Quantum yields for thymine photodimerization (Phi(TT)) have been determined for a series of short DNA single-strand and base-paired hairpin structures possessing a single thymine-thymine step with flanking purines. Values of Phi(TT) are strongly dependent upon the oxidation potential of the flanking purine, decreasing in the order: inosine > adenine > guanine > deazaguanine. The dependence of Phi(TT) on the ionization potential of the flanking purine is more pronounced when the purine of lower oxidation potential is located at the 5'- versus 3'-position in either a single strand or a hairpin. Molecular dynamics simulations for hairpin structures indicate that the TT step is pi-stacked with both the 5' and 3' purine, but that there is little pi-stacking with either purine in single-strand structures. The observation of moderately intense long-wavelength UV absorption features for hairpins having 5'-Z or G flanking purines suggests that excitation of ground state donor-acceptor complexes may account for more extensive quenching of dimerization by 5'- versus 3'-purines. The "purine effect" on Phi(TT) is attributed to a combination of ground state conformation, ground state electron donor-acceptor interactions, and excited state exciplex formation.