Download Chem 204

Name: Answer Key
1.
Part I : Multiple Choice.
Circle the appropriate answer for the following questions.
( 1 pt each)
A compound is known to have a free amino group with a pKa of 8.8, and one other
ionizable group with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this
compound at pH 8.2 was added 40 mL of a solution of 0.2 M hydrochloric acid. The pH
changed to 6.2. The pKa of the second ionizable group is:
A)
B)
C)
D)
E)
2.
The pH cannot be determined from this information.
5.4
5.6
6.0
6.2
A 1.0 M solution of a compound with 2 ionizable groups (pKa’s = 6.2 and 9.5; 100 mL
total) has a pH of 6.8. If a biochemist adds 60 mL of 1.0 M HCl to this solution, the
solution will change to pH:
A)
B)
C)
D)
E)
3.
Chem 204 - Summer 2009
Problem Set II
Student ID:
5.60
8.90
9.13
9.32
The pH cannot be determined from this information.
Three buffers are made by combining a 1 M solution of acetic acid with a 1 M solution of
sodium acetate in the ratios shown below.
1 M acetic acid
Buffer 1: 10 mL
Buffer 2: 50 mL
Buffer 3: 90 mL
1 M sodium acetate
90 mL
50 mL
10 mL
Which of these statements is true of the resulting buffers?
A)
B)
C)
D)
E)
pH of buffer 1 < pH of buffer 2 < pH of buffer 3
pH of buffer 1 = pH of buffer 2 = pH of buffer 3
pH of buffer 1 > pH of buffer 2 > pH of buffer 3
The problem cannot be solved without knowing the value of pKa.
None of the above.
4.
Amino acids are ampholytes because they can function as either a(n):
A) acid or a base.
B) neutral molecule or an ion.
C) polar or a nonpolar molecule.
D) standard or a nonstandard monomer in proteins.
E) transparent or a light-absorbing compound.
5.
In an α helix, the R groups on the amino acid residues:
A) alternate between the outside and the inside of the helix.
B) are found on the outside of the helix spiral.
C) cause only right-handed helices to form.
D) generate the hydrogen bonds that form the helix.
E) stack within the interior of the helix.
Part II: Short Answer
(4pts each)
6.
Which of the following statements concerning protein domains is true, if false explain
why (or correct the statement):
A) They are a form of secondary structure.
B) They are examples of structural motifs.
C) They consist of separate polypeptide chains (subunits).
D) They have been found only in prokaryotic proteins.
E) They may retain their correct shape even when separated from the rest of the
protein.
7.
Which of the following is least likely to result in protein denaturation, and explain why it
is the least likely?
A) Altering net charge by changing pH
B) Changing the salt concentration
C) Disruption of weak interactions by boiling
D) Exposure to detergents
E) Mixing with organic solvents such as acetone
8 . (a) Briefly define “isotonic,” “hypotonic,” and “hypertonic” solutions.
(b) Describe what happens when a cell is placed in each of these types of solutions
a) Isotonic: solution with equal osmolarity
Hypotonic: solution with higher osmolarity outside than in (i.e. cytosol)
Hypertonic: solution with lower osmolarity outside than in (i.e. cytosol)
b) In isotonic: cell neither gains nor looses water
In hypotonic: cell will shrink as water moves out
In hypertonic: cell will swell as water moves in
9.
Briefly describe the five major groupings of amino acids and how they are distinguished.
A) Non-polar aliphatic – nonpolar side chains which are phydroophobic
B) Polar – hydrophilic side chains; can form H-bonds
C) Aromatic – nonpolar and hydrophobic; with aromatic (conjugated DB) groups
D) Positively charged (basic) – have a (+) charge at pH 7; hydrophilic
E) Negatively charged (acidic) – have a (–) charge at pH 7; hydrophilic
10.
Leucine has two dissociable protons: one with a pKa of 2.3, the other with a pKa of 9.7.
Sketch a properly labeled titration curve for leucine titrated with NaOH; indicate where the
pH = pK and the region(s) in which buffering occurs.
16
Buffering regions
14
12
pKa2=9.6
pH
10
8
6
pKa1=2.3
4
2
0
0
0.5
1
1.5
2
[OH] eqivalents
11.
What is the role of ATP and ATP hydrolysis in the cycle of actin-myosin association and
disassociation that leads to muscle contraction?
ATP binding to myosin results in a conformational change that causes dissociation of
actin from the myosin. ATP hydrolysis results in a change of orientation of the myosin
relative to the actin filament which allows movement to the next actin subunit. This is
followed initially by release of the phosphate hydrolysis product and weak binding of the
myosin to this actin subunit, and, subsequently by tight binding and release of the ADP
hydrolysis product.