a–d, Overall topologies on the left and example HBNets on the right. a, A left-handed supercoiled backbone, with each monomer being helix hairpins. b, A backbone-permuted ‘3 + 1’ design; one monomer is a single helix and the other is a three-helix bundle. c, A left-handed supercoiled backbone, with each monomer being a three-helix bundle. d, A straight, untwisted backbone, with each monomer being a helix hairpin. e, Hydrogen-bond pairing in DNA bases. Top, A–T base pairing. Bottom, C–G base pairing. Green arrows point from hydrogen-bond donors to acceptors. f, Two examples of hydrogen-bond pairing in designed protein hydrogen-bond networks. g, Top-down view of antiparallel twisted (top) and parallel untwisted (bottom) backbones sampled in this study. h, Comparison of a designed protein heterodimer (right) with B-form DNA (left) on the same scale.