The (κ, α) plot and the distribution of the 23-state structural alphabet. (a) The typical (κ, α) plots of an all-α protein (Protein Data Bank [PDB] code 1J41-A; red) and an all-β protein (PDB code 1RZF-L; blue). (b) The distribution of accumulated (κ, α) plot of 225,523 segments derived from the pair database with 1,348 proteins. This plot, which comprises 648 cells (36 × 18), is clustered into 23 groups, and each cell is assigned a structure letter. (c) The average intrasegment (blue) and intersegment root mean square deviation (rmsd) values of the 23-state structural alphabet.