Accuracy and resolution in small-angle crystallographic analyses. A comparison with solution scattering studies

This paper is an extension of the preceding one and deals with the problem of assessing the potential accuracy of low-resolution crystallographic analyses restricted to the same angular range as solution scattering studies. The mathematical treatment is very similar in the two papers; similarly, the final result takes the form of a curve relating the normalized variances of the positional and chromatic structure parameters. In the absence of `small-angle' data - these are utterly disregarded in routine protein crystallographic analyses - an ideal experiment performed on a low-density serum lipoprotein, under conditions similar to those that prevailed in the experimental study of methionyl t-RNA synthetase, is used as an illustration. The potential accuracy estimated for this putative crystallographic analysis is found to be quite close to that assessed for the solution scattering experiment. In the case of protein crystallographic analyses, the projection information associated with the `small-angle' reflections is estimated and it is shown that it amounts to a substantial fraction of the whole of the experimental information. More precisely, the average information per reflection is almost 40 times larger for the `small-angle' than for the other reflections. The conclusion is reached that 'small-angle' reflections should be involved in protein crystallographic analyses and it is suggested that in the early stage of the phasing process `small-angle' crystallographic data and solution scattering experiments be combined together to locate and orient the molecule(s) in the cell and to draw the molecular envelope.