The solvent effect and identification of a weakly emissive state in nonradiative dynamics of guanine nucleosides and nucleotides — a combined femtosecond broadband time-resolved fluorescence and transient absorption study

Abstract

A combined method of femtosecond broadband time-resolved fluorescence (fs-TRF) and transient absorption (fs-TA) was employed to investigate the excited state dynamics of 2'-deoxyguanosine (dG) and 2'-deoxyguanosine 5'-monophosphate (dGMP). Comparative fs-TRF and fs-TA measurements were conducted on dG and dGMP in neutral water, deuterated water, and methanol with excitation wavelengths of 245, 267 and 285 nm. Very similar results were observed with dG and dGMP. The data provide compelling evidence for the co-existence of two nonradiative pathways. One is the generally recognized Laππ* mediated channel, the other involves an unprecedented weakly emissive state which plays a significant role in the overall deactivation processes. The Laππ* channel features biphasic dynamics with time constants (τ1/τ2) of ~0.2/0.8 ps in water and ~0.25/1.0 ps in methanol. The biphasic decay arises due to a partial transfer with τ1 of the Laππ* population to the newly identified state followed by conversion in τ2 of the remaining Laππ* molecules into the electronic ground state. The channel mediated by the weakly emissive species shows solvent-dependent dynamics with time constants (τ3) of ~2.0 ps in water, ~2.3 ps in deuterated water, and ~4.1 ps in methanol. The species features absorption at UV wavelengths (~300-400 nm) and exhibits deeply red-shifted fluorescence (λmax ~ 520 nm) with polarization direction varied markedly from that of the Laππ* but close to the Lbππ*. This species acts as an effective quenching state to the radiative decay of the brightly emissive Laππ* and Lbππ*. It sets in promptly (<~50 fs) after the photoexcitation and is further populated through nonadiabatic coupling with the Laππ*. The overall involvement of this state is facilitated with excitation at high energy and is favoured in methanol over water. According to the spectral character and the solvent effect in particular the kinetic isotope effect, the species is tentatively associated to the πσ* state with charge transfer (CT) character which is considered to be preferentially stabilized by hydrogen-bonding between the guanine amino and surrounding solvent molecules. The result of this study leads to a dramatically different picture of guanine deactivation. It demonstrates a crucial role of the solvent in shaping the nonradiative dynamics of guanine nucleosides and nucleotides. The data presented are important for understanding the detailed photophysics of not only the monomeric guanine but also DNA assemblies that contain guanine in base pairs or have a guanine tetrad as the structural motif.

Photochemistry of Nucleic Acid Bases and Their Thio- and Aza-Analogues in Solution

“…The population of 1 πσ* states has been proposed to play a role in the excited-state dynamics of adenine and guanine monomers in solution, particularly at high excitation energies [81,88,[145][146][147]. A recent study by Cheng and co-workers [81] continues to add fuel to this discussion. The authors presented experimental evidence for the co-existence of two nonradiative decay pathways in 2 0 -deoxyguanosine and 2 0 -deoxyguanosine 5 0 -monophosphate in solution.…”

“…310-330 nm [1, 65, 67, 78-80, 84, 85, 87]. In general, the fluorescence yield of the nucleic acid monomers appears to be independent of the excitation wavelength and experimental conditions used within the reported uncertainties, with the exception of the adenine and guanine monomers, in which either tautomerization from N9H to N7H contributes to the total fluorescence yield [73,88] or the yield is excitation-wavelength dependent [81,82,89]. The small fluorescence yields, together with the small photodegradation [90][91][92][93][94][95][96] and triplet quantum yields of ca.…”

“…These experiments have further revealed that multiple relaxation processes contribute to the ultrafast depopulation of the optically populated excited states [73,81,[103][104][105][106][107][108]. There is still debate regarding the specific assignment of the multiexponential decay components.…”

“…The deactivation mechanism is similar to that proposed previously by others [89,111], except for the suggestion that the 1 πσ* state is also populated; either directly upon 285 nm excitation or indirectly from the 1 L b and the 1 L a states upon either 245 nm or 267 nm excitation. Furthermore, Cheng et al [81] proposed that the population in the 1 πσ* state decays back to the ground state with a solvent-dependent fluorescence lifetime of~2 ps in water,~2.3 ps in deuterated water, and~4.1 ps in methanol. An example of the spectral evolution can be observed in Fig.…”

“…Furthermore, even if the triplet state is populated in the adenine and The participation of πσ* states in the excited-state dynamics of the monomers is a matter of intense debate [1,4,6,7,81,88,111,[145][146][147][148][149]. The population of 1 πσ* states has been proposed to play a role in the excited-state dynamics of adenine and guanine monomers in solution, particularly at high excitation energies [81,88,[145][146][147]. A recent study by Cheng and co-workers [81] continues to add fuel to this discussion.…”