![]() This generates nerve impulses which travel along the optic nerve to the brain, and we perceive them as visual signals - sight. These rapid movements of the retinal are tranfered to the protein, and from there into the lipid membrane and nerve cells to which it is attached. The linkage between retinal and opsin becomes unstable, and the molecule undergoes a series of shape changes to try and better fit the binding site, before eventually breaking free of the opsin altogether. Whereas the 11- cis-retinal fitted into the opsin binding-site perfectly, all- trans-retinal is the wrong shape. Essentially, the energy in a photon has been converted into atomic motion. Thus, the light has isomerised the molecule from cis to trans, and as it did so, it changed the shape of the retinal from curved to straight. ![]() The double bond then reforms and locks the molecule back into position in a trans configuration. ![]() This means the molecule can now rotate around this bond, which it does by swivelling through 180°. When a photon of light falls onto rhodopsin, the molecule absorbs the energy and the cis-double-bond between C-11 and C-12 in the retinal is temporarily converted into a single bond. The peak of the absorption is around 500 nm, which matches the output of the sun closely. Opsin does not absorb visible light, but when it is bonded with 11- cis-retinal to form rhodopsin, the new molecule has a very broad absorption band in the visible region of the spectrum. In real-time, retinal exists as a 'resonance hybrid' (bottom) of the left- and right-hand structures. 'Resonance structures' of retinal - the high energy electrons in the double bonds can flow quickly in the course shown by the arrows. It is this extensive delocalisation of the electrons that allow retinal to absorb light so strongly. Retinal itself is what chemists refer to as 'delocalised' - hopefully the graphic below will give you an idea of what we mean by this the electrons contained in the delocalised system exist in a high energy 'cloud' above and below the plane of the structure. The trans- ( E) version is long and straight, whereas the cis- ( Z) version is bent in two. A more modern nomenclature uses the letters E (from the German, entgegen apart) and Z (from zusammen together). The other double bonds are all - trans, or with the bulky substituents positioned on opposite sides. The - cis prefix comes from the fact that one of the double bonds (at the 11th carbon) has the two largest substituents (that is, the largest chains coming off it) on the same side. Retinal comes in two forms, 11- cis- and all- trans. The diagram on the right shows the rods (thin) and cones (more thickly-formed) as pink forms embedded in the anterior of the retina's surface. ![]() Each one can only ever be on or off, but when we consider all of the signals in unison we can see a picture - a bit like pixels in a digital camera. So what makes Rhodopsin special as a molecule - how is it that it allows us to see? Firstly, it's important to realise that there is, of course, not just one, but lots of rod cells (about 100 million) coating the retina of our eyes. The cone cells, which respond to the whole spectrum of colours, require a much higher threshold of light than is present in the dark room to be triggered. They can only respond to it in a black and white fashion though, which is why everything looks grey. Have you ever noticed when you're in bed and can't sleep that once your eyes have become accustomed to the gloom, you can actually see the outlines of items in the room quite clearly? They all look grey and colourless, but the reason we can see these outlines at all is because our rod cells are sensitive to very low levels of light. The native form of opsin from bovine rod cells. This is where the terms 'rods' (think Rhodopsin) and 'cones' come from, referring to cells in the retina of our eyes which contain rhodosin and isodoposin pigments, respectively. Together they make up rhodopsin (also known as 'visual purple', the structure of which is shown below. Retinal, or more correctly, 11- cis-retinal, is a small molecule which fits into the binding site of a large protein called opsin. Retinal is an interesting molecule because it is the reason we are able to see. Also available: HTML, Chime, and VRML versions.
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