Most beamlines have attenuators, so there's little reason to collect multiple sweeps. We always collect 360deg. Since it's a small molecule, and usually fairly large and robust, you can warm it up, nudge it in a different direction with a pin (we use sterile, disposable acupunture needle), and refreeze it in the cryostream. Then do a second sweep in a different orientation.
I recommend moving the beam energy to 15.5KeV or higher to compress the diffraction image. Collect with 5-10deg widths. We can typically get the detector to around 70-80mm. You need to get to 0.85A resolution or better for good, stable refinement, and Acta Cryst. requires that resolution for publication. Often you need the low-resolution data and data to better than 1A to help with the sigma2 relationships in direct methods. You see both primary and secondary extinction, and that extinction can be anisotropic, so the SWAT option in SHELX is most useful. Otherwise, the overall scale factor is off, typically overestimated by the strong low-resolution reflection intensities, with the result that the anisotropic Gaussian displacement parameters may become non-positive definate. Bernie On Wed, February 8, 2012 12:46 pm, Jens Kaiser wrote: > Giorgo, > We have done that routinely for quite some time now. We had problems > when using a normal CCD detector, where we had to collect two or three > sweeps to avoid overloads (see below). Since we have the PILATUS this is > not necessary anymore and the data behaves fine. "Problems" still > persisting are: we have only a single axis goniometer, which can lead to > low completeness in P1 and P-1. Highest energy (17keV) and closest > distance (188mm) at our beamline have many SM crystals (even the ones > that "don't diffract" in house -- that is a 300 or 500 u sealed tube) > with an I/sig of 5-10 at the edge of the detector. Crunch, Acorn, > ShelxCDE and ShelxS don't have any problem with any of the data we > collected to <0.9A resolution. The multipass caused some inexplicable > non definite positives during refinement. We haven't tracked that down > systematically, so it might just have happened haphazardly. > > HTH, > > Jens > > On Wed, 2012-02-08 at 11:41 +0000, Giorgio Giardina wrote: >> Hello, >> I have some interesting small molecule xtals. >> I was wondering if it is possible to collect a small molecule data-set >> using a sincrotron macromolecular xtallography beam line, maybe with a >> very low beam intensity and moving the detector as close as possible? >> Has anybody experienced that? >> And if I get the images back home, can I process them using standard >> macromolecular software or do I need ab-initio special programs? >> Will MR work for phasing? >> >> Thanks in advance, >> Giorgio > > -- Bernard D. Santarsiero Research Professor Center for Pharmaceutical Biotechnology and the Department of Medicinal Chemistry and Pharmacognosy Center for Structural Biology Center for Clinical and Translational Science University of Illinois at Chicago MC870 3070MBRB 900 South Ashland Avenue Chicago, IL 60607-7173 USA (312) 413-0339 (office) (312) 413-9303 (FAX) http://www.uic.edu/labs/bds
