What is a zwitterion?

A nucleophile is a species that donates a pair of electrons to an electron-deficient carbon atom, forming a new covalent bond. Nucleophiles are attracted to areas of positive charge or electron deficiency.

Amino acids contain two functional groups: a basic .......... group and an acidic .......... group. Because they contain both, amino acids are described as .......... .

Amino acids contain two functional groups: a basic amino (–NH 2 ) group and an acidic carboxylic acid (–COOH) group. Because they contain both, amino acids are described as amphoteric .

True or False? At a pH below the isoelectric point, an amino acid exists as a positively charged ion because the –COO - group accepts an H + ion.

True. Adding acid (lowering pH) causes the –COO - group to accept H + and reform –COOH, leaving the –NH 3 + group intact, so the amino acid carries an overall positive charge.

What is the isoelectric point of an amino acid?

Isoelectric point is the pH at which an amino acid exists predominantly as a neutral zwitterion — the pH at which the positively and negatively charged forms are in equal concentration, giving no net charge.

The general structural formula of a 2-aminocarboxylic acid is .......... . There are .......... naturally occurring amino acids, each distinguished by a different .......... group.

The general structural formula of a 2-aminocarboxylic acid is RCH(NH 2 )COOH . There are 20 naturally occurring amino acids, each distinguished by a different R group.

True or False? Amino acids are volatile liquids at room temperature because their R groups are small.

False. Amino acids are soluble crystalline solids at room temperature. The strong intermolecular forces between zwitterions (due to their charges) give them high melting points.

What happens to an amino acid zwitterion when a small amount of alkali is added?

The –NH 3 + group donates an H + ion to reform the –NH 2 group. The amino acid becomes a negatively charged ion, resisting the change in pH — it acts as a buffer solution.

What is a peptide bond?

A curly arrow is a symbol used in reaction mechanisms to show the movement of a pair of electrons. The arrow starts at the electron source (a bond or lone pair) and ends at the electron destination.

The .......... structure of a protein is the sequence of amino acids linked by covalent peptide bonds. A single change in this sequence can .......... the function of the protein.

The primary structure of a protein is the sequence of amino acids linked by covalent peptide bonds. A single change in this sequence can alter the function of the protein.

True or False? The secondary structure of a protein involves hydrogen bonds between R groups on different amino acids.

False. The secondary structure involves hydrogen bonds between the amino and carboxyl groups of the protein backbone — not the R groups. R group interactions occur in the tertiary structure.

Name the two shapes formed by hydrogen bonding in the secondary structure of a protein.

The alpha-helix (α-helix) and the beta-pleated sheet (β-pleated sheet).

Protein hydrolysis breaks .......... bonds by addition of .......... . In the laboratory this is carried out using concentrated .......... and heating for many hours.

Protein hydrolysis breaks peptide bonds by addition of water . In the laboratory this is carried out using concentrated hydrochloric acid and heating for many hours.

True or False? After hydrolysis, amino acids from a protein mixture can be identified by TLC using ninhydrin as a locating agent because amino acids are coloured.

False. Amino acids are colourless , so a locating agent such as ninhydrin is needed to stain the spots on the TLC plate and make them visible.

Why might two-dimensional TLC be needed to identify amino acids after protein hydrolysis?

Some amino acids have very similar R f values in the same solvent, making identification uncertain. Two-dimensional TLC runs the plate through two different solvents at 90° to each other, giving two R f values per amino acid and allowing more confident identification.

What bonds are found in the tertiary structure of a protein?

Hydrogen bonds, disulfide bonds (between cysteine residues), ionic bonds (between charged R groups) and hydrophobic interactions (between non-polar R groups) — all involving the R groups of amino acids.

An oxidising agent is a species that gains electrons (is reduced) in a redox reaction, causing another species to be oxidised.

Enzymes are highly specific because the .......... of the active site is complementary to the shape of a specific .......... . Extremes of heat or pH cause .......... , which alters the active site shape.

Enzymes are highly specific because the shape of the active site is complementary to the shape of a specific substrate . Extremes of heat or pH cause denaturation , which alters the active site shape.

True or False? Enzyme specificity arises from the primary structure of the protein because the sequence of amino acids determines the three-dimensional shape of the active site.

True. The order of amino acids (primary structure) determines how the enzyme folds, producing a specific tertiary structure with an active site complementary to one substrate.

What is an enzyme-substrate complex?

The temporary structure formed when a substrate binds to the active site of an enzyme. The substrate must collide with the active site at the correct orientation and speed for a reaction to occur.

Drugs bind to receptors — proteins found on enzymes, cell membranes or DNA — mainly using .......... or .......... . Many drugs are .......... , meaning only one enantiomer is biologically active.

