血红素蛋白自旋交换动力学的飞秒X射线发射研究
Femtosecond X-ray emission study of the spin cross-over dynamics in haem proteins
摘要：
在血红蛋白（由四个球状肌红蛋白样亚基组成）中，配体脱离时从低自旋（LS）六配位血红素到高自旋（HS）五配位圆顶形式的变化以及配体结合时的相反过程代表了最终驱动呼吸功能的过渡态。可见紫外光长期以来一直被用来模拟从血红素的光解离释放配体，而其重组监测使用时间分辨红外到紫外光谱工具。然而，它们既不是元素敏感的，也不是自旋敏感的。在这里，我们研究的过渡态的情况下，肌红蛋白-NO（MbNO）使用飞秒Fe K α和K β非共振X射线发射光谱（XES）在X射线自由电子激光光解的Fe-NO键。我们发现LS（S = 1/2）MbNO到HS（S = 2）脱氧肌红蛋白（deoxyMb）血红素的光诱导变化发生在约100 ℃。800 fs，并且它通过中间（S = 1）自旋状态进行。XES观测值还表明，NO复合到脱氧Mb后，返回到平面MbNO基态是一个电子从HS到LS的弛豫过程，发生在ca. 30 ps。因此，在MbNO中的整个配体解离-重组循环是自旋交叉，随后是反向自旋交叉过程。
英文摘要：
In haemoglobin (consisting of four globular myoglobin-like subunits), the change from the low-spin (LS) hexacoordinated haem to the high spin (HS) pentacoordinated domed form upon ligand detachment and the reverse process upon ligand binding, represent the transition states that ultimately drive the respiratory function. Visible-ultraviolet light has long been used to mimic the ligand release from the haem by photodissociation, while its recombination was monitored using time-resolved infrared to ultraviolet spectroscopic tools. However, these are neither element- nor spin-sensitive. Here we investigate the transition state in the case of Myoglobin-NO (MbNO) using femtosecond Fe Kalpha and Kbeta non-resonant X-ray emission spectroscopy (XES) at an X-ray free-electron laser upon photolysis of the Fe-NO bond. We find that the photoinduced change from the LS (S = 1/2) MbNO to the HS (S = 2) deoxy-myoglobin (deoxyMb) haem occurs in ca. 800 fs, and that it proceeds via an intermediate (S = 1) spin state. The XES observables also show that upon NO recombination to deoxyMb, the return to the planar MbNO ground state is an electronic relaxation from HS to LS taking place in ca. 30 ps. Thus, the entire ligand dissociation-recombination cycle in MbNO is a spin cross-over followed by a reverse spin cross-over process.
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