sure, "whether the structure can be solved" is a good criterion.but often, when trying different heavy atoms, the structure can NOT be solved, and then some criterion to judge whether to continue to try and solve it with that data, or whether to focus on other datasets, other derivatives, another crystal form, another construct or even another protein, is useful.
CCanom in scala/aimless is one of these criteria.
Fluorescence signal is obtained also if you have disordered metal, so even if you get it, it does not mean you have a good derivative, because the metal may be in the solvent or bound to a disordered region.
If you have an isomorphous native, Patterson diff maps are a way to judge the number of heavy atoms, for SAD perhaps the anomalous Patterson map, but that may be less clear (more noisy).
In the Hg-case, the number of Cys can be a first guess of the number of Hg sites.
On 16 Jan 2015, at 17:18, Keller, Jacob wrote:
How about “whether the structure can be solved” as a criterion, i.e, maybe just put it into your favorite software and see whether it works, trying various parameters? Depends on your goals, I guess—most people just want to solve the structure and move on.
JPK
From: CCP4 bulletin board [mailto:[log in to unmask]] On Behalf Of luzuok
Sent: Friday, January 16, 2015 9:33 AM
To: [log in to unmask]
Subject: [ccp4bb] Assess anomalous signal?
I'm doing Hg SAD phasing for the first time, and I met some problem:
1. How to assess the anomalous signal after I process the data and get my mtz file?
My labmate doesn't scan the fluorescence signal. Actually, I don't quite understand why we use fluorescence to detect anomalous signal.
2. How to estimate the number of heave atom in one unit cell? by soaking heavy atom derivatives.
Mark J van Raaij
Lab 20B
Dpto de Estructura de Macromoleculas
Centro Nacional de Biotecnologia - CSIC
c/Darwin 3
E-28049 Madrid, Spain
tel. (+34) 91 585 4616
http://www.cnb.csic.es/~mjvanraaij