FIG. 12-6 (a) The alpha helix form of a protein molecule. Each amino acid unit in the helix is linked by hydrogen bonds (a type of van der Waals bond) to other units above and below it. Most proteins have molecules like this. (b) The pleated sheet form of a protein molecule. Two or more chains of amino acid units are linked side-to-side along the sheet by hydrogen bonds. Fibrous proteins–such as those in hair, silk, cartilage, and horn–are of this kind. Some proteins have still other forms.