Key HW 1 Fall, 2009

# Key HW 1 Fall, 2009 - 1 of 5 Biological Sciences 102 Fall...

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1 of 5 Biological Sciences 102 Answer Key Fall, 2009 K. Hilt Homework #1 Learn the structures and one-letter codes of the twenty amino acids. Assigned problems in Biochemical Calculations (Segel), concerning weak acid/base concepts: Example 1 13 on page 18 #17, 24, 25 (a d), 29, 31, 32, 34, 36, 38 (a, d, j), 40, and 41 on pages 92 and 93. See appendix IV of Biochemical Calculations for pK a values needed in some of the above questions. Comments: 1. Example 1-13 reminds us that if you add an extremely small amount of H + or OH to a container of water, the pH is determined mainly by the H + coming from water. 2. In #17 c, we are reminded that strong bases always win when added to weak acids. 3. In #24, “amine” means that the weak acid/base is fully unprotonated. The solution is at the top of the titration curve. Use K b = x 2 / y-x to solve. 4. In #25 b and c, these are intermediate ions. Use pH = (pK a1 + pK a2 )/ 2 to solve. The K + ions have no effect on the pH. 5. In #29, do not forget that addition of 0.06 moles of H + generates 0.06 moles of NH 4 + . We also have 0.02 moles of NH 3 remaining. Use the Henderson-Hasselbalch equation to solve. 6. In #40, addition of H + causes a decrease in the pH. You will need two Henderson-Hasselbalch equations to solve this. The first equation yields the [b] and [a] at time zero. The second equation essentially works backwards: the new H-H equation is pH = 7.21 + log {[b] – [x]}/{[a] + [x]}. We know [b] and [a] from the first equation and x = 0.004 M. For the questions below, use the following pK a values for the amino acids: -COOH (2.1), -NH 3 + (9.6); side chains---see the class handout on amino acid structures For oligopeptides, pK a values are: N-terminus (7.4), C-terminus (3.6) 1. Which amino acid has a net charge of “+ 0.09” at pH 7? Show all calculations. Draw the structure of the amino acid that would predominate at this pH. Histidine. To calculate the net charge of histidine at pH 7, we can deal with two of the groups, the α -carboxyl group and α -amino group, by inspection. Their charges are “-1” and “+1”, respectively. This is because their pKa’s are more than two units away from pH 7. After having worked with the Henderson-Hasselbalch equation enough, we will realize that whenever the pH and pKa differ by two or more units, the inverse log will be at least 100/1 in favor of either “a” or “b”. For the side chain imidazole group, whose pKa is 6.0, we need to use the Henderson-Hasselbalch equation and calculate the percentage of the population in the “a” form (why?). In drawing the structure of histidine at pH 7, the

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## This note was uploaded on 10/21/2009 for the course BIS 52192 taught by Professor Enochbaldwin during the Fall '09 term at UC Davis.

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Key HW 1 Fall, 2009 - 1 of 5 Biological Sciences 102 Fall...

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