[ccp4bb] off topic - static laser light scattering

[Anastassis Perrakis]

Dear all,

The MALLS instruments on-line with an FPLC and with an RI detector,  
should provide an 'absolute MW', shape independent,
and indeed in our hands they do well. Until yesterday, where a 21kD  
protein pretends to be 25 kD. We did the mass spec
anyway, and its 21kD as we expected to the residue, but I am still  
puzzled by that result.

One central assumption for the MALLS formulas, is that dn/dc, the  
specific refractive index increment, is constant for unmodified  
proteins,
made by aa with no sugars etc. Literature suggests dn/dc values for  
proteins to be constant and between 0.189/0.190 is a good value,
with minimal buffer dependence for aqueous buffers with 'the usual'  
salts.

I am a rather bad physicist, but my reading tells me that dn/dc, and  
thus light scattering, depends to the "laser-light induced dipole in  
the molecule". Is there any reason to believe that in theory a  
molecule with a very particular charge distribution (eg a small DNA  
binding protein which is already a 'dipole') would have significantly  
different dn/dc values? Is anyone aware of such an experiment?  
Literature searches were in vain ...

Best -

Tassos