Drugs bind to receptors — proteins found on enzymes, cell membranes or DNA — mainly using intermolecular forces or ionic bonds . Many drugs are stereoselective , meaning only one enantiomer is biologically active.

True or False? Both enantiomers of a drug molecule always produce the same biological effect because they have identical physical properties.

False. Although enantiomers have the same physical properties in achiral environments, drug-receptor binding is stereoselective — only one enantiomer fits the receptor's active site in the correct orientation to produce a biological effect.

Why are computers increasingly used in drug design?

Computers can model drug-receptor interactions in three dimensions, allowing chemists to predict which molecular shapes and functional groups will bind most effectively to a receptor before synthesising the drug.

What is a nucleotide?

A functional group is an atom or group of atoms within an organic molecule that is responsible for the characteristic chemical reactions of that molecule.

In DNA, adjacent nucleotides within a strand are connected by .......... bonds between the sugar and phosphate groups, forming the .......... . The two strands are held together by .......... bonds between complementary bases.

In DNA, adjacent nucleotides within a strand are connected by phosphodiester (covalent) bonds between the sugar and phosphate groups, forming the sugar-phosphate backbone . The two strands are held together by hydrogen bonds between complementary bases.

True or False? Adenine pairs with cytosine in DNA, forming three hydrogen bonds.

False. Adenine (A) pairs with thymine (T) via two hydrogen bonds. Guanine (G) pairs with cytosine (C) via three hydrogen bonds.

What does it mean to say that the two strands of DNA are antiparallel?

The two polynucleotide strands run in opposite directions — one strand runs in one direction, the other strand runs in the opposite direction. This antiparallel arrangement is essential for complementary base pairing.

DNA replication is possible because the .......... bonds between base pairs are weaker than the .......... bonds in the sugar-phosphate backbone, so the strands can .......... during cell division.

DNA replication is possible because the hydrogen bonds between base pairs are weaker than the covalent (phosphodiester) bonds in the sugar-phosphate backbone, so the strands can separate during cell division.

True or False? The three-dimensional shape of DNA is described as a double helix.

True. The two antiparallel polynucleotide strands coil around each other to form a double helix — the three-dimensional structure of a DNA molecule.

What is complementary base pairing in DNA?

Complementary base pairing is the specific hydrogen-bonding relationship between bases in the two DNA strands: adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).

How does cis-platin prevent cancer cells from replicating?

Cis-platin binds to nitrogen atoms on DNA bases, distorting the shape of the DNA and preventing the double helix from being replicated during cell division.

Cis-platin enters a cell and undergoes .......... exchange in which the chlorine ligands are replaced by .......... . Nitrogen on a DNA base then acts as a better .......... and binds to the platinum.

Cis-platin enters a cell and undergoes ligand exchange in which the chlorine ligands are replaced by water molecules . Nitrogen on a DNA base then acts as a better ligand and binds to the platinum.

True or False? Trans-platin is as effective as cis-platin in treating cancer because both are square planar molecules.

False. Only cis-platin is effective. Trans-platin is a geometric isomer with the leaving groups in different positions; it cannot cross-link DNA strands in the same way, so it has no anticancer activity.

Why does cis-platin cause hair loss as a side effect?

Cis-platin binds to healthy cells as well as cancerous cells. Hair follicle cells replicate rapidly, so they are also affected by the drug, leading to hair loss.

Cis-platin is a .......... planar molecule and a geometric isomer of trans-platin. It forms .......... covalent bonds with nitrogen atoms on DNA bases, using the dative bond-forming ability of .......... .

Cis-platin is a square planar molecule and a geometric isomer of trans-platin. It forms dative covalent bonds with nitrogen atoms on DNA bases, using the dative bond-forming ability of nitrogen .

True or False? Cis-platin affects cancer cells more than healthy cells because it is selectively absorbed only by tumour tissue.

False. Cis-platin binds to both healthy and cancerous cells. It affects cancer cells more because cancer cells replicate faster , so they are more vulnerable to a drug that blocks DNA replication.

State the types of cancer for which cis-platin has been found effective.

Testicular, ovarian, cervical, breast, lung and brain cancer. Cis-platin was discovered in the 1960s and remains one of the most widely used anticancer drugs.

A free radical is a species with an unpaired electron. Free radicals are highly reactive and are intermediates in chain reactions such as the free radical substitution of alkanes.

A LevelChemistryAQAFlashcardsAdvanced Organic ChemistryAmino acids, Proteins & DNA

Amino acids, Proteins & DNA (AQA A Level Chemistry): Flashcards

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What is a zwitterion?

