Ultrafast electronic deactivation dynamics of the inosine dimer--a model case for H-bonded purine bases.

The structural properties and ultrafast electronic deactivation dynamics of the inosine dimer in CHCl3 have been investigated by two-dimensional (1)H NMR and static FTIR spectroscopy and by femtosecond time-resolved transient absorption spectroscopy, respectively. The (1)H NMR and IR spectra show the formation of a well-defined, symmetric dimer with an association equilibrium constant of KI·I = 690 ± 100 M(-1). The excited-state dynamics after photoexcitation at λpump = 260 nm monitored by ultrafast absorption spectroscopy show great similarity with those of the monomer inosine in an aqueous solution and are governed by a decay time of τ = 90 ± 10 fs, which is one of the shortest electronic lifetimes of all nucleobases and nucleobase dimers studied so far. On the basis of these observations, the inosine dimer is expected to follow a similar relaxation pathway as the monomer, involving an out-of-plane deformation of the six-membered ring. The importance of the C(2) position for the electronic deactivation of hypoxanthine and guanine is discussed. The obtained well-determined structure and straightforward dynamics qualify the inosine dimer as an excellent reference case for more complicated systems such as the G·G dimer and the G·C and A·T Watson-Crick pairs.