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Choice of Strand Residues

In designed b-hairpins, a variety of residues can be accommodated in the strand segments. The b-branched residues (Val, Leu, Ile) [33] and bulky nonpolar residues (Phe, Tyr, Trp) [10, 34, 38] have been successfully used in a number of cases. Positioning of aromatic residues in the non-hydrogen-bonding position confers additional stability to the hairpin structures, because the aromatic-aromatic inter

Fig. 5.13 (a) Schematic diagram ofb-hairpins with three residue loops; (b) NMR derived structure of peptide Boc-Leu-Phe-Val-DPro-LPro-DAla-Leu-Phe-Val-OMe in CDCl3 [34].

Fig. 5.13 (a) Schematic diagram ofb-hairpins with three residue loops; (b) NMR derived structure of peptide Boc-Leu-Phe-Val-DPro-LPro-DAla-Leu-Phe-Val-OMe in CDCl3 [34].

actions are weak [10]. In aqueous solution, Trp-Trp interactions can be used to promote residue clustering and hairpin formation as illustrated by extensive studies on Trp zipper peptides [10a]. Cross-strand interactions also assume additional importance in the case of multi-stranded b-sheet structures, where internal strands interact with the two flanking strands. Nonpolar amino acids like omega amino acids are readily accommodated into the strand sequences of b-hairpins [32d-g]. An interesting residue, which has been used in strand segments, is m-aminobenzoic acid. The positioning of amino and carboxyl groups onto a rigid aromatic scaffold has been exploited in incorporating the residue into strand segments of synthetic b-hairpins [41].

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