Nina K. Schwalb

Base Sequence and Higher-Order Structure Induce the Complex Excited-State Dynamics in DNA

The high photostability of DNA is commonly attributed to efficient radiationless electronic relaxation processes. We used femtosecond time-resolved fluorescence spectroscopy to reveal that the ensuing dynamics are strongly dependent on base sequence and are also affected by higher-order structure. Excited electronic state lifetimes in dG-doped d(A)20 single-stranded DNA and dG·dC-doped d(A)20·d(T)20 double-stranded DNA decrease sharply with the substitution of only a few bases. In duplexes containing d(AGA)·d(TCT) or d(AG)·d(TC) repeats, deactivation of the fluorescing states occurs on the subpicosecond time scale, but the excited-state lifetimes increase again in extended d(G) runs. The results point at more complex and molecule-specific photodynamics in native DNA than may be evident in simpler model systems.

Ultrashort Fluorescence Lifetimes of Hydrogen-Bonded Base Pairs of Guanosine and Cytidine in Solution

The optically excited electronic states of hydrogen-bonded homo- and heterodimers of guanosine (G) and deoxycytidine (C) were investigated by femtosecond fluorescence up-conversion spectroscopy. The base pairs were prepared in CHCl(3) solution by employing tert-butyldimethylsilyl (TBDMS) groups at the OH positions of the ribose (G) or deoxyribose (C) moieties to enhance the solubilities of the nucleosides in organic solvents. The H-bonded complexes that were obtained were characterized by FTIR spectroscopy. Fluorescence lifetime measurements were performed following electronic excitation at a series of UV wavelengths from lambda(pump) = 294 nm, close to the electronic origins of the bases, to lambda(pump) = 262 nm, where significant excess vibronic energy is deposited in the molecules, at nucleoside concentrations of c(0) = 0.1 and 1.0 mM. The experimental results revealed the existence of an ultrafast deactivation pathway for the optically prepared electronically excited state(s) of the G.C Watson-Crick base pair, which was found to have a lifetime of tau(GC) = 0.30(3) ps (with 2sigma error limits) irrespective of the pump wavelength. A similar short decay time, tau(GG) = 0.32(2) ps, was observed for the respective excited G.G homodimer. In contrast, the excited G monomer displayed a significantly longer-lived and wavelength-dependent deactivation, requiring three time constants, between 0.43(6) ps < or = tau(G,1) < or = 1.2(1) ps, 4.2(8) ps < or = tau(G,2) < or = 8(1) ps, and tau(G,3) = 195(32) ps. Self-complexation of C, on the other hand, led to a longer-lived excited state with a lifetime estimated between 1 ps < or = tau(CC) < or = 10 ps, compared to the dominant initial subpicosecond decay time of the C monomer of tau(C,1) = 0.80(4) ps.

A modified four-state model for the "dual fluorescence" of N(6),N(6)-dimethyladenine derived from femtosecond fluorescence spectroscopy.

The radiationless deactivation of the excited electronic states of the dual fluorescence molecule N(6),N(6)-dimethyladenine (DMAde) was investigated using femtosecond time-resolved fluorescence up-conversion spectroscopy. The molecules were studied in solution in water and in dioxane. Fluorescence-time profiles were recorded in the wide wavelength range of 290 <or= lambda(fl) <or= 650 nm. The excitation wavelengths in the region of the first UV absorption band were tuned from close to the electronic origin (lambda(pump) = 294 nm) to excess energies of approximately 5400 cm(-1) above (lambda(pump) = 258 nm). Global fits to the measured curves turned out to reflect distinctive molecular relaxation processes on five well-defined time scales. Sub-100 fs and 0.52(3) ps lifetimes were found to predominate at the shortest UV and blue emission wavelengths in water, 1.5(1) and 3.0(2) ps components at intermediate wavelengths and a 62(1) ps value in the red region of the spectrum (2sigma error limits of the last digits in parentheses). In dioxane, these lifetimes changed to <or=0.27 and 0.63(4) ps in the UV, 1.5(1) and 10.9(10) ps in a wide range of intermediate, and 1.40(4) ns at the longest wavelengths. However, little dependence of the respective time constants on lambda(pump) was observed, indicating that the ensuing relaxation processes proceed via practically barrierless pathways through conical intersections. Building on the knowledge for the parent molecule adenine (Ade), the observations were rationalized with the help of a modified four-state model for the electronic dynamics in DMAde with the pipi*(L(a)), pipi*(L(b)), and npi* states similar to those in Ade and an intramolecular charge-transfer (ICT) state, which has no counterpart in Ade, responsible for the long-wavelength fluorescence.

