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Some regions of a polypeptide may coil or fold back on themselves. this is called

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d. secondary structure … hydrogen bonds

Explanation:

The secondary structure of a polypeptide corresponds to local regions of an orderly structure stabilized by hydrogen bonds between the -NH and C = O backbone groups and there are no hydrogen bonds in which the side chains are involved. The secondary structure of the polypeptide is characterized by its ability to curl or fold, the folds being maintained by the polypeptide chains.