HL Paper 3
Liquid-crystal displays (LCDs) have many uses.
A molecule which acts as a chiral nematic thermotropic liquid-crystal is given.
Label with an asterisk, *, the chiral carbon atom.
Explain the effects of very low and high temperatures on the liquid-crystal behaviour of this molecule.
Low temperature:
High temperature:
Vision is dependent on retinol (vitamin A) present in retina cells. Retinol is oxidized to the photosensitive chemical 11-cis-retinal and isomerizes to 11-trans-retinal on absorption of light.
Outline how the formation of 11-trans-retinal results in the generation of nerve signals to the brain.
Amino acids are the building blocks of proteins.
Draw the structures of the main form of glycine in buffer solutions of pH 1.0 and 6.0.
The pKa of glycine is 2.34.
Calculate the pH of a buffer system with a concentration of 1.25 × 10−3 mol dm−3 carbonic acid and 2.50 × 10−2 mol dm−3 sodium hydrogen carbonate. Use section 1 of the data booklet.
pKa (carbonic acid) = 6.36
Sketch the wedge and dash (3-D) representations of alanine enantiomers.
UV-Vis spectroscopy can be used to determine the unknown concentration of a substance in a solution.
Calculate the concentration of an unknown sample of pepsin with an absorbance of 0.725 using section 1 of the data booklet.
Cell length = 1.00 cm
Molar absorptivity (extinction coefficient) of the sample = 49650 dm3 cm−1 mol−1
A different series of pepsin samples is used to develop a calibration curve.
Estimate the concentration of an unknown sample of pepsin with an absorbance of 0.30 from the graph.
Taxol is an anticancer drug.
State the feature of Taxol that is a major challenge in its synthesis. Use section 37 of the data booklet.
Describe how the challenge in (a) was resolved by pharmaceutical companies.
A polarimeter can be used to determine the optical rotation of an optically active substance.
Describe what happens to plane-polarized light when it passes through a solution of an optically active compound.
A mixture of enantiomers shows optical rotation.
Suggest a conclusion you can draw from this data.