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The fusion domain of the influenza coat protein hemagglutinin HA2, bound to dodecyl phosphocholine micelles, was recently shown to adopt a structure consisting of two antiparallel α-helices, packed in an exceptionally tight hairpin configuration. Four interhelical H(α) to C═O aliphatic H-bonds were identified as factors stabilizing this fold. Here, we report evidence for an additional stabilizing force: a strong charge-dipole interaction between the N-terminal Gly(1) amino group and the dipole moment of helix 2. pH titration of the amino-terminal (15)N resonance, using a methylene-TROSY-based 3D NMR experiment, and observation of Gly(1 13)C' show a strongly elevated pK = 8.8, considerably higher than expected for an N-terminal amino group in a lipophilic environment. Chemical shifts of three C-terminal carbonyl carbons of helix 2 titrate with the protonation state of Gly(1)-N, indicative of a close proximity between the N-terminal amino group and the axis of helix 2, providing an optimal charge-dipole stabilization of the antiparallel hairpin fold. pK values of the side-chain carboxylate groups of Glu(11) and Asp(19) are higher by about 1 and 0.5 unit, respectively, than commonly seen for solvent-exposed side chains in water-soluble proteins, indicative of dielectric constants of ε = ∼30 (Glu(11)) and ∼60 (Asp(19)), placing these groups in the headgroup region of the phospholipid micelle.

journal of the american chemical society

Helical Hairpin Structure of Influenza Hemagglutinin Fusion Peptide Stabilized by Charge−Dipole Interactions between the N-Terminal Amino Group and the Second Helix