Dear Petr Leiman : It could be the result of series termination in anomolous map.(not sure) The error in the position results in a shifted position in the 2Fo-Fc map. As a check it is worthwhile to see an average of the two positions (three coordinates) results in a feature consistent with Bijvoet Difference and 2Fo-Fc maps. But I do not know how this error affects just one site and not others. Best Srinivasan Raghunathan On Thu, 24 Apr 2014, Petr Leiman wrote:
Dear All, We are looking for an explanation for a very strange observation. Problem: We have two fully independent data sets (two different crystals), in which the Bijvoet Difference Fourier map peak of one particular metal site is shifted by 0.47 A from its position in the 2Fo-Fc map. Relevant information: The resolution of both data sets is 1.5-1.6 A. The 2Fo-Fc and BDP maps are calculated using the same phases. The metal ion is water hydrated and all the details are crystal clear in both 2Fo-Fc maps. The crystals are grown in the presence of Sr and the data sets are collected at the Sr K-edge. There are many other Sr sites and all strong peaks in the BDF map overlap with 2Fo-Fc map peaks perfectly. The Sr site in question is not the strongest, but it is well above the noise level of the BDF map. Additional information: The weird site is actually fully buried inside an internal cavity (it is surrounded by protein atoms from all sides), but a Sr atom is able to diffuse into this cavity somehow. All other Sr sites are on the surface of the protein. Any thoughts about why a non-noise BDF map peak would not overlap with a 2Fo-Fc map peak are welcome! Thank you very much, Petr ------------------ Petr Leiman EPFL BSP 415 CH-1015 Lausanne Switzerland Office: +41 21 69 30 441 Mobile: +41 79 538 7647 Fax: +41 21 69 30 422
