The a-helix of proteins is:
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 C=O groups of the main chain.
Reference: Vasudevan 7th  ed, pg 37
A pleated structure
Made periodic by disulphide bridges
A non-periodic structure
Stabilised by hydrogen bonds between NH and CO groups of the main chain