Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China.
Photochem Photobiol Sci. 2013 Aug;12(8):1351-65. doi: 10.1039/c3pp25450j.
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.
采用飞秒宽带时间分辨荧光（fs-TRF）和瞬态吸收（fs-TA）相结合的方法，研究了 2'-脱氧鸟苷（dG）和 2'-脱氧鸟苷 5'-单磷酸（dGMP）的激发态动力学。在中性水、氘代水和甲醇中，用 245、267 和 285nm 的激发波长对 dG 和 dGMP 进行了比较 fs-TRF 和 fs-TA 测量。在 dG 和 dGMP 中观察到非常相似的结果。这些数据为两种非辐射途径的共存提供了令人信服的证据。一种是普遍认可的 Laππ介导的通道，另一种涉及一种前所未有的弱发射态，它在整体失活过程中起着重要作用。Laππ通道的特征是双相动力学，在水中的时间常数（τ1/τ2）为0.2/0.8ps，在甲醇中的时间常数为0.25/1.0ps。双相衰减是由于 Laππ* 种群的部分转移引起的，τ1 将 Laππ* 种群转移到新识别的状态，然后在 τ2 中将剩余的 Laππ* 分子转化为电子基态。由弱发射物质介导的通道具有与溶剂相关的动力学，在水中的时间常数（τ3）为2.0ps，在氘代水中为2.3ps，在甲醇中为4.1ps。该物质在紫外波长（300-400nm）处具有吸收，发射荧光明显红移（λmax520nm），其偏振方向与 Laππ明显不同，但与 Lbππ接近。这种物质作为一种有效的猝灭态，对辐射衰减的 Laππ和 Lbππ。它在光激发后立即（<50fs）出现，并通过与 Laππ的非绝热耦合进一步填充。这种状态的整体参与是通过高能激发来促进的，并且在甲醇中比在水中更有利。根据光谱特征和溶剂效应，特别是动力学同位素效应，该物质被暂时与具有电荷转移（CT）特征的πσ态相关联，认为该态优先由鸟嘌呤氨基和周围溶剂分子之间的氢键稳定。该研究结果导致鸟嘌呤失活的画面发生了巨大变化。它表明溶剂在塑造鸟苷核苷酸碱基的非辐射动力学方面起着关键作用。所提供的数据对于理解不仅单体鸟嘌呤而且还包含碱基对中鸟嘌呤或具有鸟嘌呤四联体作为结构基序的 DNA 组装的详细光物理性质非常重要。
