Generations of parents have nagged their veggie-phobic kids to eat their carrots, promising that vegetables rich in vitamin A will give them sharper vision. The “eat-your-carrots” wives’ tale evolved in the early part of this century, when scientists discovered that a vitamin A deficient diet causes a condition called night blindness, or nyctalopia, where a person’s eyes are unable to adjust to low light levels. In World War I, infantry men would sometimes supplement their rations with foods containing vitamin A, such as liver and leafy green vegetables, in order to counteract the symptoms of night blindness and see in the trenches at night.
Assistant professor of physics Daniel Aalberts is fascinated with the mechanics of vision, and is combining the disciplines of biology and physics to better understand the crucial function of vitamin A in eyesight. Looking under a microscope at a piece of retina, the inner eye tissue where the lens focuses light, reveals a thick carpet of cells resembling rods and cones. Scores of physiologists have mapped and labeled the rods and cones, and have studied the biochemistry of nerve impulses along the optic nerve. But Professor Aalberts is studying what happens before all of that, at the first instant when light touches these cells.
An enlarged rod cell looks something like a stack of pancakes piled on top of some nerve gadgetry. Each pancake is coated with millions of proteins called rhodopsin that look something like bulbous blueberries, each filled with a single molecule of vitamin A. In the dark, the vitamin A nestled snugly inside the blueberry looks like a bent twig, with carbon atoms attached to each other in two branches that bend at a double bond. At the instant a photon of light strikes rhodopsin, the vitamin A absorbs its energy and snaps into a straight form in one trillionth of a second, which is less time than it takes light to cross the width of a strand of hair. When the bent molecule is fully straightened, it changes the shape of the blueberry around it and exposes an important binding site on this protein, catalyzing a chain of chemical reactions that produce a nerve impulse.
What’s remarkable about this lightening fast process is that it happens with an efficiency of 67 percent; when 100 rhodopsin proteins each absorb a photon of light, at least 67 of them snap into straight configurations. If we were to sit in a dark room with a “magic light bulb” that emitted individual photons of light, our highly efficient rod cells would catch almost every tiny spark of light coming from the bulb. Until recently, scientists have been perplexed by how rhodopsin changes shape so quickly and efficiently.
According to Professor Aalberts, rhodopsin has been a slippery protein to work with. Physicists have only been able to observe vitamin A-absorbing light within the past ten years, as advances in laser technology have made it possible to measure extremely fast photoreactions precisely. In addition to its fast shape changes, vitamin A is also a large molecule with many double bonds, which makes it difficult to observe what’s happening at the special “hinge” bond where the bending occurs.
Aalberts’s solution to these obstacles was to use ethyleneâ€”one of the simplest, most mundane organic moleculesâ€”as a theoretical model for the elusive rhodopsin. Describing his work with ethylene, Professor Aalberts contains his enthusiasm as well as a child at the entrance of a Toys-R-Us would. In ethylene, he explains, two carbon molecules are connected by a double bond, with two hydrogen atoms sticking off on either end. A key idea in this research is that the double bond keeps the whole ethylene molecule flat. When you clip one of the bonds between the carbons, however, the molecule is then able to twist into a more relaxed three dimensional shape.
Professor Aalberts and his thesis student, Brian Gerke ’99, have not actually conducted physical experiments on ethylene, but have instead collected data from different laboratories and made extensive calculations to create a theoretical model of how much energy is needed to bend ethylene at different angles. Even using the powerful workstations in his lab â€“ which are roughly as fast as Colrain â€“ it often takes the computers three hours to complete a batch of computations.
While Professor Aalberts was muscling through these marathon calculations for ethylene, he was struck by a revelation that gave new momentum to his research. One of the mysteries of rhodopsin that has confounded physicists in the past was that in their calculations, there simply was not enough energy in the vitamin A molecule to change into a “straight twig” and stretch the protein around it. A molecular principal that chemists have been aware of for a long time is called “sterics,” which are the forces of repulsion from different atoms within a molecule that lead to certain molecular shapes. Professor Aalberts’s epiphany was that sterics were a beautifully simple explanation for why ethylene and vitamin A have such dramatic conformational changes. Certain pieces of the vitamin A molecule have strong repulsion forces against each other, but are kept under strain because the rhodopsin protein doesn’t allow the bent molecule to straighten out. When light breaks the important double bond in vitamin A, however, all of the strain is suddenly released and the molecule can finally relax into a lower energy state. Professor Aalberts and Brian Gerke are two-thirds of the way towards publishing their work, and Aalberts is preparing to give a presentation of his research before the American Physical Society. Now that they’ve made calculations on a simplified model of vitamin A, they’re ready to make the next step forward, which involves considering the molecule as a whole.
Before hearing about with Professor Aalberts’s research, one probably has never stopped to think about what actually happens inside an eye every the eyelid opens. Eyes are miraculous machines that collect something abstract and intangible, and produce nerve impulses that end up as crystal clear images in our brains. The complexity of rod cells and twisting molecules can be a little unnerving. Maybe it is best just to listen to Mom and eat our carrots.