Secondary

• The chain of amino acids twists into a shape.
• $\alpha$-helix is a coil of 36 amino acids per 10 turns of the helix held together by hydrogen bonds between the -NH group of one amino acid and the -CO group of another 4 places ahead of it in the chain.
• $\beta$-pleated sheet folds over itself into a zig zag structure held together by hydrogen bonds between the -NH group of one amino acid and the -CO group of another further down the strand.
• The hydrogen bonds make both structures stable at optimal temperatures and pH.