Tryptophan-containing peptide helices: interactions involving the indole side chain

TTwo designed peptide sequences containing Trp residues at positions i and i + 5 ( Boc- Leu- Trp- Val- Ala- Aib- Leu- Trp- Val- OMe, 1) as well as i and i + 6 ( Boc- Leu- Trp- Val- Aib- Ala- Aib- Leu- Trp-Val-OMe, 2) containing one and two centrally positioned Aib residues,respectively, for helix nucleation, have been shown to form stable helices in chloroform solutions. Structures derived from nuclear magnetic resonance ( NMR) data reveal six and seven intramolecular lyhydrogen- bonded NH groups in peptides 1 and 2, respectively. The helical conformation of octapeptide 1 has also been established in the solid state by X- ray diffraction. The crystal structure reveals an interesting packing motif in which helical columns are stabilized by side chain - backbone hydrogen bonding involving the indole N epsilon 1H of Trp( 2) as donor, and an acceptor C=O group from Leu( 6) of a neighboring molecule. Helical columns also associate laterally, and strong interactions are observed between the Trp( 2) and Trp( 7)residues on neighboring molecules. The edge- to- face aromatic interactions between the indoles suggest a potential C- H. pi interaction involving the C zeta 3H of Trp( 2). Concentration dependence of NMR chemical shifts provides evidence for peptide association in solution involving the Trp( 2) N epsilon 1H protons,presumably in a manner similar to that observed in the crystal