[RASMB] another vbar question

[Borries Demeler]

On Thu, Sep 28, 2017 at 11:42:44AM -0700, John Sumida wrote:
> Hi Everyone,
> 
> Nice discussion on v-bar, thank you for asking the question!
> 
> I am wondering about the overall utility of the density contrast approach.
> For unknown materials such as CdSe/ZnS qdots or block-polymer micelles which
> can be highly heterogeneous and for which a calculation of v-bar may not be
> facile, the density contrast approach would seem to have great utility.  In
> these cases a densitometric measurement of the density increment can be
> severely sample limited.
> 
> However, how would the sample heterogeneity in this case affect the v-bar
> measurement?  Seems that the best possible outcome here would be some weight
> averaged quantity, which, while not precise, may have some utility
> nonetheless?
> 

For CdSe, CdTe, etc. qdots the method is likely not useful, because
the density of the qdots is too high, your extrapolation to zero s is
going to be subject to considerable "leverage" error. Depending on your
functionalization it may be possible to come up with a density model
that works quite well, see the SI from this article:

	https://www.ncbi.nlm.nih.gov/pubmed/27461742

For block-polymer micelles the extrapolation problem should be far less 
significant, since their densities are much closer to that regime which
can be modulated with D2O or H2O18, though how much success you'll have 
may depend on the precise nature of the polymer and the degree of 
heterogeneity.

Regards, -b.
---
Borries Demeler, Ph.D.
Professor
The University of Texas Health Science Center at San Antonio
Dept. of Biochemistry and Structural Biology, MC 7760
7703 Floyd Curl Drive, San Antonio, Texas 78229-3901
Voice: 210-767-3332, Email: demeler at biochem.uthscsa.edu