The average side-chain torsion angles of isoleucine as a function of resolution, secondary structure and backbone dihedrals. Error bars represent the standard errors of the mean. The nine panels in (a) show χ1 values. The nine panels in (b) show χ2 values. In both (a) and (b), the panels are placed in the same order as in Fig. 1. Dark blue and red are for isoleucine in an α-helix or β-strand, respectively, that have the corresponding backbone φ, ψ angles. The lighter coloured blue, red and green data are isoleucines that are located in a loop according to DSSP, but that have φ, ψ angles as in an α-helix, β-strand or neither, respectively. Isoleucines without missing atoms were taken from virtually every suitable X-ray structure in the PDB to obtain acceptable counting statistics. The resolution bin size was 0.2 Å, so that each dot represents the mean of at least 50 data points.