Section 13.4.9. Combining different crystal forms

Frequently, a molecule crystallizes in a variety of different crystal forms [e.g. hexokinase (Fletterick & Steitz, 1976), the influenza virus neuraminidase spike (Varghese et al., 1983), the histocompatibility antigen HLA (Bjorkman et al., 1987) and the CD4 receptor (Wang et al., 1990)]. It is then advantageous to average between the different crystal forms. This can be achieved by averaging each crystal form independently into a standard orientation in the h-cell (if the redundancy is for a given crystal form, then this simply amounts to producing a skewed representation of the p-cell in the h-cell environment). The different results, now all in the same h-cell orientation, can be averaged. However, care must be taken to put equal weight on each molecular copy. If the ith cell contains noncrystallographic copies, then the average of the densities, , is at each grid point, x, in the h-cell. Additional weights can be added to account for the subjective assessment of the quality of the electron densities in the different crystal cells.

With the h-cell density improved by averaging among different crystal forms, it can now be replaced into the different p-cells. These p-cells can then be back-transformed in the usual manner to obtain a better set of phases. These, in turn, can be associated with the observed structure amplitudes for each p-cell structure, and the cycle can be repeated.