help: search for heavy atom sites in P21

Harry Powell hrp1000 at
Mon Nov 18 05:10:05 EST 1996

On 14 Nov 1996 xd691281 at postmaster wrote:

> Hi, everyone,
> 	I am using CCP4.3 to determine heavy atom positions from anomalous
> difference patterson map of Se-Met protein.  There should be
> sixteen Se sites.  The space group is P21.  
snip, snip

> 	Any help will be highly appreciated.
> 						Joyce
Hi Joyce 

(sorry to everyone on the newsgroup who doesn't want to read this; I 
tried 'reply' in mail and it got bounced. Real mail addresses are 
wonderful things...)

My answer is "Use SHELXS direct methods to find your heavy atoms"; I use 
TREF, OMIT 0, and HKLF 3, i.e. default setting for direct methods (TREF), I 
*know* all the anomalous differences are small so I don't want a sigma 
cutoff (OMIT 0), and I have to tell the program I'm using anomalous delta-F 
data (HKLF 3).

In my (limited) experience of macromolecular delta-F work, direct methods 
seem to be less sensitive to outliers than Pattersons, and they are about 
100x faster, so you lose little by trying. With the three structures I've 
been closely involved with solving this year with Br anomalous, the 
direct methods took less than ~30seconds cpu (R44K, 150MHz) to solve for the 
Br(s) in each case. 

If you don't have SHELXS-9* (* could = 0,3,6, depending on where it comes 
from and how official it is);
         (a) ask a local small molecule crystallographer
         (b) e-mail George Sheldrick directly...

I hope this helps.

Dr Harry Powell                               
Telephone: +44 1223 
333127 Automation Office                      Fax:       +44 1223 333160
University Library
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