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Hairpins from a,o-Peptide Hybrids

New hairpin type motifs have been generated by combining turn and strand segments made of distinct backbones (Fig. 2.18).

D-Pro-Xaa and Asn-Gly dipeptide sequences are known to support the formation of type I' or II' b-turns and to promote nucleation of a-peptide hairpin structures [176-178]. By extension, the D-Pro-Gly turn motif [179] or a related depsipeptide segment l-Pro-glycolate [158] have been used to connect b-peptide

Fig. 2.18 (A) Hybrid b-peptide 35 with a D-Pro-Gly type II ' b-turn segment (grey color): X-ray crystal structure [179]. Intermolecular NH - O=C H bonds (with N-H-O) angles of 160° and 133°) connect the hairpin into an infinitely extended b-sheet. (B, C) Parallel hairpin formation in b- and g-peptides (e.g. 36, 37) by incorporation of a D-Pro-DADME turn segment [158, 183].

Fig. 2.18 (A) Hybrid b-peptide 35 with a D-Pro-Gly type II ' b-turn segment (grey color): X-ray crystal structure [179]. Intermolecular NH - O=C H bonds (with N-H-O) angles of 160° and 133°) connect the hairpin into an infinitely extended b-sheet. (B, C) Parallel hairpin formation in b- and g-peptides (e.g. 36, 37) by incorporation of a D-Pro-DADME turn segment [158, 183].

strands into new antiparallel hairpin type structures (e.g. 35). X-ray diffraction studies have shown that b-amino acids residues generally adopt the expected anti-periplanar conformation around the ethylene bond (9 @ 180°). The same holds true for b-hairpins obtained by incorporation of b- or g-amino acids at discrete positions into the strand segments (91 and 92 values @ 180°) [180-182]. Specific connectors made of several residue types such as d-Pro-(1,1-dimethyl)-1,2-diaminoethyl (d-Pro-DADME) have been reported that allow formation of parallel hairpin type structures from b- (e.g. 36) and g-peptides (made of constrained trans-3-ACPC residues, e.g. 37) strand segments [158, 183]. Conversely, studies by the groups of Gellman and Yang have shown that b-peptides (dinipecotic acid heterochiral sequence, see Section 2.5) [184] and a-aminoxy-peptides (d,l-a-aminoxy acid dimer) [116] reverse turn segments can be used to nucleate a-peptide hairpin structures.

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