[RASMB] hydrated vs. anhydrous MW

[Arthur Rowe]

Hi Karl

Sedimentation equilibrium measure the buoyant mass of a protein in (normally
aqueous) solution. Archimedes and all that. With a minor reservation which
in practice has little practical effect (see my communication of 2 days
back) the water associated with a protein has the same density as free
water. So - just as the buoyant mass of a boat is unaffected by any water
that gets stuck to it (a bit hypothetical that one - but maybe it could go
into a jelly form!), so the mass estimate from sedimentation equilibrium is
unaffected by water bound.

That applies to the equilibrium state. Transport methods - e.g.
sedimentation velocity, dynamic light scattering - are very much affected by
any bound component which increases the hydrodynamic radius.

All best

Arthur Rowe


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Arthur J Rowe
Professor of Biomolecular Technology
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University of Nottingham
School of Biosciences
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Leicestershire LE12 5RD   UK

Tel:        +44 (0)115 951 6156
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Dear RASMBers,

The typical hydration estimate for proteins of 0.3 grams water bound per
gram protein would seem to predict that proteins will have molecular weights
about 1.3 times larger than predicted by their primary sequences, yet this
does not seem to be observed experimentally by sedimentation equilibrium
techniques.  Can someone help me understand why this is not observed?

Karl Maluf
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