Applications of light induced Electron Paramagnetic Resonance to biological systems - from structure determination towards biological quantum gates

电子顺磁共振（EPR）是一项快速发展的技术，在结构生物学和量子信息处理等领域有着广泛的应用。它被用来研究含有未配对电子的系统，类似于核磁共振。了解生物系统的结构，或在不同扰动下的变化，对于确定功能是很重要的。脉冲偶极EPR可以通过测量它们之间的偶极相互作用来量化自旋中心（具有未配对电子的化学部分）之间的纳米距离。通常EPR中使用的自旋中心在其基态中具有未配对电子，例如氮氧化物、自由基或一些金属团簇/离子，并且通常通过结构修饰（诱变）和标记添加到系统中。最近，我们已经证明了光激发三重态可以通过光诱导三重态电子共振（Light - Induced Triplet-Triplet Electron Resonance, itter）来测量距离。将两个激光器耦合到EPR光谱仪中，独立形成两个三重态，利用微波脉冲控制电子自旋，探测三重态自旋中心之间的偶极相互作用。在生物系统中，特别是那些具有天然辅助因子（如血红素群）的生物系统中，利用这种辅助因子可以形成光学生成的三重态。这是有利的，因为它不需要修改或标记的生物系统，这可能会导致结构变化。该项目将探索和发展利用光诱导EPR技术来研究不同蛋白质系统的结构；例如，二聚体肌红蛋白、血红蛋白、神经红蛋白、原叶绿内酯还原酶和吗啡inioine还原酶使用天然辅助因子或发色团标签的三重态。由于辅因子在蛋白质结构内紧密结合，中心之间的距离分布和偶极相互作用将被明确定义，这使得它们的使用比附着在表面可达位置并具有一定灵活性的发色团标签具有潜在的优势。首先，我们的目标是利用EPR和其他生物物理技术，研究和表征不同天然发色团的性质，使它们适合或不适合使用新的凋落物方法进行研究。我们的目标是将这些系统中的itter结果与更成熟的生物物理方法进行比较，这些方法用于测量生物系统中的距离，例如使用稳定自旋中心和Förster共振能量转移（FRET）的脉冲偶极EPR。新型光诱导EPR光谱技术的发展将该项目置于EPSRC的职权范围内。此外，与传统的EPR方法相比，在光激发下形成的三重态自旋态居群的极化可以导致信号的增加。生物系统中辅因子与极化种群之间存在的定义良好的偶极相互作用的结合，使此类系统成为使用EPR方法编码量子信息处理算法的潜在目标。因此，第二个目标是实现基于EPR的量子信息处理脉冲序列，该脉冲序列可用于检测三重态自旋中心之间纠缠的存在。

Electron Paramagnetic Resonance (EPR) is a fast-growing technique with applications in diverse fields including structural biology and quantum information processing. It is used to study systems containing unpaired electrons, analogous to NMR. Understanding the structure of a biological system, or changes under different perturbations, is important for determining functionality. Pulsed dipolar EPR can be used to quantify nanometre distances between spin-centres, chemical moieties with unpaired electrons, by measuring the dipolar interaction between them. Usually spin-centres used in EPR have unpaired electrons in their ground states, e.g. nitroxides, radicals or some metal clusters/ions, and are often added to the system via structure modification (mutagenesis) and tagging.Recently we have shown that optically-excited triplet-states can be used to measure distances via Light Induced Triplet-Triplet Electron Resonance (LITTER). Two lasers are coupled into the EPR spectrometer, independently forming two triplet-states, and microwave pulses are used to manipulate the electron spins and detect the dipolar interaction between the triplet spin-centres. LITTER has great promise to be applied in biological systems, particularly those with native cofactors, such as heme groups, that can be used to form the optically-generated triplet states. This is advantageous as it does not require modification or tagging of the biological system, which may cause structural changes.This project will explore and develop the use of LITTER and other light-induced EPR techniques to investigate the structure of different protein systems; for example, dimeric Myoglobin, Hemoglobin, Neuroglobin, Protochlorophyllide Reductase and Morphinioine Reductase using the triplet-states of native cofactors or chromophore tags. As cofactors are tightly bound within a protein structure the distribution of distances and dipolar interactions between centers will be well-defined, making their use potentially advantageous over chromophore tags which are attached at surface accessible sites and have some flexibility. Initally we aim to investigate and characterize the properties of different native chromophores that make them suitable or otherwise for study using the novel LITTER method, using both EPR and other biophysical techniques. We aim to compare the results of LITTER in these systems to more established biophysical methods for measuring distances in biological systems such as pulsed dipolar EPR using stable spin centers and Förster Resonance Energy Transfer (FRET). The development of the novel light induced EPR spectroscopy techniques places this project within the remit of the EPSRC.Furthermore, polarization of the triplet spin-state population formed on optical excitation can lead to an increase in signal relative to conventional EPR methods. The combination of well-defined dipolar interactions that can be present between the cofactors in a biological system and the polarized populations make such systems exciting potential targets for encoding quantum information processing algorithms using EPR methods. As such a second aim is to implement a EPR based quantum information processing pulse sequence that can be used to detect the presence of entanglement between the triplet spin-centers.