Download Amino Acid Answers: 1. Determine if the amino acids shown are of

Amino Acid Answers:
1. Determine if the amino acids shown are of the D- or L-series.
O
OH
H
NH2
O
OH
H 2N
CH2 Ph
O
H
H 2N
CH 2 CH2SCH 3
phenylalanine
OH
O
OH
H
H
NH2
CH 2 CO 2H
methionine
CH3
aspartic acid
D
L
L
D
Placement of the amine group on the right of the Fischer Projection indicates a D amino acid. Placement
on the left is a carbohydrate of the naturally occurring L series.
2. Determine if the amino acids shown are acidic, basic or neutral amino acids.
O
OH
H2 N
H
CH 2CO 2H
aspartic acid
O
H2N
OH
O
OH
O
OH
H
H
NH 2
H
NH2
CH 2CH2 SCH3
methionine
CH2OH
serine
CH 2CH 2 CH 2CH 2 NH 2
arginine
acidic
neutral
neutral
basic
Compounds containing the carboxylic acid functional group in their side chain are acidic amino acids.
Compounds containing amines in their side chair are basic amino acids. Those side chains containing
alkyl groups, alcohols, or even sulfur-containing groups are neutral.
3. The zwitterion of methionine:
O
O-
H3N
H
CH2CH2SCH3
methionine
Zwitterions are drawn by transferring the proton of the carboxylic acid (not from the side chain if it is an
acidic amino acid) to the amine group.
4. Define the term : amphoterism
The ability of a compound to act as both an acid and a base.
5. Draw the products of the following acid or base reactions:
O
OH
H
NH2
H 3O+
CH 2Ph
O
OH
H
NH3
CH2 Ph
phenylalanine
O
OH
H
NH2
O
O
H
NH2
NaOH, H2O
CH2 Ph
CH2 Ph
phenylalanine
6. Define the term : isoelectric point
The pH at which the zwitterion is perfectly balanced in charges, including any effects from the side chain
present.
7. Is the isoelectric point acidic or basic for the following amino acid? Explain.
O
OH
H2N
The isoelectric point for this compound will be in the acidic range (actually
2.77) to suppress loss of the carboxylic acid proton on the side chain.
H
CH2CO 2H
aspartic acid
8. Is the isoelectric point acidic or basic for the following amino acid? Explain.
O
H
OH
NH2
CH2CH 2CH2CH 2NH2
The isoelectric point for this compound will be in the basic range
(pH = 9.74) to prevent protonation of the amine group on the side
chain.
arginine
9. Why are the isoelectric points for neutral amino acids still slightly acidic, not neutral, for a neutral
amino acid, like the one shown below, whose isoelectric point is 5.96?
O
H
O
OH
NH2
H2O
H
O
NH2
CH(CH3)2
CH(CH 3)2
valine
valine
+
+ H3 O
Under neutral conditions, the carboxylic
acid is still prone to losing the proton to
water, thus a slightly acidic solution must
be maintained to prevent loss of the
carboxylic acid proton.