[ccp4bb]: Problems with low mosaicity crystals

[<yxiong@chemistry.ohio-state.edu>]

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Dear all, I would like to know whether anyone has the experience with low 
mosaicity crystals that give troublesome data sets (unreasonably high Rsym). 
I have seen quite a few cases and this happened again in our lab recently. 
The crystals diffracted to resolutions ranging from ~3A to ~0.6A, data 
collected at various beamlines in APS and CHESS with wavelength from ~1A to 
~0.62A. In each case the diffraction pattern looked real clean with sharp and 
round spots. Seeing the diffraction image one would expect very good data. 
However, The final Rsym's are very high, starting from the lowest resolution 
bin (>10%). This problem remains even with reprocessing the data in P1 
(Fridiel pairs don't match!). These are regular protein/RNA crystals. A 
common character of them is that they all have low mosaicity (<0.3). 
Different crystals of the same sample, with larger mosaicity would give 
better statistics although the diffraction looked not as good (collected at 
the same beamline on the same trip).

People at APS beamline 19 pointed out the vibration of the loop (hence the 
crystal) in the cold stream could be the reason for this.  By aligning the 
pin and loop to the parallel direction of the cold stream (less vibration) 
this problem is reduced.

Too much description of the problem, here is the question: how the vibration 
of the loop causes this problem? Strictly speaking, all the loops we are 
using are vibrating in the cold stream, and, compared to the cell constants 
the vibration are *large* in any case. The translational component of the 
vibration therefore has no effect. The fact this only happens to low 
mosaicity crystals suggests the angular movement of the vibration might be 
the culprit. I would like to see others' experience and understanding of this 
problem. And, giving data collected this way, is there a remedy to apply some 
correction?

Many thanks!
Yong