From: C.S.RAMAN <raman@bioc01.uthscsa.edu>
  To  : Thomas M Laue <tml@christa.unh.edu>
  Date: Tue, 24 May 1994 14:34:50 -0500 (CDT)

Re: V-bar

Hi Tom:

> aspects of the structure. For example, Preston noted that Ab molecular
> weights are often "too low," indicating that the estimated v-bar is
> likewise too low. We have noted that coagulogen (unrelated to Ab in any

I have a comment and  a few questions on this matter:

	Peng and Kim (Biochemistry 33, 2136-2141 [1994]) have perfomed
equilibrium sedimentation studies on the oxidized alpha domain of lactalbumin
and find that the molecular weight decreases by 5-10%.  They have show
that this protein remains monomeric (> 90%) at [protein] < 100 uM with
no salt added.  They also do not observe non-ideality between [protein]
= 5 - 30 uM.  The apparent mol. wt. measured at [protein] > 60 uM shows
a 5-10% decrease.  The explanation provided by these authors for this
discrepancy is electrostatic interaction with the counterion gradient
(Williams et al., Chem. Rev., 58, 715-806 [1958]).  I don't know if this
criterion applies to your situation.

I read the follow-up on your original post about complications from
disulfides and would like to point out that a naturally occurring
disulfide in the a-domain (vide infra) was eliminated by mutating one of
the cysteines.  

QUESTIONS:

1)  Has anyone observed > 15% discrepancies in mol. wt. for a highly
basic protein (e.g. cytochrome c) via sed. equil. experiments?

2)  I am under the impression that v-bar values don't change much with
temperature!  Is this correct?  Some of the Zamyatnin's review articles
on volume changes in proteins point to this conclusion.

3)  While one can calculate v-bar values for protein which don't include
prosthetic groups (e.g. heme) solely from their amino acid sequence, is
there any data for changes introduced by including them?

Cheers
-raman
-- 
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