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': 'At isoelectric pH, proteins: \nSelect and explain. \nA. Have net charge ‘0’, B. Are positive charged, C. Are negative charged, D. None of the above', 'role': 'user'}, {'content': "Answer: A \nExplanation: Zwitterions\xa0 are\xa0 one\xa0 example\xa0 of\xa0 an\xa0 isoelectric\xa0 species—the\xa0form of a molecule that has an equal number of positive and\xa0negative charges and thus is electrically neutral. The isoelectric pH, also called the pI, is the pH midway between pKa\xa0\xa0values for the ionizations on either side of the isoelectric species.\nThe pKa\xa0\xa0values of all functional groups of an amino acid or of\xa0a peptide dictate its net charge at a given pH. pI, the isoelectric\xa0pH, is the pH at which an amino acid bears no net charge, and\xa0thus does not move in a direct current electrical field.\nHarper's Illustrated Biochemistry 30 th edition page no 20, 24", 'role': 'assistant'}]