RELATIONSHIP BETWEEN AMIDE PROTON CHEMICAL-SHIFTS AND HYDROGEN-BONDING IN AMPHIPATHIC ALPHA-HELICAL PEPTIDES

The relationship between amide proton chemical shift and hydrogen bond length has been investigated for a designed amphipathic alpha-helical peptide (Ac-Glu-Leu-Glu-Lys-Leu-Leu-Lys-Glu-Leu-Glu-Lys-Leu-Leu-Lys-Glu-Leu-Glu-Lys-NH2). Four important results were obtained. (1) The secondary chemical shifts (DELTA-delta(HN)) changed periodically in a 3-4 repeat pattern along the peptide chain except for the first three residues at the N-terminal end of the peptide. The amide proton exhibits a large positive chemical shift in the center of the hydrophobic face and a large negative chemical shift in the center of the hydrophilic face. For the amide protons that are hydrogen-bonded between residues in the hydrophobic and hydrophilic faces, their chemical shifts are close to the random-coil values. (2) The DELTA-delta(HN) values at positions 9 and 13 are correlated with the decrease in the hydrophobicity of the side chain substituted at position 9 (Leu, Ala, and Gly). (3) The amplitude of DELTA-delta(HN) is dependent upon the amphipathicity of the peptide; that is, a peptide with a higher hydrophobic moment has a greater amplitude of DELTA-delta(HN). (4) The hydrogen-bond lengths calculated from amide chemical shifts are similar to those measured from computer-modeling experiments of a curved alpha-helix. These results indicate that the amide proton chemical shift is related to hydrogen-bond length and that a single-stranded amphipathic alpha-helix in solution is curved and this curved structure causes the upfield shifts of amide protons in the convex side and downfield shifts of amide protons in the concave side.