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.