Hydrogen-bond lengths in polypeptide helices: No evidence for short hydrogen bonds

The long and the short of it: N⋯O distances less than 2.8 Å (see picture) have been implicated in determining the 220 nm circular dichroism band intensity in helical peptides. An analysis of helical peptide crystal structures, however, reveals the average experimentally determined hydrogen-bond lengths are 2.978-3.113 Å (α helix) and 2.907-3.211 Å (3 10 helix).