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 <CCP4BB@JISCMAIL.AC.UK> On Behalf Of Eleanor Dodson Sent: Tuesday, April 3, 2018 20:17 To: CCP4BB@JISCMAIL.AC.UK 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-requ...@jiscmail.ac.uk<mailto:00001d16aa30e8a1-dmarc-requ...@jiscmail.ac.uk>> 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
