# Problem Set

GENERAL BIOCHEMISTRY Problem Set No. 1 Answered by: Andrea Rose A. Fajardo of BSFT 2-1N 1. Calculate the pH of a dilute solution that contains a molar ratio of potassium acetate to acetic acid (pKa=4. 76) of: a. 2:1 pH = 4. 76 + log[2]/[1] = 4. 76 + 0. 3010 = 5. 06 b. 1:4 pH = 4. 76 + log[1]/[4] = 4. 76 + (-0. 6021) = 4. 16 c. 6:5 pH = 4. 76 + log[6]/[5] = 4. 76 + 0. 0792 = 4. 81 d. 2:3 pH = 4. 76 + log[2]/[3] = 4. 76 + (-0. 1761) = 4. 58 e. 9:1 pH = 4. 76 + log[9]/[1] = 4. 76 + 0. 9542 = 5. 71 2. A buffer contains 0. 01mol of lactic acid (pKa=3. 86) and 0. 05mol of sodium lactate per liter. a. What is the resulting pH of the buffer? H = 3. 86 + log[0. 05]/[0. 01] = 3. 86 + 0. 6990 = 4. 56 b. Calculate the change in pH when 5mL of 0. 5M HCl is added to 1L of buffer. 5mL of 0. 5 mol/L 5mL * 1L/1000mL = 0. 005L 0. 005L * 0. 5 mol/L = 0. 0025mol NaC3H5O3 = 0. 05mol – 0. 0025mol = 0. 0475 C3H5O3 = 0. 01mol – 0. 0025mol =0. 0075 pH = 3. 86 + log[0. 0475]/[0. 0075] = 3. 86 + 0. 8016 = 4. 66 • 4. 55-4. 66 = 0. 11 is the change in pH c. What pH change would you expect if you added the same quantity of HCl to 1L of pure water? [H+] = (0. 0025mol)(1L) = 0. 0025M pH = -log 0. 0025 = 2. 60 3. The structure of the amino acid of isoleucine is given at the right. a.

How many chiral centers does it have? 2 b. How many optic isomers? 4 c. Draw the perspective formulas for all the optical isomers of isoleucine. [pic] 4. A peptide has the sequence: Glu-His-Trp-Ser-Leu-Arg-Pro-Gly a. What is the charge of the molecules at pH 4 and 11? (Use pKa values of those in the ppt) |pH |NH3+ |Glu |His |Arg |COO- |Net Charge | |4 |+1 |-1 |0 |+1 |-1 |0 | |11 |0 |-1 |0 |+1 |-1 |-1 | . What is the pI for the peptide? | |pKaCOOH |pKaNH3 |pKaR | |Glu |2. 19 |9. 67 |4. 25 | |His |1. 82 |9. 17 |6. 04 | |Trp |2. 83 |9. 39 | | |Ser |2. 1 |9. 15 | | |Gly |2. 34 |9. 60 | | |Leu |2. 63 |9. 60 | | |Arg |2. 17 |9. 04 |12. 48 | |Pro |1. 99 |10. 60 | | |Gly |2. 4 |9. 60 | | 2. 34 6. 04 9. 67 12. 48 pI = 6. 04 + 9. 67 / 2 = 7. 85 5. Histidine has three ionisable functional groups. Write the equilibrium equations for its three ionizations and assign the proper pKa for each ionization. Draw the structure of histidine in each ionization state. What is the net charge of the histidine molecules in each ionization state? STRUCTURE OF IONIZATION IN EACH IONIZATION STATE [pic] NET CHARGE Structure |Net Charge | |+H3N–CH–(RH+)–COOH |+2 | |+H3N–CH–(RH+)–COO- |+1 | |+H3N–CH–(R)–COO- |0 | |H2N–CH–(R)–COO- |-1 | ———————– -2 +1 +2 -1 0

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