In the distant past, we did get some additional phasing signal by a simplified version of the RIP concept based on the lability of the bromines in a 5-bromo-dU substituted DNA in an x-ray beam. We collected the peak anomalous signal first and fast, and then just kept collecting (we may probably to the edge wavelength too). If my memory serves me right, we binned the data into reasonably complete subsets, and then tested which initial - decayed gave us the best difference signal (which would be a combination of delta f' and delta f at that point). We could have solved the structure without going to such lengths, but it was kind of fun and it did improve the phasing a bit. And heck, the crystal was in the beam, so why not milk it for every smidgen of signal it could provide?
________________________________________ From: CCP4 bulletin board [CCP4BB@JISCMAIL.AC.UK] on behalf of George Sheldrick [gshe...@shelx.uni-ac.gwdg.de] Sent: Saturday, April 29, 2017 2:14 PM To: CCP4BB@JISCMAIL.AC.UK Subject: Re: [ccp4bb] radiation damage-induced phasing (RIP) tutorial Dear Murpholino, I must apologize, there was a mistake in may last email. The critical parameter is, as you correctly pointed out, DSCA not RIPW and it is necessary to try a range of values for DSCA. For this reason I am CCing this to CCP4bb. As you will be discovering, RIP phasing is not easy. We treat it like SIR, but it is complicated by the presence of both positive and negative difference peaks caused by radiation damage. Also density modification is less effective for SIR and RIP than for SAD phasing. In the case of SAD, just replacing negative density with zero improves the phases, but this is not true of SIR and RIP, However using the anomalous signal as well (RIPAS) helps, similar to SIRAS. Best wishes, George -- Prof. George M. Sheldrick FRS Dept. Structural Chemistry, University of Goettingen, Tammannstr. 4, D37077 Goettingen, Germany Tel. +49-551-39-33021 or -33068 Fax. +49-551-39-22582
