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<br>
Dear RASMBers,<br>
<br>
Joachim Behlke and Tom Laue sent me the following messages
concerning<br>
my previous inquiry, which are highly instructive to me and perhaps to
some <br>
of you, too, so I would like to share them with those of you. I
thank them <br>
as well as Ariel and John, and assume both Joachim and Tom do not
mind. Fumio<br>
<br>
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From Joachim:<br>
<br>
Dear Fumio,<br>
in solution the substances are hydrated, however in many cases <br>
nobody know exactly the number of bound water <br>
molecules.Therefore, it is difficult to determine the concentration
<br>
exact. For determination of v-bar using the densitometer the <br>
density of a substance is plotted versus the concentration of the <br>
non-hydrated substance, often derived from specific parameters <br>
(extinction coefficient etc.). In this case you will get v-bar non-<br>
hydrated. <br>
When you measure the density in AUC cells and compare this with <br>
hydrated markers of known density, of cause you will get the <br>
density or v-bar of hydrated molecules.<br>
<br>
Regards,<br>
<br>
Joachim<br>
<br>
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From Tom:<br>
<br>
Hi Fumio,<br>
The values of vbar that are determined by density readings relate to how
the concentration of the solute is determined. There is a good discussion
of this in Heine Eisenberg's 2000 paper on a Gibbsian perspective of
analytical ultracentrifugation. <br>
Ultimately, the molecular weight calculated from sedimentation data
refers to the form of the material used to measure the concentration in
the vbar determination. For example, if you measure the vbar for DNA in
NaCl, then do sedimentation in concentrated CsCl, the molecular weights
you calculate will be for the Na-DNA. In the case of proteins and the
vbar used in calculations come from data obtained (or calculated using
Traube's rule) for the anhydrous amino acids. Hence, the molecular
weights obtained are for the anhydrous protein.<br>
Best wishes,<br>
Tom<br>
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Thanks, Tom. I once read Dr. Henryk Eisenberg's paper, but I need
to read it again in this context. <br>
<br>
<br>
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