Hi Jay The only consideration that matters is: DON'T KILL YOUR CRYSTAL!!!!! If you think it's dying after 15min, chances are your heavy atom's vanished after 5. You'll be surprised how weak a signal you can get away with -- as long as your data is uniform!! (i.e. non-decayed). So chill, back off on intensity (e.g. attenuate) until you're barely comfortable with the image strength, take it down another 20%, and then collect at LEAST 360 degrees. Equally (or more) importantly, when trying to see your peak, try merging different numbers of frames. Cheers Frankf
_____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Jay Thompson Sent: 06 December 2006 21:17 To: [email protected] Subject: [ccp4bb]: Data collection strategies Hi All, We have crystals (0.2 x 0.2 x 0.2 mm) that belong to the spacegroup C2, with a unit cell of 310 A x 290 A x 230 A, 90 102 90 and diffracts rather poorly at ~4 angstroms. We're trying to collect a Hg and Pt-SAD datasets, since a MAD dataset is likely not feasible. So far I've collected data on a couple of crystals (~45 minutes of total exposure time, Rsym= 0.14, I/sig(I)=7.0, redundancy=4). and I'm having trouble detecting any anomalous peaks in the Harker sections using the programs in CCP4. It looks like these crystals are exhibiting radiation decay (based on unit cell length, scale factor, and mosaicity increases), after ~10-15 minutes of exposure time on a 2nd generation synchrotron (like SSRL or ALS). If anyone has any wonderful strategies to collect SAD data on weakly/poorly diffracting and radiation sensitive crystals, that would be great!! I need to decrease the exposure time on the crystal to be able to collect a complete dataset with some anomalous signal. However, I'm worried that if I decrease the exposure time the signal-to-noise would suffer and I would still have trouble finding an anomalous peak in the Harker section (although redundancy should increase signal-to-noise). Which is better in terms of improving signal to noise for SAD or MAD datasets? Would people recommend to keep the exposure time short and just collect lots of images to increase redundancy and signal-to-noise or increase exposure time but have a less redundant dataset. So I guess the question comes down to whether multiple weak reflections is better than one strong reflections for SAD? Does anyone have any nice strategies for estimating/optimizing the exposure time for SAD/MAD datasets? Also do people still like to collect inverse beam for C2 spacegroups? Any comments would be greatly appreciated. Thank you very much. JT
