This is very closely related to the way in which I would like to think about this: if you consider adding another thin shell of data, are you adding any significant information? Unfortunately as Garib Murshudov has pointed out, we don't have any reliable way of estimating the information content of data. (Also it should be considered anisotropically.)
Another way of thinking about this is to consider that if we had perfect error models and weighted the data perfectly, then adding a shell of data with essentially no useful information should at least do no harm (ie weights are close to zero). But we do not have perfect error models, so adding too much data may in the end degrade our structural model. Much of the problem arises from our addiction to R-factors as a measure of quality, when they are unweighted and therefore very misleading. We are are also too quick to judge the quality of a structure by its nominal "resolution", whatever that means. In the end, the important question as ever is "does the experimental data support the conclusions drawn from it?" and that will depend on local information about particular atoms and groups, not on global indicators Phil On 4 Mar 2011, at 10:35, John R Helliwell wrote: > Dear Roberto, > Overnight I recall an additional point:- > In chemical crystallography, where standard uncertainties are > routinely avaliable for the molecular model from the full matrix > inversion in the model refinement, it is of course possible to keep > extending your resolution until your bond distance and angles su > values go up. Thus if you distrust or do not wish to slavishly follow > a Journal's Notes for Authors, such as for Acta Crystallographica > Section C to which I referred yesterday, you can, in this way, check > yourself the good sense of the data quality criteria required. [This > is a similar test to the one that Phil mentioned yesterday ie with > respect to scrutinising electron density maps for your protein ie do > they show more detail by adding more diffraction data.] > Best wishes, > John > > On Thu, Mar 3, 2011 at 11:29 AM, Roberto Battistutta > <roberto.battistu...@unipd.it> wrote: >> Dear all, >> I got a reviewer comment that indicate the "need to refine the structures at >> an appropriate resolution (I/sigmaI of >3.0), and re-submit the revised >> coordinate files to the PDB for validation.". In the manuscript I present >> some crystal structures determined by molecular replacement using the same >> protein in a different space group as search model. Does anyone know the >> origin or the theoretical basis of this "I/sigmaI >3.0" rule for an >> appropriate resolution? >> Thanks, >> Bye, >> Roberto. >> >> >> Roberto Battistutta >> Associate Professor >> Department of Chemistry >> University of Padua >> via Marzolo 1, 35131 Padova - ITALY >> tel. +39.049.8275265/67 >> fax. +39.049.8275239 >> roberto.battistu...@unipd.it >> www.chimica.unipd.it/roberto.battistutta/ >> VIMM (Venetian Institute of Molecular Medicine) >> via Orus 2, 35129 Padova - ITALY >> tel. +39.049.7923236 >> fax +39.049.7923250 >> www.vimm.it >> > > > > -- > Professor John R Helliwell DSc
