The high propensity of Asn residue to adopt aL conformation suggests that Asn-Gly sequences may be used to nucleate type I ' b-turn structures, which in turn can promote b-hairpin formation. Approaches to water-soluble peptide b-hairpins have used the Asn-Gly segment, as the key element in the design strategies [7-9] to promote folding. An analysis by Gellman and coworkers suggests that the DPro-Gly segment is a stronger inducer for b-hairpin formation than Asn-Gly segment [9a].
Fig. 5.11 Molecular conformations observed in crystals of peptides containing b- and g-residues in the strand segments of b-hairpins: (a) Boc-Leu-Val-b3Phe-Val-DPro-Gly-Leu-b3Phe-Val-Val-OMe (DPro-Gly; type I' b-turn) [32d]; (b) Boc-b3Phe-b3Phe-DPro-Gly-b3Phe-b3Phe-OMe (DPro-Gly; type II' b-turn) [32e]; (c) Boc-Leu-Val-g-Abu-Val-DPro-Gly-Leu-g-Abu-Val-Val-OMe (DPro-Gly; type I' b-turn) [32f].
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