If you merge data with AIMLESS I + and I - are always reported seperately and it reports a CCanom checking whether two randomly selected data halves have a correlation in the anom signal .
Often *IF* you can position the S atoms the phasing steps work, but it can be challenging to get the sites . We use SHELXD but it can be confused by spurious "anomalous signal". Phasing can more often bootstrap its way out of trouble by various density modification and averaging tricks.. Eleanor On 3 April 2018 at 16:16, Raghurama P Hegde <[email protected]> wrote: > Hi Manoj, > > > > Like Eleanor has pointed out a very accurate data may get you a solution. > If you have crystals that diffract well and give you data with high > I/sigma(I) you can attempt to find the positions of the S atoms and use > that to arrive at a structure. Using lysozyme as a test case we have shown > that structure can be solved from a routine data set collected at 1 Å (with > 10 S but a f” of ~0.28), with anomalous multiplicity ~12. Details are > published in this paper (yes it’s a shameless plug for our paper! 😊): > > > > Hegde et al. (2017), The hidden treasure in your data: phasing with > unexpected weak anomalous scatterers from routine data sets, *Acta > Crystallogr F Struct Biol Commun,* *73*, 184-195. > > http://scripts.iucr.org/cgi-bin/paper?S2053230X17002680. > > > > However the data was not collected at a home source but at a synchrotron > beamline. There could also be some weak anomalous scatterers acquired > during crystallization, from the crystallization condition, so you could > collect a data set to check for the presence of anomalous signal in it and > attempt experimental phasing. That could give you pointers on a future > course of action like going to a synchrotron beamline to collect stronger, > more accurate data or prepare heavy atom derivates, like Eleanor has > pointed out. > > > > As for data processing you’d have to process the data with Friedel mates > pairs kept separate so that the anomalous differences can be used to find > positions of the S atoms. > > > > Hope that helps, > > Raghu > > > > *From:* CCP4 bulletin board <[email protected]> *On Behalf Of *Eleanor > Dodson > *Sent:* Tuesday, April 3, 2018 20:17 > *To:* [email protected] > *Subject:* Re: [ccp4bb] Sulphur SAD at home source > > > > Well - the S f" is only ~ 0.5 at Cu Kalpha so the signal will be very > weak.. > > Very accurate data may get a solution but you first have to position the S > atoms... > > Much easier to try to make a heavy atom derivative! > > Eleanor > > > > On 3 April 2018 at 15:26, Manoj Saxena <00001d16aa30e8a1-dmarc- > [email protected]> wrote: > > Hi All, > > > > I am writing to seek advice on doing sulphur SAD data collection > > at Cu based home source for a protein that is 12 KDa and has 6 S atoms. > > I have seen some links online and some references but would be grateful if > > you can share your know-how for success with this. > > Like what multiplicity of data would be good to aim for and > > data processing tips. > > Inputs from people who have tried and failed would also be highly > appreciated. > > > > Thank you > > Manoj Saxena > > University of Puerto Rico > > > > > > >
