International
Tables for Crystallography Volume F Crystallography of biological macromolecules Edited by M. G. Rossmann and E. Arnold © International Union of Crystallography 2006 |
International Tables for Crystallography (2006). Vol. F. ch. 11.4, p. 226
Section 11.4.1. Introduction
a
UT Southwestern Medical Center at Dallas, 5323 Harry Hines Boulevard, Dallas, TX 75390-9038, USA, and bDepartment of Molecular Physiology and Biological Physics, University of Virginia, 1300 Jefferson Park Avenue, Charlottesville, VA 22908, USA |
X-ray diffraction data analysis, performed by the HKL package (Otwinowski, 1993; Otwinowski & Minor, 1997
) or similar programs (Rossmann, 1979
; Howard et al., 1985
; Blum et al., 1987
; Bricogne, 1987
; Howard et al., 1987
; Leslie, 1987
; Messerschmidt & Pflugrath, 1987
; Kabsch, 1988
; Higashi, 1990
; Sakabe, 1991
), is used to obtain the following results:
Other results, like indexing of the diffraction pattern, are in most cases only intermediate steps to achieve the above goals. The HKL system and other programs also have tools to validate the results by self-consistency checks.
The fundamental stages of data analysis are:
This order represents the natural flow of data reduction, but quite often these steps are repeated based on information obtained at a later stage.
The three basic questions in collecting diffraction data are:
These questions and steps (1)–(7)
of data analysis are intimately intertwined.
Data analysis makes specific assumptions which the collected data must, or at least should, satisfy. However, the experimenter can verify whether the data satisfy those assumptions only by data analysis. This circular logic can be broken by an iterative process. On-line data analysis provides immediate feedback during data collection and can remove the guesswork about whether, what and how from the process. The description of data analysis and algorithms that follows will make frequent references to the assumptions about the data and offer guidelines on how to make the experiment fulfil these assumptions.
This article uses the HKL package coordinate system to describe data algorithms and analysis. However, as most equations are written in vector notation, they can be easily adapted to conventions used in other programs.
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