[RASMB] Interpreting SV and SE data

[Eric Salgado]

Hello all,
   I have a system for which I've done SV experiments at multiple 
concentrations that, on  increasing the concentration, reveals a peak 
that starts at an s value of about 2, shifts to a plateau between 2.4 
and 2.6, then steadily increases at 2.6. I should point out that all of 
these peaks are very broad.
   The c(MW) analysis describes a monomer at low concentrations; dimer 
at intermediate concentrations;  something between  dimer and trimer at 
high concentrations;  and finally, as  an aggregate  starts to form at 
very high concentrations, a trimer peak is resolved. This trimer peak 
correlates to our crystal structure, but it appears as if, because of 
the apparently low association constant and fast kinetics involved with 
the trimer formation, this fact is masked in the SV data.
   In order to hopefully rid of this ambiguity, I performed SE 
experiments in the concentration regime that did not form an aggregate 
(and therefore apparently only a small amount of trimer), but find that, 
as far as a species analysis goes, both a model with monomer-dimer or 
monomer-dimer-trimer present are statistically equal. That being said, 
the statistics are lower for the later model, while the fit and 
residuals for both are equally good.
   With all of this in hand, how can I determine the best SE model to 
use, and can I say with certainty that the trimer exists in solution 
from these data?

I know this is a lot, but I would appreciate any help I can get  with 
this problem.

Thank you in advance,

Eric Salgado