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': 'Which one of the following statement is NOT TRUE for an a-helix structure of a protein? \nSelect and explain. \nA. It is one of the most impoant secondary structure, B. It has a net dipole moment, C. All hydrogen bonds are aligned in the same direction, D. Long stretches of left handed a-helices occur in proteins', 'role': 'user'}, {'content': 'Answer: D \nExplanation: The hydrogen bonds in secondary structure of proteins are hydrogen bonds and disulfide bonds. The hydrogen bonds either form alpha helix or beta pleated sheet structure. Alpha helix can be either right handed or left handed. Left handed alpha helix is less stable than right handed alpha helix. Only right handed alpha helix has been found in protein structure. Beta pleated sheet structure is formed when hydrogen bonds are formed between the carbonyl oxygen and amide hydrogen of two or more adjacent extended polypeptide chains. Thus hydrogen bonding in beta pleated sheet structure is interchain. The structure is not absolutely planar due to the angles of bonds. Ref: Textbook of Medical Biochemistry By Chatterjea page 90.', 'role': 'assistant'}]