Hi Sebastiano & Claudio,
On two separate "hard" problems (one a thulium soak, one an unhappy SeMet), I've obtained good results by first
collecting a high energy remote (~100ev up from the peak) followed by the inflection. In both cases this strategy
succeeded in phasing where peak + low E remote or continuous wavelength switch attempts failed.
As you say, with a high E remote + inf collection, at least you've collected what might be a somewhat useful anom SAD
set if your xtal dies after the first wedge. And if it survives both wavelengths, both f'' and the dispersive difference
between the two wavelengths will be relatively large, making it different from a peak + inf or peak + low E collection.
Your mileage may vary, of course, and I've had cases where the normal peak + low E worked better.
Finally, I've had the best luck finding sites with peak or high E remote SAD wedges, but obtained better phasing/maps by
then using those sites with some/all wavelengths in MAD phasing.
Jacob
PS - This might go without saying, but always do a fluo. scan before choosing wavelengths to collect. Those peaks can
shift substantially.
Sebastiano Pasqualato wrote:
Hi all,
I'm looking for some advices on some "general hints" on how to carry out
a MAD/SAD data collection.
We had SeMet crystals that diffracted to ca. 3.5 Angs, with anomalous
signal only at ca. 5-5.5 Angs, with diffraction decaying on brilliant
beamlines (ID29 or ID23 at the ESRF) in a matter of ca. 300 degrees...
Needless to say, that was not sufficient to solve the structure...
We do have improved the crystals that look now nicer and bigger, and
have some beamtime next weeks both at BM16 and ID29 at the ESRF.
Assumed that we do see diffraction higher that 3 Angs, what would people
suggest?
Collecting first at the high energy remote for a SAD experiment and then
going for peak and inflection point, or rather going for the peak first,
then remote and ip?
I personally would avoid the continuous switch of wavelengths, but I
know there are some fans of this technique, either...
Let the gurus talk!
Any advice is obviously highly appreciated!
Thanks in advance,
Sebastiano and Claudio
--
Sebastiano Pasqualato, PhD
IFOM
Istituto FIRC di Oncologia Molecolare
via Adamello, 16
20139 Milano
Italy
tel +39 02 574 303 325
fax +39 02 574 303 310
--
Jacob Corn
The Berger Lab
UC Berkeley - Molecular and Cell Biology
[EMAIL PROTECTED]
phone: 510-643-8893
fax: 510-643-9290
