Sequence determinants of the capping box, a stabilizing motif at the N‐termini of α‐helices

The capping box, a recurrent hydrogen bonded motif at the N‐termini of α‐helices, caps 2 of the initial 4 backbone amide hydrogen donors of the helix (Harper ET, Rose GD, 1993, Biochemistry 32:7605–7609). In detail, the side chain of the first helical residue forms a hydrogen bond with the backbone of the fourth helical residue and, reciprocally, the side chain of the fourth residue forms a hydrogen bond with the backbone of the first residue. We now enlarge the earlier definition of this motif to include an accompanying hydrophobic interaction between residues that bracket the capping box sequence on either side. The expanded box motif—in which 2 hydrogen bonds and a hydrophobic interaction are localized within 6 consecutive residues — resembles a glycine‐based capping motif found at helix C‐termini.