A discovery by a team of researchers has shed new light on how the vision process is initiated.
The research team, led by a Syracuse University physicist, has demonstrated that visual signals can be initiated in the absence of isomerization.
"We have demonstrated that chromophores (light-absorbing substances in retinal photoreceptor molecules), do not have to change shape in order to trigger the visual signal," said Kenneth Foster, professor of physics in SU's College of Arts and Sciences.
"The shape-change that results from isomerization is actually the second step in the process. Historically, scientists have focused on isomerization without realizing there is an earlier and more crucial first step," added Foster.
Chromophores absorb light after it enters the eye, setting off an extremely rapid series of complex molecular changes that enable light signals to be transmitted to, and interpreted by, the brain so that we can visually perceive the world around us. Visual chromophores are composed of retinal (a type of vitamin A), which attaches to a protein (opsin) to form rhodopsin.
Foster's team of researchers discovered that the visual process is initiated by the redistribution of electrons on the chromophores, which occurs during the first few femptoseconds (one-quadrillionth of a second) after light enters the eye. Their experiments showed that when a chromophore absorbs a photon of light, electrons move from the chromophore's "free" end to the place where it attaches to opsin. The movement of the electrons causes a change in the electrical field surrounding the chromophore. That change is detected by nearby amino acids that are highly sensitive to changes in the electrical field. These amino acids, in turn, signal the rest of the rhodopsin molecule to initiate the visual process.
"We found that the complete blocking of isomerization of the chromophore does not preclude vision in our model organism," said Foster.
"The signal is triggered as a result of an electronic coupling instead of a geometric change in the chromophore's structure as previously hypothesized. We believe this is a universal mechanism that activates all rhodopsins present in organisms from bacteria to mammals," added Foster.
The study has been published online in the journal Chemistry and Biology.