Exam 1 Key Fall 2007
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Chemistry 340 Exam 1 Key

Fall 2007

There are 24 questions in all. Questions 1-20 are multiple choice and are each worth 3 points. Questions 21-24 are essays and problems worth a total of 40 points.

Multiple Choice answers:

1. A

5. A

8. B

12. C

17. D

2. D

6. D

9. A

13. B

18. C

3. D

7. D

10. C

14. A

19. A

4. B

 

11. C

15. C

20. C

     

16. A

 

21-23. Use sequence M: histidine- glycine-glutamine-aspartate-isoleucine -leucine-isoleucine-arginine .

21. Fill in the rest of the table. For class use (N) nonpolar; (A) aromatic;
(P) polar uncharged; (+) positively charged; or (-) negatively charged.

name

histidine

glycine

glutamine

aspartate

isoleucine

leucine

arginine

α-COOH pKa

1.82

2.34

2.17

1.88

2.36

2.36

2.17

α-NH3+ pKa

9.16

9.60

9.13

9.60

9.68

9.60

9.04

side chain pKa

6.00

-

-

3.65

-

-

12.48

1-letter abbreviation

H

G

P

D

I

L

R

class

P/+

N

P

-

N

N

+

22. Draw the underlined residues in sequence M as a 4-residue peptide at pH 7.

      What is the pI for this peptide?    0.5 (2.36 + 3.65) = 3.005 = 3.01

23. M is the sequence for residues 24-31 in myoglobin.
a. Name two residues that would be connected by a hydrogen bond if this sequence formed an α-helix.

 histidine (α-C=O) and isoleucine (α-N-H), glycine and leucine, glutamine and isoleucine (the 2nd), aspartate and arginine:
all connected by backbone hydrogen bonds

       b. Name two other residues whose side chains could have a different weak interaction in an α-helix, and name the interaction they would have.

 side chain interactions: could be hydrophobic (glycine and leucine) or ionic (aspartate and arginine)

c. The drawing on the right represents an α-helix viewed from the top. Write the one-letter abbreviations of the first six amino acids in the sequence to show their side chain positions.

d. There is at least one reason why not all of sequence M would form an α-helix. What is it?

simplest: glycine is rarely found in an α-helix.

24. Your lab has three prepared phosphate buffers.
Buffer A: 0.75 M NaH2PO 4 + 0.25 M Na2HPO4
Buffer B: 0.25 M NaH2PO 4 + 0.75 M Na2HPO4
Buffer C: 0.45 M NaHPO4 + 0.55 M Na3PO4
The pKa 's for phosphate are 2.14, 6.86, and 12.4 .
a. Which is the best buffer to use for an experiment at pH 6.0?            A        
b. What do you need to do to the buffer, if anything, to adjust its pH?
      Use the Henderson-Hasselbalch equation to justify your answers.

            [NaH2PO4] should be 0.88 M, and [Na2HPO4] should be 0.12 M.

To calculate this: 6.0 = 6.86 + log ratio
            – 0.86 = log ratio
            ratio = 0.14

This means that for every 1.0 mole HA, there is 0.14 mole A-. To have the total concentration = 1.0 M, let [A-] = 0.14 [HA].

1.0 HA + 0.14 HA = 1.0 M
1.14 HA = 1.0 M
[HA] = 0.88 M, and [A-] = 0.12 M.

 

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