[RASMB] another vbar question

[Laue, Thomas]

Hi-
If you have measured the density increment at constant chemical potential of all diffusible components, you do not need to calculate v-bar. The density increment is correct term for 1-vp, which is a two component (particle, water) approximation. In other words, do not use the vbar approximation in this case, just use the density increment in place of 1-vp.
It is useful, too, to keep in mind what you are measuring/calculating- it is the change in volume of the system per gram of added material. When you have a high-salt solution, the water already is experiencing extensive electrostriction due to the ions. It is not surprising then, that the volume change per gram of added component is not the same as in a dilute aqueous solution.
Best wishes to all-
Tom
From: RASMB [mailto:rasmb-bounces at list.rasmb.org] On Behalf Of Rufer, Arne
Sent: Thursday, September 28, 2017 11:46 AM
To: HGSR (Holger Martin Strauss) <hgsr at novonordisk.com>
Cc: rasmb at list.rasmb.org
Subject: Re: [RASMB] another vbar question

Hi Holger,

would preferential hydration play a role at the very high NaCl concentrations you are using? Regarding anecdotes on vbar, I can add that when we worked with acylated peptides the vbar measured by densitometry turned out to be much smaller than vbar calculated from the aa sequence + modification. We had congruent AUC and SEC-MALLS data showing that certain acylations led to oligomerisation, presumably due to an hydrophobic interaction that became stronger with higher salt concentration. At that time we argued the effect on vbar could be due to a solvent envelope for packed acyl-moieties that is smaller than that of the separated acyl-moieties. Makes sense? I am curious to read Mattia's comment on this, too.

Best,

Arne





On Thu, Sep 28, 2017 at 3:48 PM, HGSR (Holger Martin Strauss) <hgsr at novonordisk.com<mailto:hgsr at novonordisk.com>> wrote:
Hello yall,

Wow, vbar trending?!

I actually have a question regarding the salt-dependency of the vbar. We are working with peptides modified with some organic linkers and a diacid (between C4-C50, to give a range). We measure the droh/dc (DMA5000) at different concentrations and calculate the vbar. Controls OK, concentration accurate and measurements reproducible, buffer dialysed. Salt-concentration is varied (NaCl, 5-1500 mM-ish). Turns out the vbar is considerably different at high salt. (And so is the self-association, btw.) No detergents or denaturants, just some additional phosphate for buffering, besides the NaCl.

·        How can this be rationalised structurally?
·        Any tests when droh/dc is more appropriate for Mw /sigma than vbar?
·        Other examples in the literature for peptides?
·        Any general comments/anectodes/remarks?

Cheers,

Holger


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