title: "Bright" and "dark" excited states of an alternating at oligomer characterized by femtosecond broadband spectroscopy.
abstract: 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 (< approximately 50 fs) "bright" S(LE) state and its ensuing evolution within approximately 3 ps into a approximately 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 approximately 0.3 ps conversion of the S(LE) to S(G) that then evolves by approximately 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.
authors.affiliation: Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. bcwmkwok@polyu.edu.hk
authors.forename: Wai-Ming
authors.initials: WM
authors.lastname: Kwok
authors.forename: Chensheng
authors.initials: C
authors.lastname: Ma
authors.forename: David Lee
authors.initials: DL
authors.lastname: Phillips
mesh_terms: D001704:Biopolymers / Q000737:chemistry*; D004247:DNA / Q000737:chemistry*; D005453:Fluorescence; D013056:Spectrophotometry, Ultraviolet / Q000379:methods*
chemical_list: D001704:Biopolymers; D004247:DNA