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

电子顺磁共振（EPR）是一种快速发展的技术，在包括结构生物学和量子信息处理在内的各个领域都有应用。它用于研究含有未成对电子的系统，类似于NMR。了解生物系统的结构，或在不同扰动下的变化，对于确定功能非常重要。脉冲偶极EPR可用于通过测量它们之间的偶极相互作用来量化自旋中心（具有未成对电子的化学部分）之间的纳米距离。EPR中的自旋中心通常在基态具有未配对电子，例如氮氧自由基、自由基或某些金属团簇/离子，并且通常通过结构修饰（诱变）和标记（tagging）加入到体系中。最近我们已经证明，光激发三重态可以通过光诱导三重-三重电子共振（LITTER）来测量距离。两个激光器耦合到EPR光谱仪中，独立地形成两个三重态，微波脉冲用于操纵电子自旋并检测三重态自旋中心之间的偶极相互作用。LITTER在生物系统中有很大的应用前景，特别是那些具有天然辅因子的系统，如血红素基团，可以用来形成光学产生的三重态。本项目将探索和开发使用LITTER和其他光诱导EPR技术来研究不同蛋白质系统的结构;例如，使用天然辅因子或发色团标签的三重态，研究二聚体肌红蛋白、血红蛋白、神经红蛋白、原叶绿素还原酶和吗啡还原酶。由于辅因子在蛋白质结构内紧密结合，中心之间的距离和偶极相互作用的分布将被明确定义，使得它们的使用潜在地优于附接在表面可接近位点处并且具有一定灵活性的发色团标签。最初，我们的目标是调查和表征不同的本地发色团，使他们适合或以其他方式使用新的LITTER方法，使用EPR和其他生物物理技术的研究的属性。我们的目标是比较在这些系统中的LITTER的结果，以更成熟的生物物理方法测量距离的生物系统，如脉冲偶极EPR使用稳定的自旋中心和福斯特共振能量转移（FRET）。新的光诱导EPR光谱技术的发展使这一项目纳入了EPSRC的职权范围。此外，相对于传统的EPR方法，光激发形成的三重态自旋态布居的偏振可以导致信号的增加。明确定义的偶极相互作用的组合，可以存在于生物系统中的辅因子和极化群体之间，使得这样的系统令人兴奋的潜在目标，用于编码量子信息处理算法，使用EPR方法。因此，第二个目的是实现基于EPR的量子信息处理脉冲序列，其可以用于检测三重态自旋中心之间的纠缠的存在。

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.