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Hi Maria, Initial questions: 1) What's present in crystallisation/purification buffers? 2) Are any other ligand visible for the 9sigma peak? 3) Does the 9 sigma peak also have a peak in an anomalous difference map? Assuming 1.7A is accurate (and with 1.5A resolution, you'd hope it would be!) a metal-ion - water interaction is looking less likely. Take a look at Marjorie Harding's papers and website for target metal ion - ligand distances, and the closest you'll see is water:Mg2+, at 2.07. http://tanna.bch.ed.ac.uk/newtargs_06.html So one would assume it is a covalently bonded compound. So back to question 1. What's in your buffers? A quick search for bond length tables suggests Carbon-Sulphur (1.8) and Carbon-Chlorine at 1.7. hope this helps David 2008/9/19 Maria Håkansson <[log in to unmask]>: > Hello All, > > I have a problem with a 9 sigma positive peak 1.7 Å away from a water > molecule (or what I believe is > a water molecule). There are several similar peaks in my map though only one > is as high as 9 sigma. > > My first thought was to exclude these too close waters. However the R-values > increased by more than 0.5 %. Could it > be carbonmonoxide or oxygen atoms? By the way, It is 1.5 Å resolution data. > Any suggestions? > > Best Regards, > > Maria Håkansson > > >> Maria Håkansson, Ph.D. >> tel: +46 (0) 76 8585 706 >> Senior Scientist, Max-lab, Lund University >> fax: +46 46 222 47 10 >> Ole Römers väg 1 (P.O. Box 188) >> www.maxlab.lu.se >> SE-221 00 Lund, Sweden [log in to unmask] >> > -- ============================ David C. Briggs PhD Father & Crystallographer http://drdavidcbriggs.googlepages.com/home AIM ID: dbassophile ============================