乙二醇均相及其TX-100胶束溶液中光敏分子菲醌的激光光解研究

用时间分辨电子自旋共振（TR-ESR）和瞬态吸收光谱技术,研究了菲醌在乙二醇均相及其TX-100含水胶束溶液中的光化学反应机理。化学诱导动态电子极化（CIDEP）谱和瞬态吸收光谱都表明,在乙二醇均相溶液中,菲醌光激发三重态3PAQ* 夺取氢原子形成中性自由基PAQH.,三重态机理是CIDEP形成的主要机理。在TX-100含水胶束溶液中,光解主要得到菲醌负离子基PAQ-．,PAQ-．由PAQH.解离形成,解离过程中伴随着极化转移。     

微波诱导光学核极化进展

微波诱导光学核极化（Microwave-Induced Optical Nuclear Polarization,MIONP）技术利用光激发三重态样品来极化电子,再用微波将处于非热平衡态的电子极化转移到待检测原子核,将原子核的检测灵敏度提高几个量级甚至更多．这种灵敏度极化增强方法可以用来进行蛋白质结构和动力学检测、光化学和光物理进程的基础研究、量子计算和低场核磁共振（Low-field Nuclear Magnetic Resonance,Low-field NMR）与磁共振成像（Magnetic Resonance Imaging,MRI）应用研究．该文简要分析了MIONP的物理原理及其在核极化增强中的优势,结合实验条件综述了一些重要的成果．最后,对微波诱导光学核极化的前景作了展望．     

核苷酸和芳香酮的电子转移和质子转移反应研究

利用时间分辨的激光闪光光解方法在1∶1乙腈/水溶液中得到了4种核苷酸和芳香酮的瞬态吸收光谱,通过瞬态吸收光谱的变化研究了鸟苷酸、腺苷酸、胞苷酸、胸腺苷酸猝灭二苯甲酮、呫吨酮激发三重态的反应。由于实验中生成了抽氢自由基和负离子自由基,以及核苷酸正离子在水中的快速抽氢反应,推断出芳香酮和鸟苷酸、腺苷酸的反应机理是先发生电子转移后发生质子转移。而在芳香酮和胞苷酸、胸腺苷酸的反应中没有观察到相应的抽氢自由基和负离子自由基的瞬态吸收峰,由此推断出它们和胞苷酸、胸腺苷酸没有发生电子转移和质子转移反应。对瞬态吸收峰处的时间衰减曲线进行拟合得到了核苷酸猝灭芳香酮的速率常数,可以看到随着反应自由能变ΔG的增大,反应速率常数逐渐减小。     

光诱导呫吨酮电子转移和氢转移反应的激光闪光光解研究

用时间分辨激光闪光光解的方法研究了在乙腈溶剂中呫吨酮的激发三重态的性质，并得到了呫吨酮激发三重态和胺类、醇类以及酚类反应的瞬态吸收光谱和猝灭速率常数(kq)．除了苯胺和3-硝基苯胺被认为是能量转移外，呫吨酮和其余胺类的反应随着自由能变的减校lgkq逐渐增大，由此认为发生了电子转移反应．而对于二甲基-对甲苯胺、3,5,N,N-四甲基苯胺、N,N-二甲基苯胺、三乙胺来说，通过瞬态吸收光谱的变化可以知道既有电子转移反应又有氢转移反应发生．呫吨酮和醇类只发生氢转移反应，其猝灭速率常数和醇的?-C?H的键能有关．由     

基于电子顺磁共振的ZnTPP激发态及其TEMPO各向异性的研究

为了研究卟啉类敏化剂的光致激发态能量转移和电子转移问题,本文基于紫外可见光谱仪和电子顺磁共振波谱仪,构建了以锌卟啉为研究对象的"锌卟啉-稳态自由基-二甲苯"实验体系.锌卟啉的紫外可见光谱显示,在可见光谱的B带和Q带出现明显的特征峰,是锌卟啉分子中电子由基态能级跃迁至激发态能级产生的.低温条件下受紫外可见光辐照的实验体系的电子顺磁共振波谱,检测到了稳态自由基四甲基哌啶氧化物的增强吸收电子顺磁共振波谱.根据分子激发态相关理论、光化学物理反应理论和化学诱导电子自旋极化理论对实验结果进行了分析,结果表明,四甲基哌啶氧化物稳态自由基电子顺磁共振波谱的增强吸收对应于锌卟啉光致激发态的能量转移和电子转移;四甲基哌啶氧化物在低温(143 K)下的电子顺磁共振波谱表现出的各向异性特征现象来源于氮氧自由基电子与氮核的各向异性超精细相互作用.     

