<prompt> The a-helix of proteins is:A pleated structureMade periodic by disulphide bridgesA non-periodic structureStabilised by hydrogen bonds between NH and CO groups of the main chain  <response>: The alpha helix is the most common and stable conformation for a polypeptide chain. In proteins like hemoglobin and myoglobin, the alpha helix is abundant, whereas it is virtually absent in chymotrypsin. The structure is stabilized by hydrogen bonds between NH and CO groups of the main chain.Reference: Vasudevan 7th ed, pg 37 