Identification of N-terminal helix capping boxes by means of 13C chemical shifts

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Abstract
We have examined the 13Cα and 13Cβ chemical shifts of a number of proteins and found that their values at the N-terminal end of a helix provide a good predictor for the presence of a capping box. A capping box consists of a hydrogen-bonded cycle of four amino acids in which the side chain of the N-cap residue forms a hydrogen bond with the backbone amide of the N3 residue, whose side chain in turn may accept a hydrogen bond from the amide of the N-cap residue. The N-cap residue exhibits characteristic values for its backbone torsion angles, with ϕ and ψ clustering around 94±15° and 167±5°, respectively. This is manifested by a 1–2 ppm upfield shift of the 13Cα resonance and a 1–4 ppm downfield shift of the 13Cβ resonance, relative to their random coil values, and is mainly associated with the unusually large value of ψ. The residues following the N-cap residue exhibit downfield shifts of 1–3 ppm for the 13Cα resonances and small upfield shifts for the 13Cβ ones, typical of an α-helix.