title: Aromatic interactions in tryptophan-containing peptides: crystal structures of model tryptophan peptides and phenylalanine analogs.
abstract: 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.
authors.affiliation: Department of Physics, Indian Institute of Science, Bangalore 560 012, Karnataka, India.
authors.forename: A
authors.initials: A
authors.lastname: Sengupta
authors.forename: R
authors.initials: R
authors.lastname: Mahalakshmi
authors.forename: N
authors.initials: N
authors.lastname: Shamala
authors.forename: P
authors.initials: P
authors.lastname: Balaram
mesh_terms: D003460:Crystallization; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D008958:Models, Molecular; D010455:Peptides / Q000737:chemistry*; D010649:Phenylalanine / Q000737:chemistry*; D011487:Protein Conformation; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D010455:Peptides; D010649:Phenylalanine; D014364:Tryptophan