Re: Why does DNA absorb UV light?

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