REMPI spectroscopy of laser desorbed guanosines
REMPI spectroscopy of laser desorbed guanosines
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DOI：
10.1021/ja000502c
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发表时间：
2000-08-23
影响因子：
15
通讯作者：
de Vries, MS
中科院分区：
文献类型：
--
作者：
Nir, E;Imhof, P;de Vries, MS
为了观察DNA结构单元的基本性质，需要在气相中研究单个核苷而不受溶剂分子或大分子结构的干扰。作为第一步，我们最近报道的第一个振动光谱的碱基鸟嘌呤，通过激光解吸，喷射冷却，共振增强多光子电离（REMPI）的组合。1.鸟嘌呤作为DNA中的重要发色团，研究核苷对理解DNA的光化学作用更为现实。那些更难完整地蒸发，因为它们在热上更不稳定，并且具有更大的分子量，具有更低的蒸气压。利用激光解吸，我们现在已经成功地形成了核苷的分子束，我们报告了一系列单独的鸟苷，即鸟苷（Gs），2′脱氧鸟苷（2 ′ deoxyGs），和3′脱氧鸟苷（3′ deoxyGs）的第一个REMPI光谱。我们比较我们的结果与计算在HF 6- 31 G（d，p）水平。结果表明，发生两种不同的构象，每个可能是由内部氢键稳定。这两种构象中的一种在2′脱氧G中不存在，这意味着2′羟基是其稳定所必需的。利用拉曼光谱技术对鸟苷类化合物的光谱性质进行了初步研究。2-9大量的注意力已经给予潜在的拉曼标记的氢键和结构构象。通过拉曼光谱观察氢键需要识别强烈依赖于鸟嘌呤中作为质子供体或受体的那些特定原子的振动。然而，大多数振动涉及多个原子的协同运动，因此标记频率与特定氢键位点的相关性并不简单。如果鸟苷振动的频率对核糖环的褶皱或围绕糖-碱基键的旋转敏感，则鸟苷振动涉及沿着糖苷键的运动，可以提供构象标记。解释这些标记需要仔细分析复杂的振动模式。另一方面，当它们产生多个起源时，可以通过电子振动光谱更直接地观察到不同的构象。正如我们将在下面展示的那样，我们在我们的光谱中观察到两个起源，我们可以将其与碱基相对于核糖部分的顺向和反向取向联系起来。我们已经在其他地方发表了我们的激光解吸喷射冷却REMPI光谱的设置细节。样品制备包括将粉末形式的纯材料沉积在石墨基底上。我们在获取光谱的同时缓慢移动基底，逐渐暴露新鲜的材料。对于解吸，我们使用来自Nd：YAG激光器的1064 nm的脉冲，其能量密度为1 mJ/cm 2。脱附的中性分子被夹带在超声膨胀与Ar驱动气体，通过脉冲电磁阀注入。在下游，夹带的分子被单色双光子光电离，并在反射飞行时间质谱仪中检测离子。第一个光子共振激发分子，而来自同一激光的第二个光子电离激发的分子。通过改变波长，同时监测特定的质量峰，我们获得了质量选择的激发光谱。典型的电离激光能量密度为0.1 mJ/cm 2的量级。图1显示了（a）Gs、（B）3′脱氧Gs和（c）2′脱氧Gs的REMPI光谱。我们分配的最低能量峰在每个光谱作为一个0-0跃迁的S1激发态。仔细扫描以降低...
To observe fundamental properties of DNA building blocks it is desirable to study individual nucleosides in the gas phase without interference from solvent molecules, or macromolecular structure. As a first step, we have recently reported the first vibronic spectrum of the nucleobase guanine, obtained by a combination of laser desorption, jet cooling, and resonance enhanced multiphoton ionization (REMPI). 1 Although guanine is important as a chromophore in DNA, it is more realistic for understanding the photochemistry of DNA to study the nucleosides. Those are even harder to vaporize intact because they are thermally more labile and, with their larger molecular weights, have still lower vapor pressures. Using laser desorption, we have now succeeded in forming a molecular beam of nucleosides, and we report the first REMPI spectra of a series of individual guanosines, namely guanosine (Gs), 2′ deoxyguanosine (2′ deoxyGs), and 3′ deoxyguanosine (3′ deoxyGs). We compare our results with computations at the HF 6-31G (d, p) level. The results suggest the occurrence of two different conformations, each probably stabilized by internal hydrogen bonds. One of those two conformations is absent in 2′ deoxyGs implying that the 2′ hydroxyl group is required for its stabilization. Spectroscopic properties of guanosines have been studied primarily by Raman techniques in solution. 2-9 A great deal of attention has been given to potential Raman markers for hydrogen bonding and for structural conformation. Observation of hydrogen bonding by Raman spectroscopy requires identification of vibrations that depend strongly on those specific atoms in guanine, that serve as either proton donor or acceptor. However, most vibrations involve the concerted motion of multiple atoms, and therefore correlation of marker frequencies with specific hydrogen bonding sites is not straightforward. Guanosine vibrations involving motion along the glycosidic bond may provide conformational markers if their frequencies are sensitive to puckering of the ribose ring or for rotation around the sugar-base bond. Interpretation of these markers requires careful analysis of complex vibrational modes. On the other hand, different conformations can be observed much more directly by vibronic spectroscopy when they produce multiple origins. As we will show below, we observe two origins in our spectra, which we can associate with the syn and the anti orientations of the base relative to the ribose moiety.We have published details of our setup for laser desorption jet cooling REMPI spectrometry elsewhere. 10 Sample preparation consisted of depositing neat material in powder form on graphite substrates. We moved the substrate slowly while acquiring spectra, gradually exposing fresh material. For desorption we used pulses from a Nd: YAG laser at 1064 nm with fluences on the order of 1 mJ/cm2. Desorbed neutral molecules were entrained in a supersonic expansion with Ar drive gas, injected by a pulsed solenoid valve. Downstream, the entrained molecules were onecolor two-photon photoionized, and the ions were detected in a reflectron time-of-flight mass spectrometer. The first photon resonantly excites the molecule, while a second photon from the same laser ionizes the excited molecule. By varying the wavelength while monitoring specific mass peaks we obtained mass selected excitation spectra. The typical ionization laser fluence was on the order of 0.1 mJ/cm2. Figure 1 shows the REMPI spectra of (a) Gs,(b) 3′ deoxyGs, and (c) 2′ deoxyGs. We assign the lowest-energy peak in each of the spectra as a 0-0 transition to the S1 excited state. Careful scans to lower …