For the first time, the fundamental chemical process that underlies sight has been reconstructed in detail. It is one of the fastest processes in nature. It lasts just a trillionth of a second and takes place every time light hits the retina.
The important scientific milestone was reached by a group of scientists led by the Swiss Paul Scherrer Institute (PSI). The study, published in the journal Nature, literally sheds light on a molecular process that until now no one had been able to observe in real time, but which is the very first step in our ability to perceive light.
How to turn on the 'eye switch'
The protagonist of the scene is the human receptor of light, rhodopsin: fixed at the center of this protein there is a small molecule, the retinal, which is a derivative of vitamin A.
When light hits rhodopsin, the retinal absorbs some of the energy and changes its three-dimensional shape in a fraction of a second, thus 'turning on' the eye switch and triggering the cascade of reactions that underlie the sense of sight. The starting and ending point of this mechanism has long been known, but researchers led by Valérie Panneels and Gebhard Schertler have now also revealed what happens during the very rapid transition from one form of the protein to another.
The study authors found that when rhodopsin absorbs light, it uses some of the energy to 'swell' slightly, much like our chest does when we inhale air. During this sort of 'breathing', the protein momentarily loses contact with the retinal molecule that is inside it, thus leaving it free to rotate. Immediately after the rhodopsin returns to contract and 'trap' the retinal, only now it must adapt to the new conformation: the process lasts in all only a picosecond, that is a trillionth of a second.