[RASMB] Equilibrium Data Analysis with WinNonlin

[Blasie, Cheryl [CNTUS]]

Good afternoon, everyone.
 
        I have a system that appears to have two species in it when the
velocity data is modeled with SedFit, one at 4.0 S (monomer) and the other
at 5.5 S (dimer).  Velocity data collected at different concentrations do
not seem to indicate a reaction boundary.  Based on the peak heights and
areas from the c(S) distribution, the dimer is 14 times higher than the
monomer.
        Recently, I have been trying to fit equilibrium data with Nonlin to
confirm the molecular weight of the two species and determine the
equilibrium constant.  I have a large range of concentrations (1.0 mg/ml to
0.015 mg/ml) and many rotor speeds (4K to 40K).  I have tried different
scans in the fitting routine, but I can't seem to fit the data without
nonrandom residuals.  Some of the residuals indicate nonideality, some
indicate reversible association with n=2, and some indicate aggregation.
These residuals are all when trying to fit to a 1-2 system.  I will add that
even fitting single data sets (3 concentrations, 1 speed) show similar
residual patterns as for the global fitting when fitting to a monomer
system; single sets do not fit to a 1-2 system.  
        Some of my parameters:  sigma is calculated and held constant.
DeltaY is set to 0, then allowed to float after first "fit", LnA starts off
at 3, and I set LnK2 to -1, N2 floats. 
        I realize I may have left out pertinent info, so please let me know
if more is needed.  I am wondering if I am missing something with the
program or the concentrations or the speeds... Any advice would be
appreciated, as I have struggled with this problem way too long.  Thank you
very much.
 
Sincerely,
 
Cheryl
 
 
******************************
Cheryl A. Blasie, Ph.D.
Centocor
200 Great Valley Parkway
Malvern, PA 19355
 
Mail Stop M-1-6
Phone: 610-889-4447
FAX:  610-889-4771
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