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HW1 Key - Biological Sciences 102 Homework#1 Spring 2008...

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Biological Sciences 102 Spring, 2008 Homework #1 Prof R Holland Cheng 1 9 Learn the structures and the codes of the twenty amino acids. In Biochemical Calculations (Segel), go through example 1 13 (stating in page 18) concerning weak acid/base concepts. Work out the assigned practice problem #17, 24, 29, 32, 34, 36, 40, 45 and 48 (pages 92 – 93). Comments: o 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. o In #17 c, note that strong bases always win when added to weak acids. o 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. o In #29, the 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. o In #40, adding H + causes a change in 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. ** Use the following table of pK a ’s to answer problems 6 - 10. Compound pK a ’s Phosphate 2.12, 7.21, 12.32 Tris 8.21 Amino acid side chains C 8.3 D 3.9 E 4.2 H 6.0 K 10.5 R 12.5 Y 10.1 Amino acid α -carboxyl group 2.1 Amino acid α -amino group 9.6 Oligopeptide N-terminal amino group 7.4 Oligopeptide C-terminal carboxyl group 3.6 1.
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