Dear Francis, I am very interested in your work with this. In the paper :- A. Mukherjee, J.R. Helliwell and P. Main 'The use of MULTAN to locate the positions of anomalous scatterers'. (1989) Acta Cryst. *A45*, 715-718. you will see that missing centric reflections was not limiting. However, the more dilute the anomalous scatterers versus the protein atoms led to the practical consequence of increasing the number of required iterations to a successful answer. That said the potential effect of missing centric data and the influence of random and systematic errors on the charge flipping may of course be different. Best wishes, John Prof John R helliwell
On Sat, May 22, 2010 at 3:23 PM, Francis E Reyes <francis.re...@colorado.edu > wrote: > Hi all > > I've been playing around with charge flipping for macromolecular > substructure determination with pretty promising results. I'm particularly > attracted to the fact that it solves structures in P1, with no space group > assumptions and curious how it would handle some of the pseudosymmetry cases > I've come into in my time. > > I'd like to know if anyone's had experience with this method, and open up > the discussion with the following questions: > > As the algorithm starts with completely random phases and charge flips the > map in P1, what is the importance of measuring (good or any) anomalous > signal at all (for the sole purpose of finding the heavy atoms)? At first > pass it would seem that just as long as you have an incorporated heavy atom > and the density of that region is greater than delta, that this alone would > be sufficient for locating the position of the heavy atom. In other words > just as long as your heavy atom is sufficiently higher in contrast than your > protein/rna it would be a good enough criteria. > > In the above regime, would the importance of measuring anomalous data be > more important for substructure refinement (via phaser, mlphare, sharp, > solve/resolve)? > > Now to a more specific question for those who've had experience (or maybe > the authors are subscribed here): > > Orthorhombic C2221 using SUPERFLIP heavy atoms are found with great > peakiness (before noise suppression: peakiness = 5, after noise suppression > peakiness >25, good separation of heavy atom peaks from noise peaks in > resulting pdb). Yet the space group check via the sym operators is rather > poor (overall agreement close to 100). My interpretation is that the heavy > atoms are found, but the space group is wrong? > > > > Thanks! > > F > > --------------------------------------------- > Francis Reyes M.Sc. > 215 UCB > University of Colorado at Boulder > > gpg --keyserver pgp.mit.edu --recv-keys 67BA8D5D > > 8AE2 F2F4 90F7 9640 28BC 686F 78FD 6669 67BA 8D5D > -- Professor John R Helliwell DSc
