[RASMB] single species fit to heterodimer system

[Joel Mackay]

hi all,
I was just wondering if anyone happened to know off the top of their head 
what you might expect to see in the following situation.

We have a mixture of two proteins that we think form a heterodimer. They 
are 11 kDa and 8 kDa. We have a 1:1 mixture of the two (in molar terms) and 
they have similar extinction coefficients (but not the same). We have run 
equilibrium experiments at several different speeds (22k, 27k, 35k), 
initially just to get a rough idea of the state of the interaction. I was 
expecting that if it was a very strong interaction, a fit to a simple 
single species model, using an averaged vbar, would yield a reasonable fit 
with a mass of ~19 kDa. If the interaction was weaker, i expected to see 
(using the same fit) a mass of somewhere between 10 kDa and 19 kDa, with 
perhaps a poorer fit.

So, my question is: if the interaction was a moderate one (~10^5-10^6 M^-1, 
say) and it was well-behaved, what would you expect to happen to the 
molecular mass that you get from a single species model as you increase the 
speed? Would you expect it to increase? (This is what i think should 
happen, based on thinking and on having a play with Allan Minton's twocompsim).

cheers
joel
*****************************************************************
Dr Joel Mackay
Senior Lecturer
School of Molecular and Microbial Biosciences
(formerly Department of Biochemistry)
Building G08
University of Sydney,
Sydney, NSW 2006
Australia
ph +61-2-9351-3906
fax +61-2-9351-4726
WWW: http://www.biochem.usyd.edu.au/staff/mackay/
Sydney Protein Group Website:
http://www.biochem.usyd.edu.au/spg/
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