You probably know it as: Retinal
Last week I wrote about betacarotene, the chemical in carrots responsible for their orange color. I mentioned that although betacarotene was necessary vision, eating a ton of it won't give you supervision. As I said, betacarotene is converted to Vitamin A, which is converted to retinal. Retinal is a cool chemical with even cooler chemistry. You may know that in your eyes you have light receptors called rod cells, or "rods". These cells are responsible for low light vision. Your rods are so sensitive that in a perfectly dark room you would be able to detect a single photon! This is what rod cell looks like:
You'll see that the outer segment has membrane shelves that are lined with rhodopsin. Rhodopsin is a protein containing 348 amino acids. Retinal sits wrapped neatly inside rhodpsin, bound to the amino acid lysine. In a completely dark room the retinal in your eyes will be in the "11-cis" conformation. This is a bent conformation which allows retinal to fit inside rhodopsin. When a photon is absorbed by retinal it changes the conformation to the "all-trans" conformation, and retinal no longer fits inside or rhodopsin.
Rhodopsin then unravels and this sets off a cascade of chemical reactions that end with a signal being sent to the brain. What's even more amazing is how well we understand that cascade. But that's a story for a different day...