Experiment 4: Titration Curve of Amino Acids Prelab Questions 1. ...

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Experiment 4: Titration Curve of Amino Acids Prelab Questions 1. Draw the titration curve of an amino acid having only two ionizable groups (e.g., glycine). Indicate on the curve the pKa values of the -COOH (pKai) and a-NH3 (pKa2) groups, and the pl of the amino acid. 2. The prevailing structure of the molecules in the 0.2 M amino acid solution used in today's experiment before titrating with 1 M HCl or 1 M NaOH is H3N-CH-COO CH2 R What is the structure produced after the amino acid is titrated with, i) 1 MHCI? ii) 1M NaOH? 3. Why must the magnetic stirrer be stopped each time before reading the pH? 3. Consider the following amino acid for questions 4a-4d. - + H1N-CH-COO` CH2 CH2 I CH2 I NH - "NH2 + NH2 a. Is this a diprotic or triprotic amino acid? b. Draw the titration curve if this molecule were titrated with 1M NaOH. 5. Consider the amino acid D having pKa's of 1.99 (pKai), 3.90 (pKaR), and 9.90 (pKa2). What is its pl? Show work. 6. At what pH are the anion and zwitterion species of equal concentration for an amino acid having no ionizable group in the side chain? 7. Why is the amino acid Y, which has three ionizable groups 2.20 (pKai), 9.21 (pKa2), and 10.5 (pKaR), considered a simple amino acid? 8. At what pH will the amino acid containing a negative group in the side chain not migrate in an electric field? 9. Consider the amino acid H (pKar = 6.00) for a - c below. a. What is its predominant ionic form at pH 7.40? b. Draw the structure of the conjugate base/weak acid pair that exists in solution at pH 7.00. c. What is the ratio of conjugate base to weak acid in an H buffer if the pH drops to one unit below pKaR? 10. If one is given an amino acid such as lysine to titrate and construct a titration curve, one might observe two, not three, buffer regions. Why? 11. The initial pH of a 0.2M arginine solution is 14. a. What is the predominant structure of arginine in solution at this pH? b. Draw the titration curve that would result if this solution were titrated with 1M HCI to pH 7. c. Where would the pKa values appear on this curve? d. What group is ionizing at each pKa?

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