Wyer Jean Ann, Ehlerding Anneli, Zettergren Henning, Kirketerp Maj-Britt S, Brøndsted Nielsen Steen
Department of Physics and Astronomy, Aarhus University, Ny Munkegade, DK-8000 Aarhus C, Denmark.
J Phys Chem A. 2009 Aug 20;113(33):9277-85. doi: 10.1021/jp904053d.
Photodissociation of protonated Tyr, Ala-Tyr, Tyr-Ala (Ala = alanine, Tyr = tyrosine), and their complexes with 18-crown-6-ether (CE) was performed in an electrostatic ion storage ring using a tunable laser system. While the three bare ions all absorb strongly at 222 nm, absorption at higher wavelengths was barely visible from sampling the neutrals formed in delayed dissociation. A band at 270 nm was introduced, however, as a consequence of CE attachment to the bare ions. To understand the difference between bare ions and complexes, electronically excited states are considered: The initially reached pipi* state on phenol couples with the dissociative pisigma* state on ammonium, which leads to direct hydrogen loss. Cold radical cations are formed that at high wavelengths do not have enough energy for further dissociation. Excitation within the 222-nm band on the other hand leads to delayed dissociation of stored radical cations that is monitored in the present setup. The pisigma* state moves out of the spectral region upon CE attachment, and instead statistical dissociation is sampled on the microsecond to millisecond time scale at all wavelengths. Our data demonstrate the strength of using supramolecular complexes for action spectroscopy experiments to prevent erroneous spectra as a result of undesired dissociation (H loss) from electronically excited states. The gas-phase absorption spectra firmly establish the perturbations of the phenol electronic structure by a water solvent: The 270-nm band red shifts by approximately 5 nm, whereas the 222-nm band changes by approximately 3 nm. Both transitions occur in the phenol group. These results may be useful for protein dynamics experiments that rely on electronic excitations. Product ion mass spectra of [Tyr + H]+, [Ala-Tyr + H]+, [Tyr-Ala + H]+, [Ala-Tyr + H]+(CE), and [Tyr-Ala + H]+(CE) significantly depend on the excitation wavelength from 210 to 310 nm and on whether the ionizing proton is mobile or not.
使用可调谐激光系统在静电离子储存环中对质子化的酪氨酸（Tyr）、丙氨酰 - 酪氨酸（Ala - Tyr）、酪氨酰 - 丙氨酸（Tyr - Ala，Ala = 丙氨酸，Tyr = 酪氨酸）及其与18 - 冠 - 6 - 醚（CE）的配合物进行光解离。虽然这三种裸离子在222 nm处都有强烈吸收，但从延迟解离中形成的中性物质采样来看，更高波长处的吸收几乎不可见。然而由于CE与裸离子结合，在270 nm处出现了一个吸收带。为了理解裸离子和配合物之间的差异，考虑电子激发态：苯酚上最初达到的ππ态与铵上的解离性πσ态耦合，这导致直接氢损失。形成了冷自由基阳离子，在高波长下它们没有足够的能量进行进一步解离。另一方面，在222 nm波段内的激发会导致储存的自由基阳离子延迟解离，这在当前装置中进行监测。CE附着后，πσ*态移出光谱区域，取而代之的是在所有波长下在微秒到毫秒的时间尺度上对统计解离进行采样。我们的数据证明了使用超分子配合物进行作用光谱实验以防止由于电子激发态的不期望解离（氢损失）导致错误光谱的优势。气相吸收光谱牢固地确立了水溶剂对苯酚电子结构的扰动：270 nm波段红移约5 nm，而222 nm波段变化约3 nm。这两个跃迁都发生在苯酚基团中。这些结果可能对依赖电子激发的蛋白质动力学实验有用。[Tyr + H]+、[Ala - Tyr + H]+、[Tyr - Ala + H]+、[Ala - Tyr + H]+(CE)和[Tyr - Ala + H]+(CE)的产物离子质谱显著取决于210至310 nm的激发波长以及电离质子是否可移动。
