From: Dr A.J. Rowe <ajr@leicester.ac.uk>
  To  : rasmb@bbri.harvard.edu
  Date: Wed, 23 Apr 1997 14:48:19 +0100 (BST)

vbar/urea/swollen trimer

Hi RASMBers !

A little while back I raised the question as to what the partial specific
volume might be for a trimeric protein which *swelled* but DID NOT DISSOCIATE
in 9M urea.

I probably put the question badly, as all but one respondants quoted the well
known references as to what happens in general in high concs of chaotropic
media - namely the polypeptide chain assumes a conformation close to random
coil, and since internal voids  now become solvent-accessible, vbar decreases,
by the approximate order of 0.02 ml/g.

But in our own example, since overall rH was changing by less than 10%, there
was only one component present, and the apparent mass was (less, but only
10-15% less than) the trimer value, the question was, *what happens to vbar
under these unusual conditions* ?

My own (non-externally-broadcast) thought was that whilst a high degree of
chain disorder might be present, the internal voids would be INCREASED rather
than abolished, and hence vbar would likewise increase, probably by a quite
measurable amount.

OK - one can speculate for years. Sooner or later experiments are called for. A
set of very careful DDM expts using solutions of concentration determined with
fanatical care, has given us the answer:

	vbar for CAT (swollen trimer)  =  0.762 +/- 0.001  ml/g

(precision quoted - absolute accuracy will be a bit more modest)

For reference, CAT in normal salt solutions, vbar = 0.736 ml/g.

Pleasantly, the above vbar value returns the correct value for M(trimer) when
shoved into sed equil data.

So there we are. Did I hear anyone murmer 'molten globule' ? Thanks to all who
contributed to the discussion. The above is part of a study currently being
submitted on CAT by several of us here.

Arthur Rowe

***************************************************
Dr Arthur J Rowe
Director
UK National Centre for Macromolecular Hydrodynamics
Leicester Laboratory
Adrian Building
University of Leicester
Leicester LE1 7RH    UK

Tel: +44 (0)116 252 3448
Fax: +44 (0)116 252 5602
ajr@leicester.ac.uk
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