Researchers at Emory, USA, and Kyoto, Japan, Universities used hybrid quantum mechanics/molecular mechanics to investigate the structure and stability of pigments in the eye. They hoped to understand why evolution naturally selected the 11-cis form of retinal in the visual pigment rhodopsin, rather than any of the other possible forms of 7-cis-, 9-cis-, or 13-cis-retinal. This question has remained a basic and unresolved puzzle in vision biochemistry for many years.

Their study suggests that electrostatic interactions between retinal and opsin predominate in the natural selection of 11-cis-retinal over other cis isomers in the dark state. This makes the light-sensitive pigment rhodopsin strongest with the 11-cis isomer.

• Why 11-cis-Retinal? Why Not 7-cis-, 9-cis-, or 13-cis-Retinal in the Eye?
  S. Sekharan, K. Morokuma,
  J. Am. Chem. Soc. 2011.
  DOI: 10.1021/ja208789h