Chemists at Ohio State University have probed an unusual high-energy state produced in single nucleotides — the building blocks of DNA and RNA — when they absorb ultraviolet (UV) light. This is the first time scientists have been able to probe the “dark” energy state — so called because it cannot be detected by fluorescence techniques used to study other high-energy states created in DNA by UV light.
The study suggests that DNA employs a variety of means to dissipate the energy it absorbs when bombarded by UV light.
Scientists know that UV light can cause genetic alterations that prevent DNA from replicating properly, and these mutations can lead to diseases such as cancer.
The faster a DNA molecule can dissipate UV energy, the lesser the chance that it will sustain damage — so goes the conventional scientific wisdom. So the dark states, which are much longer lived than previously known states created by UV light, may be linked to DNA damage.
The existence of this dark energy state — dubbed np* (pronounced “n-pi-star”) — had previously been predicted by calculations. Other experiments hinted at its existence, but this is the first time it has been shown to exist in three of the five bases of the genetic code — cytosine, thymine and uracil.
The detection of this dark state in single bases in solution increases the chances that it may be found in the DNA double helix, said Bern Kohler, associate professor of chemistry at Ohio State and head of the research team.
The Ohio State chemists determined that, when excited by ultraviolet light, these three bases dissipate energy through the dark state anywhere from 10-50 percent of the time.
The rest of the time, energy is dissipated through a set of energy states that do fluoresce in the lab. These “bright” energy states dissipate the energy much faster, in less than one picosecond.
A picosecond is one millionth of one millionth of a second — an inconceivably short length of time. Light travels at 186,000 miles per second, but in twenty picoseconds it would only travel just under a quarter of an inch. Still, a picosecond is not so fast compared to the speed of some chemical reactions in living cells.
In tests of single DNA bases, the dark state lasted for 10-150 picoseconds — much longer than the bright state. The chemists reported their results in the Proceedings of the National Academy of Sciences.