Your case might be different, but it could also be a true Se signal from
Se-Cys incorporated into your protein during Se-Met expression. Depending
on the protocol you used, Se may get incorporated into Cys, especially if
only source of "sulfur" is Se-Met. We have seen such signals from Cys-
containing proteins produced in E.coli. Nice thing is that you get extra
sites to use in phasing!
Konstantin
K.Korotkov,
University of Washington
Department of Biochemistry
Box 357742
Seattle, WA 98195
On Thu, 25 Sep 2008, George M. Sheldrick wrote:
Unexpected peaks in a S-SAD experiment sometimes turn out to be chloride,
sulfate or a metal ion. I would suggest that you run shelxd with and
without the disulfide option (or with different numbers of disulfides)
to see which is best, and also run SHELXE with the -b flag set. This will
produce an analysis of the anomalous density at the sites that you
inputted and a list of peaks in the anomalous map. I also suggest that
you look at the anomalous density by feeding the .pha file from SHELXE
(generated if -b is set) into e.g. Coot.
George
Prof. George M. Sheldrick FRS
Dept. Structural Chemistry,
University of Goettingen,
Tammannstr. 4,
D37077 Goettingen, Germany
Tel. +49-551-39-3021 or -3068
Fax. +49-551-39-22582
On Thu, 25 Sep 2008, Michael Jackson wrote:
Hello,
I had recently collected and solved the phases for a protein molecule using
CCP4 and the ShelXCDE SAD method in it. What I was wondering was that the
peaks for the three SE incorporated methionines are there as expected, but
there is one peak scored roughly as the second largest where the disulfide is
based on ShelXD's HA search algorithm. This data was collected at 0.97960
Angstroms which is close to the peak Xray absorption edge for Se but does
anyone know if a disulfide has any absorption edge overlapping here?
