Assuming it wasn't clear from purification/crystallisation reagents... Maybe try a high multiplicity anomalous dataset collected in house / at long wavelength?
P has ~ 75% the f" of S at CuKa. If you can figure out roughly what anomalous peak height an S atom gives from a Cys or a Met with similar ADP to your mystery atom, you might be able to distinguish PO4 & SO4. You'd need decent data (well & oft recorded anomalous differences), and usual caveats about occupancy and ADPs apply. Dave. On Fri, Nov 11, 2016, 17:50 Gulcin Gulten <gulcingul...@gmail.com> wrote: > Similarly, how do you differentiate a phosphate ion than sulfate just > based on electron density if data is not at atomic resolution? > > Thanks! > > > On Fri, Nov 11, 2016 at 3:52 AM, Harry Powell <ha...@mrc-lmb.cam.ac.uk> > wrote: > > Hi all > > Sticking to the first question, if you don't restrict yourself to _X-ray_ > crystallography but use your local neutron source instead, it should be > straightforward (subject to all the normal caveats). > > On 10 Nov 2016, at 23:02, Tim Gruene wrote: > > Dear JPK, > > to answer your first question, at atomic resolution you would notice a > density > difference between N and C. At a little less resolution you might still > measure difference in bond length. > > Regrds, > Tim > > On Thursday, November 10, 2016 8:41:43 PM CET Keller, Jacob wrote: > > Dear Crystallographers, > > > I don't think there is any feasible way crystallographically to distinguish > > between nitrate and carbonate or bicarbonate-correct? But that is not my > > main question. > > > My main question is: given that nitrate and carbonate are both very > > important and also very different physiologically, and therefore they must > > be distinguished/recognized by cells, how is this done, since the ions are > > so similar in structure? Is there some aspect of these ions that differs > > dramatically of which I am not aware? What kind of "handles" could a > > protein grab onto to distinguish between nitrate and carbonate/bicarbonate? > > > JPK > > > > ******************************************* > > Jacob Pearson Keller, PhD > > Research Scientist > > HHMI Janelia Research Campus / Looger lab > > Phone: (571)209-4000 x3159 > > Email: kell...@janelia.hhmi.org<mailto:kell...@janelia.hhmi.org > <kell...@janelia.hhmi.org>> > > ******************************************* > > > > -- > -- > Paul Scherrer Institut > Tim Gruene > - persoenlich - > OFLC/102 > CH-5232 Villigen PSI > phone: +41 (0)56 310 5297 > > GPG Key ID = A46BEE1A > > > Harry > -- > Dr Harry Powell > Chairman of International Union of Crystallography Commission on > Crystallographic Computing > Chairman of European Crystallographic Association SIG9 (Crystallographic > Computing) > > > > > > > > > > > > > > > -- > Post-Doctoral Research Associate > Texas A&M University > Dept. of Biochemistry and Biophysics > Interdisciplinary Life Sciences Building > 301 Old Main Drive, Lab 2138 > College Station, Texas 77843 > -- [image: --] David Briggs PhD [image: https://]about.me/david_briggs <https://about.me/david_briggs?promo=email_sig&utm_source=email_sig&utm_medium=email_sig&utm_campaign=external_links>
