Distribution of writhing numbers across protein structures. The distribution of writhing numbers from segments of all proteins in the PDB using a window size of 4, 5, 6 and 10. The histrogram was broken up into twenty regions of constant population (area under the curve). These 20 regions were used to define a topological alphabet. Notice that the range of writhing number increases with segment size.
Chang et al. BMC Bioinformatics 2006 7:346 doi:10.1186/1471-2105-7-346