it is shown that photoexcited propanodeoxyguanosine (PdG), in which out-of-plane deformation of the six-membered ring of the guanine moiety is hindered, does not exhibit the subpicosecond internal conversion characteristic of the naturally occurring purine bases. The result supports the theoretical conclusion that the ultrafast internal conversion occurs via the conical intersection of the electronic ground state with the biradical state, which either evolves from pi pi* state! for large out-of-plane deformation or crosses the pi pi* state at higher energy. (C) 2007 Elsevier B.V. All rights reserved.