From: Jim Cole <jim_cole@merck.com>
  To  : RASMB <rasmb@bbri.harvard.edu>
  Date: 2 Nov 1995 13:56:37 U

Nonideality

                       Subject:                               Time:2:51 PM
  OFFICE MEMO          Nonideality                            Date:11/2/95
Dear RASMB members:

In the course of  sedimentation equilibrium studies of a small dimerizing
protein we have observed surprisingly nonideal behavior.  I would be
interested if anyone knows of any precedence.  The protein has a monomer
molecular weight of 30 kDa and weakly self associates to a dimer (Kd = 30 uM)
. Addition of various low molecular weight solutes  enhances dimerization (Kd
=  0.5 to 4 uM). In all cases,  we can only obtain good fits  over a range of
loading concentrations of 0.008 to 1.5 mg/ml by allowing for nonideality.  B
converges to about 10^-6 l mole/g^2,  which is about 10-fold higher than we
would expect for a small protein. B is not decreased by addition of up to 2 M
NaCl, so it would not appear to be due to the Donnan effect.  It doesn't
appear that the protein has an unusually elongated shape. The Stokes radius of
the monomer is about 25 angstroms, i.e.,   about what you would expect for a
protein of this mass.  The apparent nonideality is not an artifact associated
with nonlinearity in our XLA absorption optics or with a concentration
dependent extinction coefficient.  

Thanks in advance for the help.

Jim Cole  (jim_cole@merck.com)