Flashcards · 9620 · A Level Topic 31

Amino acids and proteins (A Level) flashcards

12 cards on A Level Topic 31, subtopics 3.3.13.1 to 3.3.13.3. Pick a subtopic or study them all.

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Amino acid (general structure): A molecule with –NH₂ and –COOH on the same carbon: H₂N–CH(R)–COOH.
Zwitterion: A species with both a positive and a negative charge in the same molecule, neutral overall: ⁺H₃N–CH(R)–COO⁻.
Form of an amino acid (simple R) in excess acid and in excess alkali: Excess acid: ⁺H₃N–CH(R)–COOH (charge +1). Excess alkali: H₂N–CH(R)–COO⁻ (charge −1).
Formula and charge of glutamic acid, HOOC–CH₂CH₂–CH(NH₂)–COOH, in excess NaOH: ⁻OOC–CH₂CH₂–CH(NH₂)–COO⁻, charge −2 (the side-chain –COOH also reacts).

Peptide link and how it forms: –CO–NH–, formed by condensation of –COOH with –NH₂, eliminating water. n amino acids give (n − 1) links and lose (n − 1) H₂O.
Primary, secondary and tertiary structure of a protein: Primary: sequence of amino acids. Secondary: α-helix or β-pleated sheet, held by hydrogen bonds between backbone C=O and N–H. Tertiary: overall 3-D folding, held by side-chain interactions such as hydrogen bonds and S–S bonds.
Disulfide (S–S) bonds in proteins: Covalent –S–S– links formed by oxidation of the –SH groups of two cysteine residues. They help fix the tertiary structure.
How are amino acids from a hydrolysed protein identified by TLC?: Spot the hydrolysate and standards on a pencil baseline; run in a lidded tank; locate spots with ninhydrin (purple) or UV light on a fluorescent plate; compare Rf values with the standards.
Rf value: Distance moved by the spot ÷ distance moved by the solvent front, both from the baseline. Example: 3.9 cm ÷ 6.5 cm = 0.60.

Structure of cisplatin: cis-[Pt(NH₃)₂Cl₂]: square planar, platinum(II), with the two Cl⁻ ligands next to each other (at 90°).
How does cisplatin act as an anticancer drug?: By ligand replacement: Cl⁻ ligands are replaced by nitrogen atoms in DNA, forming co-ordinate bonds to Pt. The distorted DNA cannot be copied, so the cell cannot divide.
Adverse effects of cisplatin, and why they happen: It is not selective: it also binds to DNA in healthy cells. Side effects include kidney damage and hearing loss. Society weighs the benefits of a drug against its harm.