Re: [ccp4bb]: Can I find a zinc in a haystack?

[Pawel Grochulski <pawel@bch.umontreal.ca>]

***  For details on how to be removed from this list visit the  ***
***    CCP4 home page http://www.dl.ac.uk/CCP/CCP4/main.html    ***

> 
> Depends upon the quality of the crystal and your data quality and 
> redundancy. We are able to easily measure anomalous differences at the
> Fe K-edge for a protein of similar size (soybean L-1 lipoxygenase, 94 kDa,
> typical diffraction to 1.6 angstroms), so I wouldn't hesitate to try.
> 
> Diana

Hi Diana,
	The Bijvoet signal can be estimated from
the formula:
 
sqrt(1/2)sqrt(Na/Np)(2f''/Zef) where:

Na - number of Zn(Fe) atoms per protein molecule
Np - number of non-H atoms in the protein molecule
Zef - average atomic number in proteins (~6.7)

Assuming ~94kD protein (about 7015 non-H atoms) and one Fe atom (f''~3.9)
the signal will be at the level of ~1%, unless the f'' at peak is much
higher due to the presence of the "white lines", but this is usually
observed at the L edges.
I wonder how strong was your Bijvoet signal and if this would be enough
to solve your structure doing MAD at the Fe K-edge?

Pawel