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Unformatted text preview: ) (20) Histidine can be used as a buffer in solutions used in biochemistry. How much of a 1 M
solution of the monosodium salt of histidine (NaHis) and how much 1 M HCl or 1M NaOH
would you have to add to make one liter of 0.1 M histidine buffer at pH 6.0?
His+2 <–> His+1 <–> His0 <–> His-1
9.2 are the pKa s
Must add 0.1 liter of 1 M NaHis to make 1 liter of 0.1 M buffer. This is the His-1form.
Final buffer is at pH 6.0 so it will have equal concentrations of His+1 and His0 .
Therefore [His+1] = [His0 ] = 1/2 • 0.1 M = 0.05 M
Must add HCl to interconvert forms of His. Add 0.1 mole to convert all His-1 to His0 , then
add additional 0.05 mole to convert 0.05 moles of His0 to 0.05 moles of His+1. This produces
the correct ratio of the different forms to make the buffer at pH 6.0.
Added HCl = 0.1 mole + 0.05 mole = 0.15 mole , since it is 1 M, this is 0.15 liter 1.0 M NaHis = 0.1 liter
Units 1.0 M HCl or NaOH =
(circle one) 0.15 liter
Units Name KEY
First BIS102-03, Winter '03, Page 4 of 7
C. S. Gasser 4. Write the one and three letter codes for all of the amino acids whose side chains (R-groups) fit
into the following categories.
a) (4) The neutral form can act as a proton donor.
Asp D, Glu E, Cys C, Tyr Y
b) (4) Contains sulfur.
Cys C, Met M
c) (4) Is aromatic and can also form hydrogen bonds.
Tyr Y, Trp W
d) (4) Can have a positive charge.
His H, Lys K, Arg R
5. (25) Place an X next to the ONE correct answer to each of the following questions.
a) Which of the following could form an H-bond with the carbonyl oxygen of a peptide bond?
X The hydrogen in an alcohol group.
One of the hydrogens in the side chain of amino acid F.
A nitrogen in the side chain of Asparagine
One of the hydrogens in the side chain of Leu
An oxygen in the side chain of amino acid N b) The reaction A <––> B will always go in the forward direction if:
DG°' for the reaction is less than 0
The concentration of A is greater than that of B
B is an amino acid
X DG for the reaction is less than zero at the current concentrations of A and B
DG for the reaction is greater than zero at the current concentrations of A and B Name KEY
First BIS102-03, Winter '03, Page 5 of 7
C. S. Gasser (problem 5, continued)
c). The weak acid HA, like all weak acids, undergoes the following reaction to a limited extent in
HA + H2 O <––> A- + H3 O+
The ratio between the Ka for this acid and the Keq for this reaction (Ka/Keq) would be equal to:
X 55.6 M [H + ][A - ] [H+ ][A - ]
K a /Keq =
= [H2 O] = 55.6M
[HA][H2 O] 1/55.6 M
Kb for NaA
cannot tell from this information
d) The reaction A <––> B will go in the forward direction if:
DS is positive.
X TDS is positive and greater than DH (this ensures that DG is negative)
DH is positive and DS is positive
DH is positive and DS is negative
A is more organized than B e) Van der Waals (induced dipole) interactions:
are most important for charged molecules
act only between h...
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This note was uploaded on 09/19/2013 for the course BIS 102 taught by Professor Hilt during the Spring '08 term at UC Davis.
- Spring '08