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Cards in this collection (37)

What is a zwitterion?
A nucleophile is a species that donates a pair of electrons to an electron-deficient carbon atom, forming a new covalent bond. Nucleophiles are attracted to areas of positive charge or electron deficiency.
Amino acids contain two functional groups: a basic .......... group and an acidic .......... group. Because they contain both, amino acids are described as .......... .
Amino acids contain two functional groups: a basic amino (–NH₂) group and an acidic carboxylic acid (–COOH) group. Because they contain both, amino acids are described as amphoteric.
True or False?
At a pH below the isoelectric point, an amino acid exists as a positively charged ion because the –COO^- group accepts an H⁺ ion.
True.
Adding acid (lowering pH) causes the –COO^- group to accept H⁺ and reform –COOH, leaving the –NH₃⁺ group intact, so the amino acid carries an overall positive charge.
What is the isoelectric point of an amino acid?
Isoelectric point is the pH at which an amino acid exists predominantly as a neutral zwitterion — the pH at which the positively and negatively charged forms are in equal concentration, giving no net charge.
The general structural formula of a 2-aminocarboxylic acid is .......... . There are .......... naturally occurring amino acids, each distinguished by a different .......... group.
The general structural formula of a 2-aminocarboxylic acid is RCH(NH₂)COOH. There are 20 naturally occurring amino acids, each distinguished by a different R group.
True or False?
Amino acids are volatile liquids at room temperature because their R groups are small.
False.
Amino acids are soluble crystalline solids at room temperature. The strong intermolecular forces between zwitterions (due to their charges) give them high melting points.
What happens to an amino acid zwitterion when a small amount of alkali is added?
The –NH₃⁺ group donates an H⁺ ion to reform the –NH₂ group.
The amino acid becomes a negatively charged ion, resisting the change in pH — it acts as a buffer solution.
What is a peptide bond?
A curly arrow is a symbol used in reaction mechanisms to show the movement of a pair of electrons. The arrow starts at the electron source (a bond or lone pair) and ends at the electron destination.
The .......... structure of a protein is the sequence of amino acids linked by covalent peptide bonds. A single change in this sequence can .......... the function of the protein.
The primary structure of a protein is the sequence of amino acids linked by covalent peptide bonds. A single change in this sequence can alter the function of the protein.
True or False?
The secondary structure of a protein involves hydrogen bonds between R groups on different amino acids.
False.
The secondary structure involves hydrogen bonds between the amino and carboxyl groups of the protein backbone — not the R groups. R group interactions occur in the tertiary structure.
Name the two shapes formed by hydrogen bonding in the secondary structure of a protein.
The alpha-helix (α-helix) and the beta-pleated sheet (β-pleated sheet).
Protein hydrolysis breaks .......... bonds by addition of ........... In the laboratory this is carried out using concentrated .......... and heating for many hours.
Protein hydrolysis breaks peptide bonds by addition of water. In the laboratory this is carried out using concentrated hydrochloric acid and heating for many hours.
True or False?
After hydrolysis, amino acids from a protein mixture can be identified by TLC using ninhydrin as a locating agent because amino acids are coloured.
False.
Amino acids are colourless, so a locating agent such as ninhydrin is needed to stain the spots on the TLC plate and make them visible.
Why might two-dimensional TLC be needed to identify amino acids after protein hydrolysis?
Some amino acids have very similar R_f values in the same solvent, making identification uncertain.
Two-dimensional TLC runs the plate through two different solvents at 90° to each other, giving two R_f values per amino acid and allowing more confident identification.
What bonds are found in the tertiary structure of a protein?
Hydrogen bonds, disulfide bonds (between cysteine residues), ionic bonds (between charged R groups) and hydrophobic interactions (between non-polar R groups) — all involving the R groups of amino acids.
What is an enzyme?
An oxidising agent is a species that gains electrons (is reduced) in a redox reaction, causing another species to be oxidised.
Enzymes are highly specific because the .......... of the active site is complementary to the shape of a specific ........... Extremes of heat or pH cause .........., which alters the active site shape.
Enzymes are highly specific because the shape of the active site is complementary to the shape of a specific substrate. Extremes of heat or pH cause denaturation, which alters the active site shape.
True or False?
Enzyme specificity arises from the primary structure of the protein because the sequence of amino acids determines the three-dimensional shape of the active site.
True.
The order of amino acids (primary structure) determines how the enzyme folds, producing a specific tertiary structure with an active site complementary to one substrate.
What is an enzyme-substrate complex?
The temporary structure formed when a substrate binds to the active site of an enzyme.
The substrate must collide with the active site at the correct orientation and speed for a reaction to occur.
