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computer programs
An anisotropic scaling program, ASLSQ, to scale two sets of three-dimensional single-crystal X-ray diffraction data has been developed. The data were divided into blocks according to the spatial location of the reflections. The ratios between the sums of the structure factors in each block were then calculated. The spatial variation of these ratios was modeled by a set of spherical harmonics. The radial component of the variation in the diffracted intensity was modeled by an exponential function. A least-squares minimization procedure was used to calculate the best-fit ratio between the corresponding data blocks. This program has been used to calculate the scale factors between the native data of E. coli aspartate transcarbamoylase (ATCase) wild type and the native data of one of its mutants, ATCase 231. Several sets of data were artificially made to test the program. The program ASLSQ fits better at large deviations for various spherical coordinates in reciprocal space than does a polynomial-fit program.