“Bright” and “Dark” Excited States of an Alternating AT Oligomer Characterized by Femtosecond Broadband Spectroscopy

The nature and dynamics of DNA excited states is of pivotal importance in determining both DNA ultraviolet photostability and its vulnerability toward photodamage. The complexity regarding the involvement of “bright” and “dark” excited states, their molecular origin, and the roles played by these states in the course of electronic energy relaxation constitute an active and contentious area in current research of DNA excited states. As a case study, we report here a combined broadband femtosecond time-resolved fluorescence (TRF) and transient absorption (TA) study on a self-complementary d(AT)₁₀ oligomer and a reference system of an equal molar mixture of the constituent bases represented by adenosine and thymidine (A+T). Comparison of the spectral character and temporal evolution of the TRF and TA data for 267 nm excited d(AT)₁₀ and A+T provides evidence for a base-localized excitation feature for an early (<∼50 fs) “bright” S_LE state and its ensuing evolution within ∼3 ps into a ∼72 ps “dark” S_E exciplex in d(AT)₁₀. Combined analysis of the d(AT)₁₀ TRF and TA results suggests the presence of a weakly fluorescent transient S_G state that acts as a gateway to mediate the excitation transfer and energy elimination. A distinct base conformation-dependent model involving an ultrafast ∼0.3 ps conversion of the S_LE to S_G that then evolves by ∼3 ps into the S_E has been proposed to account for the collective deactivation character of d(AT)₁₀. This presents a novel excited-state picture that can unify the seemingly conflicting time-resolved results reported previously for related AT DNAs. The direct spectral and dynamical data provided here contributes important photophysical parameters for the description of the excited states of AT oligomers. The possible connection between the energy transfer giving the S_E and the photostability vs photodamage of A/T DNAs is briefly discussed.