MCAT AAMC ACTUAL EXAM REVIEW 2026
SOLVED QUESTIONS AND DETAILED
SOLUTIONS GRADED A+
◉ 1A.1) amino acids. Answer:
◉ * description. Answer: - Amino acids contain two functional groups,
an amino group (-NH2) and a carboxyl group (-COOH)
- Alpha amino acids are those where both functional groups are attached
to the same carbon.
- Side chain/R group and hydrogen atom are also attached to alpha
carbon.
- Side chain determines the properties of amino acids and its functions.
◉ Absolute configuration at the α position. Answer: * Alpha carbon is
usually a chiral center since it has four different groups attached to it.
- Thus most amino acids are optically active
- Glycine is the only exception since it has a hydrogen as its R group.
* All chiral amino acids are L-amino acids, which means that the amino
group is drawn on
,the left side for the Fischer projection.
- Translates to an (S) absolute configuration for almost all chiral amino
acids.
- Cysteine is the only amino acid that has an L-amino acid configuration
but has an (R) absolute configuration. This is because the carboxyl
group is not the highest priority functional group.
◉ hydrophobic and hydrophilic amino acids. Answer: * Amino acids
with long alkyl side chains - alanine, isoleucine, leucine, valine, and
phenylalanine - are strongly hydrophobic, and are thus more likely to be
found in the interior of proteins
* All amino acids with charged side chains - histidine, arginine, lysine,
glutamate, and aspartate - are hydrophilic
- So are the amides asparagine and glutamine
* Remaining amino acids are somewhere in the middle between being
philic and phobic.
◉ Acid Base Chemistry of Amino Acids. Answer: * Amino acids are an
amphoteric species since they have an acidic carboxyl group and a basic
amino group.
* Ionizable groups tend to gain protons under acidic conditions and lose
them under basic conditions.
- i.e. at low pH an ionizable group will be protonated
,* The pKa of a group is the pH at which half of the molecules of the
species are deprotonated or [HA]=[A-]
- If pH is lower than pKa then the majority of the species will be
protonated.
◉ protonation and deprotonation. Answer: * All amino acids have at
least two pKa values.
- pKa1 is the pKa of the carboxyl group and is usually around 2
- pKa2 is the pKa of the amino group and is usually around 9-10
* If the amino acid has an ionizable side chain, then there will be three
pKa values.
◉ positively charged under acidic conditions. Answer: - At pH below 1,
the pKa is far below that of the amino group, so the amino group is fully
protonated (-NH3+)
- Additionally, the carboxylic acid group is fully protonated (-COOH)
- At very acidic pH values, amino acids tend to be positively charged.
◉ zwitterions at intermediate pH. Answer: At pH of 7.4, carboxylic acid
pKa has been moved past. As such, you will not find amino acids with
their carboxylate group protonated and the amino group unprotonated
- Amine group stays protonated since the pKa still is not high enough.
, - Resulting molecule has both a positive charge and a negative charge,
but is overall
neutral.
- These are called dipolar ions or zwitterions. These exist in water as
internal salts.
◉ negatively charged under basic conditions. Answer: - pKa of the
amino group is below the pKa at higher pH's>10, which means that the
amino group deprotonates to NH2
- So the molecules become negatively charged at high pH
◉ titration of amino acids. Answer: - The titration curve should look like
a combination of two monoprotic acid curves or three curves if the side
chain is charged.
- Starting at low pH, the amino acid is fully protonated. As the pH
approaches pKa1, the solution begins acting like a buffer and this is
characterized by a straight line on the graph.
- When pH= pKa1, then [HA]=[A-]
- Isoelectric Point (pI): the pH at which the molecule is electrically
neutral which means that the amino acid exists exclusively in its
zwitterion form2.
- Can be calculated for neutral amino acids by the following: (pka1 +
pka2)/2
SOLVED QUESTIONS AND DETAILED
SOLUTIONS GRADED A+
◉ 1A.1) amino acids. Answer:
◉ * description. Answer: - Amino acids contain two functional groups,
an amino group (-NH2) and a carboxyl group (-COOH)
- Alpha amino acids are those where both functional groups are attached
to the same carbon.
- Side chain/R group and hydrogen atom are also attached to alpha
carbon.
- Side chain determines the properties of amino acids and its functions.
◉ Absolute configuration at the α position. Answer: * Alpha carbon is
usually a chiral center since it has four different groups attached to it.
- Thus most amino acids are optically active
- Glycine is the only exception since it has a hydrogen as its R group.
* All chiral amino acids are L-amino acids, which means that the amino
group is drawn on
,the left side for the Fischer projection.
- Translates to an (S) absolute configuration for almost all chiral amino
acids.
- Cysteine is the only amino acid that has an L-amino acid configuration
but has an (R) absolute configuration. This is because the carboxyl
group is not the highest priority functional group.
◉ hydrophobic and hydrophilic amino acids. Answer: * Amino acids
with long alkyl side chains - alanine, isoleucine, leucine, valine, and
phenylalanine - are strongly hydrophobic, and are thus more likely to be
found in the interior of proteins
* All amino acids with charged side chains - histidine, arginine, lysine,
glutamate, and aspartate - are hydrophilic
- So are the amides asparagine and glutamine
* Remaining amino acids are somewhere in the middle between being
philic and phobic.
◉ Acid Base Chemistry of Amino Acids. Answer: * Amino acids are an
amphoteric species since they have an acidic carboxyl group and a basic
amino group.
* Ionizable groups tend to gain protons under acidic conditions and lose
them under basic conditions.
- i.e. at low pH an ionizable group will be protonated
,* The pKa of a group is the pH at which half of the molecules of the
species are deprotonated or [HA]=[A-]
- If pH is lower than pKa then the majority of the species will be
protonated.
◉ protonation and deprotonation. Answer: * All amino acids have at
least two pKa values.
- pKa1 is the pKa of the carboxyl group and is usually around 2
- pKa2 is the pKa of the amino group and is usually around 9-10
* If the amino acid has an ionizable side chain, then there will be three
pKa values.
◉ positively charged under acidic conditions. Answer: - At pH below 1,
the pKa is far below that of the amino group, so the amino group is fully
protonated (-NH3+)
- Additionally, the carboxylic acid group is fully protonated (-COOH)
- At very acidic pH values, amino acids tend to be positively charged.
◉ zwitterions at intermediate pH. Answer: At pH of 7.4, carboxylic acid
pKa has been moved past. As such, you will not find amino acids with
their carboxylate group protonated and the amino group unprotonated
- Amine group stays protonated since the pKa still is not high enough.
, - Resulting molecule has both a positive charge and a negative charge,
but is overall
neutral.
- These are called dipolar ions or zwitterions. These exist in water as
internal salts.
◉ negatively charged under basic conditions. Answer: - pKa of the
amino group is below the pKa at higher pH's>10, which means that the
amino group deprotonates to NH2
- So the molecules become negatively charged at high pH
◉ titration of amino acids. Answer: - The titration curve should look like
a combination of two monoprotic acid curves or three curves if the side
chain is charged.
- Starting at low pH, the amino acid is fully protonated. As the pH
approaches pKa1, the solution begins acting like a buffer and this is
characterized by a straight line on the graph.
- When pH= pKa1, then [HA]=[A-]
- Isoelectric Point (pI): the pH at which the molecule is electrically
neutral which means that the amino acid exists exclusively in its
zwitterion form2.
- Can be calculated for neutral amino acids by the following: (pka1 +
pka2)/2
