Stereochemistry in biomolecules

Nature of science:

Theories used to explain natural phenomena/evaluate claims—biochemistry involves many chiral molecules with biological activity specific to one enantiomer. Chemical reactions in a chiral environment act as a guiding distinction between living and non-living matter. (2.2)

• With one exception, amino acids are chiral, and only the L-configuration is found in proteins.
• Naturally occurring unsaturated fat is mostly in the cis form, but food processing can convert it into the trans form.
• D and L stereoisomers of sugars refer to the configuration of the chiral carbon atom furthest from the aldehyde or ketone group, and D forms occur most frequently in nature.
• Ring forms of sugars have isomers, known as α and β, depending on whether the position of the hydroxyl group at carbon 1 (glucose) or carbon 2 (fructose) lies below the plane of the ring (α) or above the plane of the ring (β).
• Vision chemistry involves the light activated interconversion of cis- and trans- isomers of retinal.

Applications and skills:

• Description of the hydrogenation and partial hydrogenation of unsaturated fats, including the production of trans-fats, and a discussion of the advantages and disadvantages of these processes.
• Explanation of the structure and properties of cellulose, and comparison with starch.
• Discussion of the importance of cellulose as a structural material and in the diet.
• Outline of the role of vitamin A in vision, including the roles of opsin, rhodopsin and cis- and trans-retinal.

• Different countries have very different standards of food labelling with respect to its chemical content, including the type of fats present.

Utilization:

• Syllabus and cross-curricular links:
• Topic 10.1—organic functional groups
• Topic 20.1—organic reactions
• Topic 20.3—stereoisomerism
• Option A.4—intermolecular/London forces

Aims:

• Aim 8: Ethical questions arise through the use of saturated and trans-fats, particularly in the fast-food industry.

• Names of the enzymes involved in the visual cycle are not required.
• Relative melting points of saturated and cis-/trans-unsaturated fats should be covered.