Structure determination of tubular crystals of membrane proteins.: I. Indexing of diffraction patterns

Indexing of diffraction patterns is the starting point of every crystallographic analysis. The diffraction patterns from helical particles consist of a series of layer-planes, the indexing of which involves the assignment of the start numbers of the helices that contribute to the layer-planes. The indexing could be challenging if the diameter of the helical particle is large and if the conventional indexing methods based on selection rule is used. Presented here is a tutorial on how to index them using (h, k; n) indexing method, which is particularly useful for tubular crystals of membrane proteins.