From: Michael Jacobsen -BIOCHEM <jacobsen@biosci.uq.edu.au>
  To  : Jim Cole <jim_cole@merck.com>
  Date: Fri, 3 Nov 1995 11:47:09 +1000 (EST)

Re: Nonideality

On 2 Nov 1995, Jim Cole wrote:

>                        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)
> 
> 

Hi Jim,

The article on the affect of small inert solutes on the nonideality of
associating systems by

Shearwin and Winzor, Biophys. Chem., 31, (1988), 287-294

may provide some clues.

Kind Regards


Mike Jacobsen

Centre for Protein Structure Function and Engineering,
Biochemistry Department,
University of Queensland.