Drugs bind to receptors — proteins found on enzymes, cell membranes or DNA — mainly using .......... or ........... Many drugs are .........., meaning only one enantiomer is biologically active.
Drugs bind to receptors — proteins found on enzymes, cell membranes or DNA — mainly using intermolecular forces or ionic bonds. Many drugs are stereoselective, meaning only one enantiomer is biologically active.
True or False?
Both enantiomers of a drug molecule always produce the same biological effect because they have identical physical properties.
False.
Although enantiomers have the same physical properties in achiral environments, drug-receptor binding is stereoselective — only one enantiomer fits the receptor's active site in the correct orientation to produce a biological effect.
Why are computers increasingly used in drug design?
Computers can model drug-receptor interactions in three dimensions, allowing chemists to predict which molecular shapes and functional groups will bind most effectively to a receptor before synthesising the drug.
What is a nucleotide?
A functional group is an atom or group of atoms within an organic molecule that is responsible for the characteristic chemical reactions of that molecule.
In DNA, adjacent nucleotides within a strand are connected by .......... bonds between the sugar and phosphate groups, forming the ........... The two strands are held together by .......... bonds between complementary bases.
In DNA, adjacent nucleotides within a strand are connected by phosphodiester (covalent) bonds between the sugar and phosphate groups, forming the sugar-phosphate backbone. The two strands are held together by hydrogen bonds between complementary bases.
True or False?
Adenine pairs with cytosine in DNA, forming three hydrogen bonds.
False.
Adenine (A) pairs with thymine (T) via two hydrogen bonds. Guanine (G) pairs with cytosine (C) via three hydrogen bonds.
What does it mean to say that the two strands of DNA are antiparallel?
The two polynucleotide strands run in opposite directions — one strand runs in one direction, the other strand runs in the opposite direction. This antiparallel arrangement is essential for complementary base pairing.
DNA replication is possible because the .......... bonds between base pairs are weaker than the .......... bonds in the sugar-phosphate backbone, so the strands can .......... during cell division.
DNA replication is possible because the hydrogen bonds between base pairs are weaker than the covalent (phosphodiester) bonds in the sugar-phosphate backbone, so the strands can separate during cell division.
True or False?
The three-dimensional shape of DNA is described as a double helix.
True.
The two antiparallel polynucleotide strands coil around each other to form a double helix — the three-dimensional structure of a DNA molecule.
What is complementary base pairing in DNA?
Complementary base pairing is the specific hydrogen-bonding relationship between bases in the two DNA strands: adenine (A) always pairs with thymine (T) and guanine (G) always pairs with cytosine (C).
How does cis-platin prevent cancer cells from replicating?
Cis-platin binds to nitrogen atoms on DNA bases, distorting the shape of the DNA and preventing the double helix from being replicated during cell division.
Cis-platin enters a cell and undergoes .......... exchange in which the chlorine ligands are replaced by ........... Nitrogen on a DNA base then acts as a better .......... and binds to the platinum.
Cis-platin enters a cell and undergoes ligand exchange in which the chlorine ligands are replaced by water molecules. Nitrogen on a DNA base then acts as a better ligand and binds to the platinum.
True or False?
Trans-platin is as effective as cis-platin in treating cancer because both are square planar molecules.
False.
Only cis-platin is effective. Trans-platin is a geometric isomer with the leaving groups in different positions; it cannot cross-link DNA strands in the same way, so it has no anticancer activity.
Why does cis-platin cause hair loss as a side effect?
Cis-platin binds to healthy cells as well as cancerous cells.
Hair follicle cells replicate rapidly, so they are also affected by the drug, leading to hair loss.
Cis-platin is a .......... planar molecule and a geometric isomer of trans-platin. It forms .......... covalent bonds with nitrogen atoms on DNA bases, using the dative bond-forming ability of ...........
Cis-platin is a square planar molecule and a geometric isomer of trans-platin. It forms dative covalent bonds with nitrogen atoms on DNA bases, using the dative bond-forming ability of nitrogen.
True or False?
Cis-platin affects cancer cells more than healthy cells because it is selectively absorbed only by tumour tissue.
False.
Cis-platin binds to both healthy and cancerous cells. It affects cancer cells more because cancer cells replicate faster, so they are more vulnerable to a drug that blocks DNA replication.
State the types of cancer for which cis-platin has been found effective.
Testicular, ovarian, cervical, breast, lung and brain cancer.
Cis-platin was discovered in the 1960s and remains one of the most widely used anticancer drugs.
Cis-platin
A free radical is a species with an unpaired electron. Free radicals are highly reactive and are intermediates in chain reactions such as the free radical substitution of alkanes.

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