AW: [3dem] PCS vs cryoEM

[Frank Polzer]

Hey Kimberley,

your colleague is definitely right about that. Having a kind of rod-like
structure, the diffusion coefficient (which is taken to actually calculate
the hydrodynamic radius) of the particles has a translational and rotational
component. So imagine a rod moving in one direction while spinning around
his center of mass, this'll slow down its movement in solution which makes
him appear bigger by PCS (of course this is very simplified picture).

One can determine the aspect ratio of anisotropic particles using
depolarized dynamic light scattering (DDLS). But this is totally
non-straight forward especially for low-scattering material. And you will
need a depolarizator on your PCS setup as well.

So the fact of having polydisperse sample including an anisotropic part
could easily lead to deviations in cryoTEM - PCS!

Cheers,
Frank

Frank Polzer
TEM Group
Insitute of Physics
Humboldt Universität zu Berlin
Newtonstraße 15
12489 - Berlin

frank.polzer at physik.hu-berlin.de

Tel.: +49 30 2093-7905 (office)
Tel.: +49 30 2093-7829 (TEM)
Fax:  +49 30 2093-7760

-----Ursprüngliche Nachricht-----
Von: 3dem-bounces at ncmir.ucsd.edu [mailto:3dem-bounces at ncmir.ucsd.edu] Im
Auftrag von Kimberley Cheng
Gesendet: Montag, 31. Oktober 2011 12:51
An: fabian.kebbel at unibas.ch
Cc: 3dem at ncmir.ucsd.edu
Betreff: SV: [3dem] PCS vs cryoEM

Thank you all for responding so quickly to my e-mail! 

The cryoEM images show that the liposomes have different shapes, everything
from spherical to elongated. A colleague of mine suggested that the
difference in size determined by the two methods could be due to this issue.
She said the PCS method works best on samples that are spherical, and
results from samples containing different shapes could be misleading. 

Any thoughts about this?

Thanks!
/Kimberley



________________________________________
Från: fabian.kebbel at unibas.ch [fabian.kebbel at unibas.ch]
Skickat: den 31 oktober 2011 11:50
Till: Kimberley Cheng
Ämne: Re: [3dem] PCS vs cryoEM

Hi,

did you take the viscosity of the sample into account? This is part of
the Stokes-Einstein equation. When you enter a viscosity, which is too
low, you would yield too big hydrodynamic diameters.

All the best,

Fabian

Zitat von Kimberley Cheng <Kimberley.Cheng at ki.se>:

> Dear all,
>
> I recently got the following question "The particle size of our
> liposome product determined by cryoEM is about 75 nm, but the
> particle size determined by photon correlation spectroscopy (PCS) is
> about 110 nm. So we were wondering why the size determined by cryoEM
> is smaller than by PCS method?".
>
> Does anyone have a good explanation to this?
>
> Many thanks in advance!
> /Kimberley
>
>



Fabian Kebbel, PhD student
Center for Cellular Imaging and Nano Analytics (C-CINA)
Biozentrum, University of Basel
Mattenstrasse 26
CH-4058 Basel
Switzerland

fabian.kebbel at unibas.ch
http://www.c-cina.unibas.ch

phone: +41 61 38 73214


----------------------------------------------------------------
This message was sent using IMP, the Internet Messaging Program.


_______________________________________________
3dem mailing list
3dem at ncmir.ucsd.edu
https://mail.ncmir.ucsd.edu/mailman/listinfo/3dem