Dear Engin, On Tue, May 12, 2009 at 12:20:31PM -0700, Engin Ozkan wrote: > The take home message for me was that noone agrees on the best data > collection strategy
No - since you have to factor in at least half a dozen parameters: unfortunately there is no silver bullet :-( > Another point is the success rate of software - be it direct methods > based or Patterson based - with such data (where anomalous signal > would die at even lower resolution) at solving the substructure. In general you need better data for finding the HA substructure this way then to solve it (where 'solve' can mean a lot of things, e.g. breaking the phase ambiguity and getting some meaningful map). > In another case, they confirmed the correct selenium sites with a > platinum derivative data to further proceed with phasing. To be > honest, I can't ever get shelx to find my platinums with 6 A data :) How do you know SHELXD hasn't found them? In my experience at this kind of resolution you have to be careful to trust the usual criteria for a good solution (CCall > 40% etc). Maybe looking at good old fashioned anomalous Patterson Harker sections? Also: I've never had good experiences with Pt derivatives ... low occupancy, VERY high B-values and generally a pain. I've seen people using them very successfully though. As Jim said: it's all about signal and noise. If you have lousy crystals (large noise) you need to go for a large signal: one of those Ta/W clusters maybe - and bootstrapping your way to the other derivatives from there? Depending on the quality of your crystals (and data collection) a weak Pt derivative might not be enough. > Overall, it appears that with such data, there are too many places for > failure. Yes, such data isn't forgiving - but that makes success even sweater. Remember that what we now see as a straightforward and nearly trivial project was one of those 'really difficult structures' only a few years back (and people did solve those structures). Cheers Clemens -- *************************************************************** * Clemens Vonrhein, Ph.D. vonrhein AT GlobalPhasing DOT com * * Global Phasing Ltd. * Sheraton House, Castle Park * Cambridge CB3 0AX, UK *-------------------------------------------------------------- * BUSTER Development Group (http://www.globalphasing.com) ***************************************************************
