Tryptophan-derived peptides. 1: Crystal structure and solution conformation of Boc-Gly-Trp-Ala-OtBu

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

Two 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 intramolecularly hydrogen-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 Nepsilon1H 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 Czeta3H of Trp(2). Concentration dependence of NMR chemical shifts provides evidence for peptide association in solution involving the Trp(2) Nepsilon1H protons, presumably in a manner similar to that observed in the crystal.

Aromatic interactions in tryptophan-containing peptides: crystal structures of model tryptophan peptides and phenylalanine analogs

The crystal structures of the peptides, Boc-Leu-Trp-Val-OMe (1), Ac-Leu-Trp-Val-OMe (2a and 2b), Boc-Leu-Phe-Val-OMe (3), Ac-Leu-Phe-Val-OMe (4), and Boc-Ala-Aib-Leu-Trp-Val-OMe (5) have been determined by X-ray diffraction in order to explore the nature of interactions between aromatic rings, specifically the indole side chain of Trp residues. Peptide 1 adopts a type I beta-turn conformation stabilized by an intramolecular 4-->1 hydrogen bond. Molecules of 1 pack into helical columns stabilized by two intermolecular hydrogen bonds, Leu(1)NH...O(2)Trp(2) and IndoleNH...O(1)Leu(1). The superhelical columns further pack into the tetragonal space group P4(3) by means of a continuous network of indole-indole interactions. Peptide 2 crystallizes in two polymorphic forms, P2(1) (2a) and P2(1)2(1)2(1) (2b). In both forms, the peptide backbone is extended, with antiparallel beta-sheet association being observed in crystals. Extended strand conformations and antiparallel beta-sheet formation are also observed in the Phe-containing analogs, Boc-Leu-Phe-Val-OMe (3) and Ac-Leu-Phe-Val-OMe (4). Peptide 5 forms a short stretch of 3(10)-helix. Analysis of aromatic-aromatic and aromatic-amide interactions in the structures of peptides, 1, 2a, 2b are reported along with the examples of 14 Trp-containing peptides from the Cambridge Crystallographic Database. The results suggest that there is no dramatic preference for a preferred orientation of two proximal indole rings. In Trp-containing peptides specific orientations of the indole ring, with respect to the preceding and succeeding peptide units, appear to be preferred in beta-turns and extended structures.