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Helical by nature: Urea-based peptidomimetics with proteinogenic side chains are fully helical in the crystalline state (see picture). Four acyclic residues are sufficient to drive complete helix formation with all complementary H-bonding sites being satisfied (up to 14 for a 8-mer). Helices pack head-to-tail to create infinite H-bonded networks with different topologies.
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Fischer et al. (2009) studied this question.
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