From: Allen Minton <minton@helix.nih.gov>
  To  : Diane Wakeham , rasmb@alpha.bbri.org
  Date: Tue, 12 Sep 2000 12:25:27 -0400

Re: MW Determination for Rod- Shaped Protein(Axial Ratio?)

At 06:04 PM 09/11/00 -0700, Diane Wakeham wrote:

>I am a rather inexperienced user of an XL-A analytical ultracentrifuge
>to assess the assembly state of a highly nonspherical protein (approx
>dimensions 115 x 28 x 24 angstroms, essentially rod shaped and rather
>large) likely to form dimers or possibly tetramers.  Using the simplest
>fits to my sedimentation equilibrium data with Optima software the
>molecular weight I obtain is about 10- 20 kD less than I would expect
>the actual protein size to be.

Jack Correia has correctly identified non-ideality as the likely source of 
your discrepancy.  It is essential for you to characterize the dependence 
of the apparent molar mass upon protein concentration and to extrapolate 
your results to zero concentration.  The nonideality may be due to excluded 
volume, in which case shape would play a factor, but in the absence of 
information about the pI of this particular protein and the pH and ionic 
strength of the solution, electrostatic repulsion cannot be ruled out as a 
contributor.

Allen Minton