Fig. 8. Sequence distributions for secondary structure. The four graphs show alignment statistics for (A) a-helices, (B) 310-helices, (C) b-sheets and (D) b-bridges by the Kullback-Leibler information at positions surrounding the one assigned (position 0). The number of aligned segments are: (A) 41803, (B) 4952, (C) 27320, (D) 1851. These segments were retrieved from a data set of 707 non-homologous protein chains using the DSSP assignment. At a given position, we therefore observed the 20 amino acids with a certain frequency; the Kullback-Leibler information calculates the information content of the observed frequencies with respect to the background frequencies (irrespectively of the structure). The more an observed set of frequencies differs from the background, the high the respective letter. If an amino acid at a given position is observed less frequently than in the background, it is drawn upside-down and hollow.