Electronic Deactivation of Guanosine in Extended Hydrogen-Bonded Self-Assemblies

Guanosine (G) derivatives in nonpolar aprotic solvents self-assemble to intricate hydrogen-bonded supramolecular architectures, including dimers, ribbons, and cyclic quartets. Considerable interest exists in the nature of the excited electronic states, their lifetimes and the radiationless deactivation mechanisms of the molecules in those environments. Here, we report on the electronic relaxation of G in the extended H-bridged networks in solution in n-hexane. The resulting architectures were sampled by FTIR, UV, and CD spectroscopies. The dynamics after 260 nm photoexcitation were investigated by femtosecond fluorescence up-conversion, broadband UV-vis absorption, and single-color deep-UV measurements. The observed temporal profiles reveal a hierarchy of relaxation processes, with lifetimes τ1 = 0.63 ± 0.03 ps, τ2 = 5.9 ± 0.3 ps, and τ3 = 62 ± 7 ps. Moreover, about 10% of the photoexcited molecules transform to much longer-lived product states with lifetime τ4 ≈ 3.6 ± 1.0 ns. These excited-state lifetimes are much longer than in the G monomer or the G·G dimers studied previously, hinting at sizable energy shifts among the excited ππ* and nπ* states and trapping of excited-state population in the supramolecular networks by potential energy barriers along the optimal electronic deactivation pathways of the molecules.

A femtosecond time-resolved investigation of dual fluorescence from N6,N6-dimethyladenine.

The excited electronic state dynamics of N(6),N(6)-dimethyladenine (DMAde), a molecule known to emit dual fluorescence, has been studied in aqueous solution using femtosecond fluorescence up-conversion spectroscopy. Time profiles of the fluorescence of DMAde excited at lambda= 258 nm were measured at a series of wavelengths in the range 320 nm <or=lambda(fl)<or= 650 nm. At wavelengths in the near UV (lambda(fl)<or= 420 nm), the time profiles were dominated by a very short-lived decay component with a lifetime between 0.2 ps <or=tau(1)<or= 0.6 ps, depending on the detection wavelength. Two other components with lifetimes of tau(2) approximately 3.5 ps and tau(3) approximately 60 ps gave minor contributions. In the emission observed at longer wavelengths (lambda(fl)>or= 500 nm), which appeared slightly delayed compared to the UV fluorescence, the long-lived fluorescence component (tau(3)) dominated, the second component (tau(2)) disappeared. The results are consistent with the assumption that DMAde is primarily excited to a short-lived local excited (LE) electronic state that fluoresces mostly in the UV and decays rapidly, on a approximately 0.5 ps timescale, to an intramolecular charge transfer (ICT) state that emits only at longer wavelengths in the visible spectrum. The fluorescence-time profiles and transient fluorescence spectra reconstructed from the time profiles provided further information on secondary relaxation processes within and between the excited states and their non-radiative relaxation to the electronic ground state.

Femtosecond Fluorescence Spectroscopy of N6,N6-Dimethyladenine: New Explanation of the “Dual Fluorescence” Dynamics from Decay and Rise Time Measurements at Threshold

Femtosecond time-resolved fluorescence measurements on N6,N6-dimethyladenine in a wide wavelength range following excitation at threshold and much higher show identical dynamics, requiring a new explanation for the so-called “dual fluorescence” of the molecule.