2-甲基蒽醌光敏氧化芳香类氨基酸的激光光解

以355 nm激光为激励光源,研究 2-甲基蒽醌(MAQ)光敏氧化芳香氨基酸(色氨酸、酪氨酸、苯丙氨酸)的光化学行为。在乙腈-水体积比1∶1的混合体系中,2-甲基蒽醌三重激发态(3MAQ*)的吸收峰位于390,450和590 nm,利用3MAQ*与电子给体的电子转移反应确定其阴离子自由基的特征吸收峰为390和490 nm。2-甲基蒽醌与色氨酸、酪氨酸、苯丙氨酸的瞬态反应都显示随着590 nm激发三重态的衰减,出现490 nm 2-甲基蒽醌阴离子自由基的生成,并进一步确定了2-甲基蒽醌与色氨酸、酪氨酸和苯丙氨酸的电子转移反应的速率分别为3.0×109,1.1×109和1.8×108 L·mol-1·s-1。同时吉布斯自由能(ΔG)的计算结果也表明2-甲基蒽醌的激发三重态与色氨酸、酪氨酸、苯丙氨酸电子转移反应在热力学上是可行的。     

芘四磺酸四钠盐光物理性质的研究

本文利用吸收光谱、荧光光谱以及时间分辨瞬态吸收光谱研究芘四磺酸四钠盐的光物理特性.研究结果表明,芘四磺酸四钠盐在水溶液中的荧光量子产额高达0.54.在纳秒时间分辨瞬态吸收光谱研究中发现芘四磺酸四钠盐具有很长的三重态寿命(3.5ms),这有利于将其作为光敏化分子把激发能传给其它受体分子;芘四磺酸四钠盐被光激发后,可能发生双光子吸收过程并产生芘四磺酸四钠盐阳离子,其吸收峰为460nm和505nm.由实验结果,我们给出了其动力学过程的解释.     

基于散射实验极化靶的2.5T/1.3K的动态核极化(DNP)系统的开发

动态核极化法(Dynamic Nuclear Polarization, DNP)是利用热平衡下的电子在磁场中的高自旋极化率转移到原子核自旋的技术,从而极大的提高原子核自旋极化率。多种动态极化靶材料已广泛的用于自旋物理散射实验。本文介绍一种简单实用,共同开发的日本山形大学DNP系统,包括超导磁场,氦4蒸发恒冷器,微波系统以及NMR核磁共振检测系统,测得中子靶材料氘带丁醇(D-butanol)中氘核的极化率在2.5T/1.3K达到+6.5%。     

吩噻嗪-1,4萘醌-乙二醇体系光化学过程的时间分辨ESR研究

用时间分辨ＥＳＲ方法研究了吩噻嗪（ＰＴＨ）－１，４－萘醌（ＮＱ）－乙二醇（ＲＨ）体系中的光化学过程．只观察到萘醌的中性半醌自由基ＮＱＨ．＊的全发射极化的ＣＩＤＥＰ（化学诱导动态极化）信号，并未观察到极化的萘醌负离子ＮＱ－·＊的ＣＩＤＥＰ信号，表明ＲＨ与三重态ＮＱ间的质子转移反应远比ＰＴＨ与ＮＱ间的电子转移反应为快．ＮＱＨ．＊的ＣＩＤＥＰ信号随ｐＨ值的变化表明体系中同时有极化自由基ＮＱＨ．＊与ＮＱＨ．＋＊２存在，并且其间有质子交换．     

基于散射实验极化靶的2.5T/1.3K的动态核极化(DNP)系统的开发

动态核极化法(Dynamic Nuclear Polarization,DNP)是利用热平衡下的电子在磁场中的高自旋极化率转移到原子核自旋的技术,从而极大的提高原子核自旋极化率.多种动态极化靶材料已广泛的用于自旋物理散射实验.本文介绍一种简单实用,共同开发的日本山形大学DNP系统,包括超导磁场,氦4蒸发恒冷器,微波系统以及NMR核磁共振检测系统,测得中子靶材料氘带丁醇(D-butanol)中氘核的极化率在2.5 T/1.3 K达到+6.5%.     

Light induced radical pair intermediates in photosynthetic reaction centres in contact with an observer spin label: spin dynamics and effects on transient EPR spectra

A novel strategy is discussed using site directed spin labelling to study the electron transfer process in photosynthetic reaction centres. An algorithm is presented for numerical simulations of the time resolved EPR spectra of radical pair states in the presence of an observer spin label. This algorithm accounts for spin dynamics, charge recombination and relaxation processes. It is shown that satisfactory agreement between experimental and simulated EPR spectra of the first stabilized radical pair state in photosystem I is achieved for various microwave frequencies. Transient EPR spectra for the radical pair state P^?+Q^?- in photosystem I were simulated for various distances and positions of the observer spin label with respect to the acceptor quinone molecule. It is shown that distances up to more than 20 Å give rise to observable changes in the transient EPR spectra. Both the additional spin-spin coupling between the quinone radical and the label and the polarization transfer processes contribute to the changes. Furthermore, the shape and intensity of the EPR spectrum of the spin label is altered by the coupling with the radical pair spins for distances up to 25 Å. Experiments on site directed spin labelled photosystem I are thus expected to provide valuable information on the dynamics of electron transfer in photosystem I.     

