Electron microscopy of proteins

Dr. Peter Gegenheimer PGegen at UKans.edu
Fri Aug 7 18:38:24 EST 1998


On Thu, 6 Aug 1998 14:25:37, 
klenchin at facstaff.REMOVE_TO_REPLY.wisc.edu (Dima Klenchin) wrote:

> In article <35C98E51.7F7C at uni-koeln.de>, "Dr. Christian Kupfer" <christian.kupfer at uni-koeln.de> wrote:

--- snip ---
> >Is there anybody who can tell me how to determine the molecular 
> >mass just by knowing the radius in angstroms? Or is there any good 
> >review/ book out there where I can find answers?
> 
> As far as I know, the density of all proteins is ~ the same. Hence, if you
> know dimensions (is it _really_ sphere?), you know mass. 

Another response gave protein density as ~1.3 g/ml. This refers to 
the *buoyant* density (theta) of protein (e.g., in solutions of 
CsCl, etc.). This is not the same as the physical density (rho), 
which is the reciprocal of the partial specific volume (nu). 

I looked around for a value of nu and couldn't find one--it should 
be in any biological physical chemistry text. I did have at hand ME 
Hamilton, Meth Enz. 20:512-521 (1971). This gives a value for 
anhydrous ribosomal protein of nu=0.74, said to have been calculated
from the amino acid composition. The corresponding density=1.35 
g/ml. I have no idea how to calculate this, but the partial specific
volumes of amino acids are not secret. 

(For the same protein in Cs salt solutions, an extrapolated value of
nu=~0.8, so density=1.25. My understanding is that a protein's 
apparent density will decrease as it is solvated by H2O, and 
increase as it binds various salts.) 

Density of 1.30 corresponds to nu=0.77, 1.32 to nu=0.76, and 1.33 to
nu=0.75. My bottom line would be that using a density of 1.33 g/ml 
(nu=0.75) would be a reasonable estimate. 

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