|MadSci Network: Molecular Biology|
John, I will assume that you know how to use a spectrophotometer to determine the concentrations of compounds dissolved in solution, and I will tell you why DNA absorbs UV light. All spectrophotometers, including those that measure UV, operate on the same principle. The electronic structure of matter determines its chemical properties and the ability of matter to absorb energy, whether the matter is in the form of molecules, atoms, or ions, and whether the matter is solid, liquid, gaseous, or dissolved in solution. According to quantum theory, the electrons in matter that make up its electronic structure can exist in only certain discrete states. The lowest possible energy level of a molecule is called its ground state, and as one or more of the electrons in the molecule become excited by the absorption of energy, the overall electronic energy of the molecule moves to a higher energy state. One of the ways a molecule can absorb energy is by absorbing electromagnetic radiation in the form of a photon, provided that the energy of photon matches the difference between two different energy states of the molecule’s electronic structure. You will remember from physics that electromagnetic radiation exists as a continuous spectrum, from very high-energy cosmic radiation to very low energy radio waves. The reason that DNA absorbs UV radiation is that the energy of a UV photon matches the difference between two different energy states of DNA’s electronic structure. DNA is a very complex molecule electronically; the part of the molecule that is thought to absorb most of the UV radiation is the aromatic ring structure of the purine and pyrimidine bases. Chris
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