Central to protein architecture is the local arrangement or secondary structure of the polypeptide backbone. Thirty to forty percent of protein domains are alpha-helices with 3.6 residues per turn. pi-Helices, in which the peptide chain is more loosely coiled (4.4 residues per turn), have also been proposed. However, such structures necessitate an energetically unfavorable similar to 1 angstrom central helical hole. We show that rather than being composed of idealized pi-helices, helical regions formed from putative pi-helices actually consist of a series of concatenated wide turns with unique elliptical configurations. These structures have a larger helical radius akin to that of a pi-helix, but without the loss of favorable cross-core van der Waals interactions. This not only obviates the helical void, but also endows proteins with important functionalities, including metal ion coordination, enhanced flexibility and specific enzyme-substrate binding interactions. Crown Copyright (C) 2010 Published by Elsevier Inc. All rights reserved.