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$\beta$-sheets

The most regular form of extended polypeptide chain seen in protein structures is the $\beta$-strand. $\beta$-strands are stabilised though interactions with other $\beta$-strands to form a $\beta$-sheet, which is more strictly a tertiary structure. Like helices, sheets too are stabilised by hydrogen bonds between CO and NH groups, but in this case they are much more distantly separated along the chain. Both parallel and anti-parallel $\beta$-sheets exist in protein structures. Due to the geometry of the peptide backbone, the amino acid side chains of $\beta$-strands alternate on either side of the sheet.