Geometric isomerism 2.

It was already mentioned that rotation of atoms around a C=C double bond (cis↔trans conversion) costs energy and only happens if the compound is heated strongly. Rotation around a C=N or N=N double bond is somewhat easier. The process can be accelerated by adding a catalytic amount iodine or using UV light. For example the natural carotinoids have all-trans configuration, but exposure to light transforms them to a mixture of cis and trans isomers.

Similarly, the antifungal macrolide antibiotics (amphotericin, nystatin, etc.) are biosynthethized in all-trans form. Exposure to light slowly converts them to a mixture of cis/trans isomers with reduced biological activity.

The cis/trans conversions of polyene chains result in characteristic changes of their UV spectrum.

Excercises

The cis/trans transformation of the carotinoid derivative shown below has a very important physiological role. Tick the correct answer.

 Important factor of signal transduction in neurons.
 Crucial role in animal vision.
 Tanning of the skin by exposure to the sun.

Which of the unsaturated fatty acids shown below are much more frequent in the nature?
Tick the correct answer.

 Oleic acid and arachidonic acid?
 Elaidic acid and linoelaidic acid?