Tracking DNA excited states by picosecond-time-resolved infrared spectroscopy: Signature band for a charge-transfer excited state in stacked adenine-thymine systems

UV photoexcitation of an adenine-thymine heterodimer (ApT) in D 2O yields a complex transient infrared signature in the 1500-1600 cm-1 spectral region. The spectral dynamics fit well to a biexponential decay assignable to two transient species. The first, a short-lived species with a lifetime of ca. 5 ps, originates from the vibrationally hot electronic ground state of the unstacked form of the dinucleotide. The second species is longer-lived (ca. 75 ps), and its yield correlates to the amount of stacked dinucleotide present in solution. We assign the longer-lived component to a charge-transfer (A•+pT •-) state by comparison with calculated spectra for the adenine radical cation and thymine radical anion. Significantly, the CT feature is also identified in UV-excited [poly(dA-dT)]2. This experimental observation gives a powerful insight into how base-base interactions lead to extended-lifetime electronic excited states of the nucleic acid bases and how a dimeric structure controls the relaxation pathway. © 2013 American Chemical Society.