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

Sengupta, A and Mahalakshmi, R and Shamala, N and Balaram, P (2005) Aromatic interactions in tryptophan-containing peptides: crystal structures of model tryptophan peptides and phenylalanine analogs. In: The Journal of Peptide Research, 65 (1). pp. 113-129.

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## 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 $P2_12_12_1$ (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 $3_1_0$-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.

Item Type: Journal Article The Copyright belongs to Blackwell. aromatic interactions;crystal structures;peptide conformations;superhelical structure;tryptophan peptides Division of Biological Sciences > Molecular Biophysics UnitDivision of Physical & Mathematical Sciences > Physics 27 Feb 2006 19 Sep 2010 04:24 http://eprints.iisc.ernet.in/id/eprint/5668