Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy
Probing the Excited State of Methylcobalamin Using Polarized Time-Resolved X-ray Absorption Spectroscopy
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使用偏振时间分辨 X 射线吸收光谱法探测甲钴胺的激发态
DOI：
10.1021/acs.jpcb.9b05854
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发表时间：
2019
期刊：
影响因子：
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通讯作者：
Meadows, Joseph H.
中科院分区：
文献类型：
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作者：
Michocki, Lindsay B.;Miller, Nicholas A.;Alonso-Mori, Roberto;Britz, Alexander;Deb, Aniruddha;Glownia, James M.;Kaneshiro, April K.;Konar, Arkaprabha;Koralek, Jake;Meadows, Joseph H.
我们使用皮秒时间分辨的偏振X射线吸收近边结构（XANES）测量探测的结构的长寿命的光激发态的甲钴胺（MeCbl）和钴（II）维生素的光激发后形成的腺苷钴胺素（MACBL，辅酶B12）的光产物。对于MeCbl，我们使用520 nm激发和100 ps的时间延迟以避免形成cob（II）alamin。我们发现只有很小的光谱变化，在赤道和轴向方向，我们解释为产生的小（<0.05毫米）的变化，在赤道和轴向距离。这证实了基于先前的紫外-可见瞬态吸收测量和理论模拟的预期。我们没有发现Subramanian [J. Phys. Chem. Lett.2018，9，1542-1546]报道的Co-C键在400 nm激发后显著伸长的证据。对于XANCbl，我们解决了不同的XANES贡献沿着三个独特的分子轴，通过激发与540和365 nm的光，表明光谱变化主要是偏振沿着轴向方向，轴向结扎的损失一致。这些数据表明，由Subramanian等人鉴定的微秒“重组产物”实际上是在MeCbl在400 nm激发下的键均裂后产生的cob（II）alamin光产物。我们的研究结果突出了显着的优势，使用偏振选择性瞬态X射线吸收隔离系统中的结构动力学进行原子位移，强烈相关的激发光偏振。
We use picosecond time-resolved polarized X-ray absorption near-edge structure (XANES) measurements to probe the structure of the long-lived photoexcited state of methylcobalamin (MeCbl) and the cob(II)alamin photoproduct formed following photoexcitation of adenosylcobalamin (AdoCbl, coenzyme B12). For MeCbl, we used 520 nm excitation and a time delay of 100 ps to avoid the formation of cob(II)alamin. We find only small spectral changes in the equatorial and axial directions, which we interpret as arising from small (<∼0.05 Å) changes in both the equatorial and axial distances. This confirms expectations based on prior UV–visible transient absorption measurements and theoretical simulations. We do not find evidence for the significant elongation of the Co–C bond reported by Subramanian [J. Phys. Chem. Lett.2018, 9, 1542–1546] following 400 nm excitation. For AdoCbl, we resolve the difference XANES contributions along three unique molecular axes by exciting with both 540 and 365 nm light, demonstrating that the spectral changes are predominantly polarized along the axial direction, consistent with the loss of axial ligation. These data suggest that the microsecond “recombination product” identified by Subramanian et al. is actually the cob(II)alamin photoproduct that is produced following bond homolysis of MeCbl with 400 nm excitation. Our results highlight the pronounced advantage of using polarization-selective transient X-ray absorption for isolating structural dynamics in systems undergoing atomic displacements that are strongly correlated to the exciting optical polarization.