailing boundary is the free concentration of the excess component.  This
is actual what we call stoichiometric binding because "all" of the
component not in excess it in complex, ie. there is no free material, or
not enough to measure!  So under these conditions, very large K, you can
not calculate an equilibrium constant by taking ratios of areas.  That is
unless it is frozen as Walter suggested.  

	For most systems you get an reaction boundary where everything exist and
equilibrium is obeyed at each radial position in the cell.  For these
systems you can try looking at the concentration dependence of the weight
average sedimentation coefficient and fit it to an equilibrium constant.
You must know the S values of all the complexes, or have reasonable
estimates, and you must obviously have a good idea about the
stoichiometries or the model to use.
-------------------------------------------------|
| Dr. John J. "Jack" Correia			 |
| Department of Biochemistry                     |
| University of Mississippi Medical Center       |
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