THE USE OF PARTIAL REFLECTIONS FOR SCALING AND AVERAGING X-RAY AREA DETECTOR DATA

Frozen crystals of proteins, nucleic acids or other biological macromo lecules often have mosaic spreads comparable to the maximum useful osc illation angles. It is, therefore, necessary to develop scaling method s that are independent of the exclusive use of full reflections. The H amilton, Rollett and Sparks [Acta Cryst (1965), 18, 129-130] procedure for scaling frames of X-ray area-detector data has been generalized t o utilize partial reflections by adding intensities of partial reflect ions from consecutive frames (method 1) or by correcting intensities o f partial reflections, using a model to calculate the reflection parti ality (method 2). Both methods have been applied to scaling and averag ing of data-sets collected on crystals of biological macromolecules. T he agreement factors of the scaled data are better when using method 1 , although it often fails when there are rotation gaps between success ive images or when the data redundancy is low. Method 2 is more stable and versatile, allowing scaling of data-sets with incompletely measur ed reflections and low redundancy. The major drawback of method 2 is i ts sensitivity to inaccuracy in calculated partiality. The actual valu es of the scale factors obtained with the two methods are within 5%. H owever, when the true value of the scale factor changes dramatically b etween consecutive frames (e.g. due to beam dumps and refills at a syn chrotron source), the results of the two methods can differ by as much as 15% because method 1 produces physically wrong results. The scalin g algorithm implemented in the commercially available program SCALEPAC K is vulnerable to the same problems as method 1.