This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
[{'content': 'The a-helix of proteins is: \nSelect and explain. \nA. A pleated structure, B. Made periodic by disulphide bridges, C. A non-periodic structure, D. Stabilised by hydrogen bonds between NH and CO groups of the main chain', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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\xa0C=O groups of the main chain.\nReference: Vasudevan 7th\xa0 ed, pg 37', 'role': 'assistant'}]