Shedding light on retinal. A contribution to the understanding of the primary process of vision
In our eyes light is absorbed by a polyene molecule called retinal. This molecule contains six double bonds and is bonded to a protein. The protein-chromophore complex is called rhodopsin and has been structurally determined by crystallography.
It was early suggested that the main event in vision is a cis-trans isomerization of retinal. By using time resolved spectroscopy it has been shown that the initial excited state relaxation is completed in 200 femtoseconds. The excited state relaxation has been suggested to involve a downhill reaction path leading to an avoided crossing connecting the excited state and the ground state. At this avoided crossing one of the double bonds is twisted 90 degrees, i.e "half" of the cis-trans isomerization is complete.
However, for retinal to reach such an avoided crossing large geometrical changes are required. This is unlikely to take place in 200 femtoseconds inside the tight protein cavity where retinal is located. The short time of 200 femtoseconds indicates that the nuclei of retinal or the nearby amino acids are not moved any significant distance. In the present thesis it has been found that a downhill reaction path exists also for a constrained motion consistent with crowded conditions. This reaction path was found by imposing a minimal motion of the nuclei and can be used during the short time of 200 femtoseconds.
quantum mechanics/molecular mechanics