Femtosecond Time- and Wavelength-Resolved Fluorescence and Absorption Spectroscopic Study of the Excited States of Adenosine and an Adenine Oligomer

Abstract

By employing broadband femtosecond Kerr-gated time-resolved fluorescence (KTRF) and transient absorption (TA) techniques, we report the first (to our knowledge) femtosecond combined time- and wavelength-resolved study on an ultraviolet-excited nucleoside and a single-stranded oligonucleotide (namely adenosine (Ado) and single-stranded adenine oligomer (dA)(20)) in aqueous solution. With the advantages of the ultrafast time resolution, the broad spectral and temporal probe window, and a high sensitivity, our KTRF and TA results enable the real time monitoring and spectral characterization of the excited-state relaxation processes of the Ado nucleoside and (dA)(20) oligonucleotide investigated. The temporal evolution of the 267 nm excited Ado KTRF spectra indicates there are two emitting components with lifetimes of approximately 0.13 ps and approximately 0.45 ps associated with the L(a) and L(b) pipi* excited states, respectively. These Ado results reveal no obvious evidence for the involvement of the npi* state along the irradiative internal conversion pathway. A distinct mechanism involving only the two pipi* states has been proposed for the ultrafast Ado deactivation dynamics in aqueous solution. The time dependence of the 267 nm excited (dA)(20) KTRF and TA spectra reveals temporal evolution from an ultrafast "A-like" state (with a approximately 0.39 ps decay time) to a relatively long-lived E(1) "excimer" (approximately 4.3 ps decay time) and an E(2) "excimer-like" (approximately 182 ps decay time) state. The "A-like" state has a spectral character closely resembling the excited state of Ado. Comparison of the spectral evolution between the results for Ado and (dA)(20) provides unequivocal evidence for the local excitation character of the initially photoexcited (dA)(20). The rapid transformation of the locally excited (dA)(20) component into the delocalized E(1) "excimer" state which then further evolves into the E(2) "excimer-like" state indicates that base stacking has a high ability to modify the excited-state deactivation pathway. This modification appears to occur by suppressing the internal conversion pathway of an individually excited base component where the stacking interaction mediates efficient interbase energy transfer and promotes formation of the collective excited states. This feature of the local excitation that is subsequently followed by rapid energy delocalization into nearby bases may occur in many base multimer systems. Our results provide an important new contribution to better understanding DNA photophysics.

Ultrafast excited-state dynamics of RNA and DNA C tracts

“…However, this appears unlikely given the sensitivity of excited-state dynamics to secondary structure seen here and elsewhere [7][8][9]20]. Calculations aimed at understanding how excited states of nucleobase multimers depend on local structure are just beginning to appear [61][62][63][64], and these will help to resolve these issues.…”

“…In recent years, ultrafast laser spectroscopy has provided many new insights into excited states in nucleic acids [3][4][5][6][7][8][9][10]. Experiments on single bases are easier to interpret than those on multiple base systems ("base multimers"), and are more amenable to quantum chemical modeling [11][12][13][14][15].…”

Proton/Hydrogen Transfer Mechanisms in the Guanine–Cytosine Base Pair: Photostability and Tautomerism

“…43,44,45,46 It is interesting to mention that measurements on NAB monomers in aqueous phase provided low fluorescence quantum yields, which suggests that the NABs ultrashort lifetimes are intrinsic molecular properties. 47,48,49,50,51 New photochemical paths have been more recently determined in π-stacked NABs, 52,53,54,55,56,57,58,59,60,61,62,63,64 which compete with the photostable routes present in the nucleobase monomers and explain the formation of the CPD lesions 59,60,61,62,64 found in UV-irradiated DNA polynucleotides. 65 Excited-state dimers of adjacent NABs (bioexcimers) have shown to be the precursors in the photoreactive process leading to the lesion.…”

Influence of base stacking on excited-state behavior of polyadenine in water, based on time-dependent density functional calculations

“…Indeed, time-resolved spectroscopic studies (1) and quantum mechanical calculations (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11) agree in suggesting the existence of an ultrafast nonradiative pathway between the bright excited state and the ground electronic state for both pyrimidine and purine nucleobases, which can explain their extremely short (Յ1-ps) excited-state lifetimes. However, the excited-state behavior of DNA and nucleobase multimers (the systems of actual interest in vivo) appears much more complex, showing multiexponential decays with very different time constants (12)(13)(14)(15)(16)(17)(18). Recent experimental studies on single-stranded polyadenine (polyA) oligomers [(dA) n ] and double-stranded thymine adenine oligonucleotides [(dT) n /(dA) n ] provide deeper insight into the behavior of the multimer excited states and the factors tuning their decay (12)(13)(14)(15)(16)(17)(18).…”

“…However, the excited-state behavior of DNA and nucleobase multimers (the systems of actual interest in vivo) appears much more complex, showing multiexponential decays with very different time constants (12)(13)(14)(15)(16)(17)(18). Recent experimental studies on single-stranded polyadenine (polyA) oligomers [(dA) n ] and double-stranded thymine adenine oligonucleotides [(dT) n /(dA) n ] provide deeper insight into the behavior of the multimer excited states and the factors tuning their decay (12)(13)(14)(15)(16)(17)(18). In (dA) 18 single-strand and (dA) 18 ⅐(dT) 18 double-strand, a fast initial decay, in the subpicosecond time range, is indeed followed by slower relaxations (lifetimes of Ϸ0.8 and Ϸ126 ps, respectively) (12,13).…”