My 2 cents: Cent #1 "But they are slightly different ( may be 50 ev) in different papers." This statement itself contains the answer - always scan, but it was already suggested by many. I'd like to get to slightly different issue here. No oxidation or any other interaction in protein crystals moves the Se edge 50 ev from its theoretical value. It is obvious that in that particular experiment the monochromator was not calibrated properly. Ethan already touched on this but I wanted to emphasize again: when a scan identifies the edge to be way out of reasonable, the beamline support staff should be asked to check the monochromator calibration before publishing such exotic results.
On our beamlines at GM/CA-CAT at APS, we do calibration at the start of each user group. It's easy, fast, and Se edges from sample to sample, project to project, user to user, are remarkably stable. Cent #2 Often orientation of your sample crystal has far greater effect on the way your scan as well as f' and f" curves look than oxidation or other issues in the protein molecule. Check out Bricogne's publications few years back. Cheers, Nukri Ruslan Sanishvili (Nukri), Ph.D. GM/CA-CAT Biosciences Division, ANL 9700 S. Cass Ave. Argonne, IL 60439 Tel: (630)252-0665 Fax: (630)252-0667 rsanishv...@anl.gov ________________________________ From: CCP4 bulletin board [mailto:ccp...@jiscmail.ac.uk] On Behalf Of Jerry McCully Sent: Thursday, July 16, 2009 12:16 PM To: CCP4BB@JISCMAIL.AC.UK Subject: [ccp4bb] MAD wavelength Dear All: Next week we are going to try some seleno-Met labeled crystals. We checked the literature to try to find out the peak wavelength that has been used for SAD or MAD data collection. But they are slightly different ( may be 50 ev) in different papers. I guess this is due to the discrepancy between the fluorescence scanning and the theoretical vaules of f' and f''. When we collect the data, which wavelength should we use? Should we trust the scanning results? Thanks a lot for your comments. All the best, Jerry McCully ________________________________ Insert movie times and more without leaving Hotmail(r). See how. <http://windowslive.com/Tutorial/Hotmail/QuickAdd?ocid=TXT_TAGLM_WL_HM_T utorial_QuickAdd_062009>
