From: Henryk Eisenberg <BPEISENB@WEIZMANN.WEIZMANN.AC.IL>
  To  : RASMB Group <rasmb@bbri.org>
  Date: Sun, 27 Jun 99 13:02:15 +0300

Equilibrium sedimentation

Dear Friends:

Re. Karen Fleming
In following Emory's suggestion the gospel for equilibrium sedimentation
is  dlnc/drsq=omegasq(drho/dc)/2(dpi/dc), all at constant chem.potential
along r. That's it.
In two component systems the density increment is the Archimedes
term directly related to (1-vbar.rho). In more than two-component
systems interaction terms appear which are not covered by the calculated
vbar. However (drho/dc) at dialysis equilibrium is an easily measured
experimental term. In non-ionic systems the osmotic pressure derivative
can be expanded in a virial series and the gospel tells us that the number
of molecules per unit volume are counted. Molecular weights are only the
result of whatever concentration units are used. The virial coefficientsd
measure attractive or repulsive interactions. With ionic systems one has
to be more careful. In the absence of added electrolyte (dpi/dc) cannot
be expanded in a virial series and in practice the number of molecules
counted is an average of all ions, the large and the small, and the tall.
It has really little to do with the calculated vbar value. OH and H ions
are always available in aqueous solutions and very low concentrations of
added electrolyte normalize the situation and allow virial expansion of
(drho/dc). However one should be careful. This is all true for all sizes
of peptides, limited of course by the ultracentrifuge practical upper
speed limit for small macromolecules. Most of this is discussed in
H. Eisenberg, Biological Macromolecules and Polyelectrolytes in Solution,
Oxford, Clarendon Press, 1976, based on the Casassa and Eisenberg 1964
article in vol. 19 of Adv.Prot.Chem.
I hope I have been of help.

Re. Raz Jelinek
Analytical ultracentrifugation is best performed with the state
of the art Beckman XLA. However, unfortunately not a single one
of these wonderful machines is now available in Israel, although
there were more than half of a dozen classical Model E's here in
the days of my youth. I am now keeping my fingers crossed because
a good possibility exists that the Weizmann Institute will soon acquire
an XLA and other Institutions will follow this nucleation process lead.
If you want to use a preparative ultracentrifuge to perform experiments
better performed in the analytical ultracentrifuge you should follow
the lead of Allen Minton, recently summarized by Saleh Darawshe and
Allen Minton in analytical Biochemistry, 220, 1-4 (1994). Saleh now
lives in Kfar Araba in lower Galilee and can tell you all about this.
I hope I have been of help