From: Jim Cole <jim_cole@merck.com>
  To  : RASMB <rasmb@bbri.harvard.edu>
  Date: Tue, 16 Dec 1997 10:29:24 -0500

Re: attractive forces

Dear T.Aerts and Julius Clauwaert:

As Jack Correia mentioned, a negative second virial coefficient means
that you have association. it may be useful to apply the usual
analysis methods for associating systems. If you fit each data channel
to single ideal species model, do the weight-average molecular
weights, tends towards a plateau  at a value corresponding to
dimer,tetramer etc ,  as you increase the protein concentrations? If
so, I would try globally fitting the data  using the appropriate
discrete association model. In his monograph on short column
sedimentation equilibrium (available on the beckman web site, I
think), Tom Laue describes some useful diagnostic plots to
characterize associating systems to check for association, (positive)
nonideality and heterogeneity.
Best wishes,
Jim Cole