β-Hairpins, triple-strand β-sheets and βαβ-units represent simple structural motifs closed into cycles by systems of hydrogen bonds. Secondary closing of these simple motifs into large cycles by means of different superhelices, split β-hairpins or SS-bridges results in the formation of more complex structural motifs having unique overall folds and unique handedness such as abcd-units, φ-motifs, five- and seven-segment α/β-motifs. Apparently, the complex structural motifs are more cooperative and stable and this may be one of the main reasons of high frequencies of occurrence of the motifs in proteins. © 2010 Elsevier Inc.
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