Le 06/03/2010 00:52, Dhirendra K Simanshu a écrit :I try answering your question without knowing exactly what has already been sent to CCP4BB.
Sorry, if I duplicate previous answers.We have a good practice with bromine as an anomalous scatterer for solving nucleic acids structures. In fact, we are using brominated bases, which is extremely comparable to using selenomethionins with proteins. What we are sure about is that the covalent bond linking bromine to the base is cleaved under X-rays, which kills the anomalous signal (AS). We have many examples of that. [see Ennifar et al., Acta Cryst D58 (2002) 1262; Schiltz et al., Acta Cryst D60 (2004) 1024; Olieric et al., Acta Cryst D63 (2007) 759]
This is a good example of 'radiation damage'.
First question: what about the stability of your cluster ? If it is cleaved as well, then you have to expect a strong variation of the AS. This should appear well with the variation of the Chi2 value of the AS. Thus a question: did you try using only a subset of your data (i.e. cutting your data after, say, 180 frames with the same crystal). The peak and the inflection with the same crystal might be too much irradiation of your crystal.... !!!???
Second remark: do you take fully into account that your anomalous scatterer is not made of a single AS ? Twelve bromine atoms certainly correspond to a huge anomalous component that need to !
Did you try phasing at low resolution ?Third remark: pay attention to a strong effect that may well happen in your case and that is related to the orientation of covalent bonds relative to the polarization of the beam [see Bricogne et al., J. Appl. Cryst. 38(2005) 168; Olieric et al., Acta Cryst D63 (2007) 759]
I hope this will help. Philippe Dumas IBMC-CNRS, Strasbourg
Dear all,I am trying to phase 172 aa protein using tantalum bromide (Ta6Br12) cluster. Data belongs to space group C2221 with 4 molecules (2 dimers) in the asymmetric unit. Phenix.xtriage indicates presence of pseudo-translation (peak height= 39). I have collected data at Ta peak (1.2550 A) to a resolution of 2.9A, Ta inflection (1.2553 A) at 2.8 A using one crystal. I also collected one high energy remote (1.059 A) data at 2.6 A using another soaked crystal. All three datasets shows presence of very good anomalous signal till the highest resolution shell.Soaking with Tantalum bromide has changed the unit cell axes "a" and "b" by 4-5 A so it is no longer isomorphous with the native data (2.7 A). But since anomalous signal is very high in all these Ta soaked datasets, I have been trying to find a solution using SAD/MAD but without any success. Processing data in lower symmetry P21 and C2 also doesn't help.Another noticeable thing is..... "scalepack2mtz" fails when I try to run to convert my sca file into mtz (using CTRUNCATE option ON) with error saying "Anisotropic correction failed - negative eigen value".Since finding a solution has not been possible for reasons unclear to me. I guess I am having problem because of presence of pseudo-translational symmetry in my crystal. I would be grateful for any helpful advice or suggestions in this regard.Thanks in advance! Regards Dhirendra
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