help: search for heavy atom sites in P21
hrp1000 at cus.cam.ac.uk
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.
> Any help will be highly appreciated.
(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
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
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