An energetically favorable electrostatic interaction between the chromophore 11-cis-retinal and the apoprotein opsin—the two major components of the ubiquitous visual pigment protein rhodopsin—causes nature to select the 11-cis isomer over the 7-cis-, 9-cis-, or 13-cis-retinal isomers, a computational study indicates (J. Am. Chem. Soc., DOI: 10.1021/ja208789h). Scientists have long wondered why rhodopsin evolved to favor 11-cis-retinal, given that exposure to light in all of the isomers causes only a conformational change in the alkenyl chain, rather than chemical bond breakage. Emory University’s Sivakumar Sekharan and Keiji Morokuma used a blend of quantum mechanics and molecular mechanics to investigate retinal isomer structure, stability, and energetics. They found that 11-cis-retinal is the most stable. However, the researchers note that the 9-cis isomer’s energy is only slightly higher than that of 11-cis, which they say makes a case for the possible existence of 9-cis-rhodopsin in nature.