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I agree with Savvas that 7 angstroms should be sufficient (assuming all else is ok) for finding the heavy-atom sites. However, instead of Fo(soak)-Fc(MR) phase(MR); I'd use Fo(soak)-Fo(native 2.4 or 7 angstrom) phase(MR) (isomorphous difference fourier). If you get peaks in this map, then it confirms your MR solution (I haven't used phaser, and don't remember offhand how to rate Z-scores). If there are no good peaks, then probably either the MR solution is off, or the Hg signal isn't good enough to phase from (or there's non-isomorphism). Pete > Hi Nick, > 7 angstrom data should be enough to determine Hg sites with high > occupancy. A difference Patterson map should do the trick very well > provided of course your 'aged and soaked' crystals are still > quasi-isomorphous to the original native or to the 'aged and unsoaked' > native crystals. In fact, I even think that your molecular replacement > solution would be even better to locate the sites in a difference > fourier synthesis [Fo(soak)-Fc(MR); alpha(MR]. > > However, with such high Z-scores in Phaser your MR solution appears to > be enough. Have you refined the structure at all? Simulated annealing > could help quite a bit with the overlapping loops, and several rounds of > model (re)building with arp-warp starting with your MR model could help > you get there as was pointed out a couple of days ago here on the > CCP4-bb (Kerr, Kostrewa, Perrakis et al.). Your 2.4 angs data should be > enough for that option! > > Best wishes > Savvas > > > > -----Original Message----- > From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of > Noinaj > Sent: donderdag 6 april 2006 23:52 > To: [email protected] > Subject: [ccp4bb]: Low Resolution Heavy Metal Derivatives > > > Hi, > > I have previously collected a native dataset at 2.4 angstroms (a few > months ago). I recently used some old crystals for heavy metal soaks > using MeHgCl and was only able to collect to around 7 angstroms. The > crystals seem to have 'died' over time, since I also tried some that > weren't soaked for comparison and they didn't diffract past 7 angstroms > either. We were able to observe a fluorescent signal from the crystals > indicating the presence of Hg, but my question is, is 7 angstroms enough > resolution to find the Hg sites with confidence? > > I intended to use this in combination with a molecular replacement > solution, that I feel is very good (Z-score is 31 in Phaser), but the > crystal packing has significant overlap in some regions. I am currently > soaking new crystals with MeHgCl for better resolution, and trying to > grow SelenoMet crystals, so I don't want to waste my time if the data > that I have now just isn't worth the effort. > > Thanks for any and all help in advance. > > > > Cheers, > Nick > > > > > > Pete Meyer Fu Lab BMCB grad student Cornell University
