Characterization of a type of β‐bend ribbon spiral generated by the repeating (Xaa–Yaa–Aib–Pro) motif: The solution structure of harzianin HC IX, a 14‐residue peptaibol forming voltage‐dependent ion channels

The three‐dimensional solution structure of harzianin HC IX, a peptaibol antibiotic isolated from the fungus Trichoderma harzianum , was determined using CD, homonuclear, and heteronuclear two‐dimensional nmr spectroscopy combined with molecular modeling. This 14‐residue peptide, Ac Aib 1 Asn 2 Leu 3 Aib 4 Pro 5 Ala 6 Ile 7 Aib 8 Pro 9 Iva 10 Leu 11 Aib 12 Pro 13 Leuol 14 (Aib, α‐aminoisobutyric acid; Iva, isovaline; Leuol, leucinol), is a main representative of a short‐sequence peptaibol class characterized by an acetylated N‐terminus, a C‐terminal amino alcohol, and the presence of three Aib‐ L ‐Pro motifs at positions 4–5, 8–9, and 12–13, separated by two dipeptide units. In spite of a lower number of residues, compared to the 18/20‐residue peptaibols such as alamethicin, harzianin HC IX exhibits remarkable membrane‐perturbing properties. It interacts with phospholipid bilayers, increasing their permeability and forming voltage‐gated ion channels through a mechanism slightly differing from that proposed for alamethicin. Sequence‐specific 1 H‐ and 13 C‐nmr assignments and conformational nmr parameters ( 3 J NHCαH coupling constants, quantitative nuclear Overhauser enhancement data, temperature coefficients of amide and carbonyl groups, NH–ND exchange rates) were obtained in methanol solution. Sixty structures were calculated based on 98 interproton distance restraints and 6 Φ dihedral angle restraints, using high temperature restrained molecular dynamics and energy minimization. Thirty‐seven out of the sixty generated structures were consistent with the nmr data and were convergent. The peptide backbone consists in a ribbon of overlapping β‐turns twisted into a continuous spiral from Asn 2 to Leuol 14 and forming a 26 Å long helix‐like structure. This structure is slightly amphipathic, with the three Aib–Pro motifs aligned on the less hydrophobic face of the spiral where the Asn 2 side chain is also present, while the more hydrophobic bulky side chains of leucines, isoleucine, isovaline, and leucinol are located on the concave side. The repetitive (Xaa–Yaa–Aib–Pro) tetrapeptide subunit, making up the peptide sequence, is characterized by four sets of (Φ,Ψ) torsional angles, with the following mean values: Φ i = −90°, Ψ i = −27°; Φ i +1 = −98°, Ψ i +1 = −17°; Φ i +2 = −49°, Ψ i +2 = −50°; Φ i +3 = −78°, Ψ i +3 = +3°. We term this particular structure, specifically occurring in the case of (Xaa–Yaa–Aib–Pro) n sequences, the (Xaa–Yaa–Aib–Pro)‐β‐bend ribbon spiral. It is stabilized by 4 → 1 intramolecular hydrogen bonds and differs from both the canonical 3 10 ‐helix made of a succession of type III β‐turns and from the β‐bend ribbon spiral that has been described in the case of (Aib–Pro)n peptide segments. © 1999 John Wiley & Sons, Inc. Biopoly 50: 71–85, 1999