χ1 (top) and χ2 (bottom) as a function of the resolution and refinement software employed for isoleucines in an α-helix (left) and in a β-strand (right). These χ1 and χ2 values correspond to section 6, which is the most populated section in all cases (more than half of all isoleucines fall in this section). REFMAC (Murshudov et al., 2011; red) and SHELXL (Sheldrick, 2008; light blue) behave similarly and so do the packages that explicitly use molecular dynamics-based force fields (X-PLOR, Brünger, 1992, dark blue; CNS, Brünger et al., 1998, green). PHENIX (Adams et al., 2010) results are in purple. Error bars represent the standard errors of the mean.