Decay Pathways of Pyrimidine Bases: From Gas Phase to Solution

Abstract

We use a variation of the pump-probe technique to unravel the photodynamics of nucleic acid bases and their water complexes. Our work aims at bridging studies from the gas phase with those in the solution phase. Our results indicate that the intrinsic properties of the pyrimidine bases can be dramatically modified by the surrounding environment. As isolated species, the bases exhibit fast internal conversion into a long lived dark state. We present evidence and discuss the nature of this electronic state. When surrounded by water molecules, however, the bases in the dark state can be quenched effectively, and the dark state becomes unobservable to our nanosecond laser system. Although contradictive to the long held belief that DNA bases possess intrinsic photostability under UV irradiation, our conclusion offers a consistent explanation to all reported experimental and theoretical results, both prior to and after our work. The long lifetime of the dark state implies that in the early stages of life’s evolution prior to the formation of the ozone layer, the abundant UV flux should have limited the existence of these bases, let alone their evolution into complex secondary structures. A protective environment, such as water, is crucial in the very survival of these carriers of the genetic code