Exploring hyperfine interactions in spin-correlated radical pairs from photosynthetic proteins: High-frequency ENDOR and quantum beat oscillations

Electron paramagnetic resonance (EPR) and related spectroscopic tools remain among the most important probes of structure and function of natural photosynthetic systems. Indeed, the challenging questions in the study of photosynthesis have to a great extent dictated the directions taken in the development of EPR and associated spectroscopies. In this overview we demonstrate, with recent examples from our laboratories, the potential of high-frequency and time-resolved EPR spectroscopy to reveal unique information about electron transfer processes and the structure of photosynthetic systems. A common feature of these experiments is that they probe hyperfine interactions of the spincorrelated radical pair. Thus, the analysis of the results requires consideration of three interacting spins: two correlated electron spins with one nuclear spin. The results illustrate the importance of resolving nuclear hyperfine structure for obtaining details of structure-function relationships in photosynthetic electron transfer.     

Selective two-dimensional exchange NMR spectroscopy and its application to the study of molecular dynamics processes: MUSEX EXSY experiment

A novel technique is suggested which allows one to study molecular dynamics processes on the basis of multiplet-selective excitation (MUSEX) of coupled spin systems. It is a general approach to exchange NMR spectroscopy. There are thirty two variants in this method depending upon combination of both selective pulses and nonselective ones into three-pulse sequence. Each of them is suitable for a concrete dynamic NMR problem. Printed in Austria     

Ultrafast spin dynamics in magnetic semiconductor quantum wells studied by magnetization modulation spectroscopy

1-x Mn_xTe quantum well structures at room temperature using time-resolved magnetization modulation spectroscopy. Access to the different electron and hole spin dynamics is obtained by carefully measuring the spectroscopic changes of the magneto-optical response during the first hundreds of femtoseconds after excitation. Experimental results are discussed in the framework of a simple model for a two-dimensional band structure. The spectroscopy is shown to be intimately related to the spectral band width of the applied ultrashort laser pulses. The general potential of the method for fundamental studies on other materials and systems is addressed. Received: 20 September 1998     

Solvent effects on the intrinsic enhancement factors of the triplet exciplex generated by photoinduced electron transfer reaction between eosin Y and duroquinone

The spin dynamics of the duroquinone anion radical (DQ^?-) generated by photoinduced electron transfer reactions from triplet eosin Y (³EY^2-) to DQ have been studied by using transient absorption and pulsed EPR spectroscopy. Unusual net-absorptive electron spin polarization plus net-emissive polarization were observed, suggesting the production of the triplet exciplex or contact radical pair as the reaction intermediate. The kinetic parameters and intrinsic enhancement factors of the electron spin polarization were determined in various alcoholic solvents. The net-absorptive electron spin polarization was also observed in ethanol-water mixed solvents. The solvent effects on the radical yield are analysed on the basis of a stochastic Liouville equation established for the magnetic field effects on the radical yield. The zero-field splitting constants of the triplet exciplex are estimated from the solvent viscosity dependence of the enhancement factors due to spin-orbit coupling induced depopulation of the reaction intermediate.     

ENDOR and related EMR methods applied to flavoprotein radicals

Flavoproteins are involved in a wide range of biological processes, owing to the versatility of the isoalloxazine moiety of flavin, which can undergo both one- and two-electron reactions with the formation of three oxidation states. Paramagnetic semiquinone radical states are stabilised in some flavoproteins and appear as transient intermediates in the reaction of many others. The apoprotein controls the reactivity of flavin semiquinones, including redox potentials, protonation states and access to substrates. Most flavoproteins are involved in oxidation-reduction processes, but some catalyze different types of reactions involving radical intermediates. Anionic and neutral flavin radicals are found in flavoproteins and are distinguished by their line widths in X-band electron paramagnetic resonance. Electron-nuclear double resonance, electron spin echo envelope modulation and hyperfine sublevel correlation spectroscopy make it possible to observe biological electron transfer and catalysis at the level of the electronic structure of the intermediate states. They provide information about the protein environment of flavin semiquinone radicals and their interactions with nearby nuclear and electron spins. Hyperfine couplings, particularly to the 8-methyl protons on the flavin ring, are a sensitive probe of perturbations of the flavin environment. They demonstrate differences in polarity of the flavin binding site and changes that occur in flavoenzymes during binding of substrates.     

RF resonance techniques for continuous muon beams

RF excitation was first used in μSR in 1958 [1]. However, even with the advent of modern meson facilities the use of RF techniques in μSR has not flourished, in part due to the muon's short lifetime, but mainly because unlike NMR RF fields are simply not used to detect the muon's polarization. Nevertheless, a cursory survey of the relative power and sophistication of these two related fields leaves little doubt that the pursuit of RF resonance within μSR will add significantly to its analytical capability. At TRIUMF over the past few years we have worked toward developing RF capabilities suitable for use in a CW meson facility. Several spectroscopic applications utilizing time integral techniques have been tested. These include detection of final states in semiconductor and substituted radical systems, the very pretty two photon absorption (TPA) experiments, and the use of low field swept frequency excitation to measure nuclear hyperfine parameters in Mu like system. Of course, the most celebrated use of RF in magnetic resonance is its application to spin dynamics via various spin echo techniques. The adaptation of the spin echo to μSR is discussed and examples of both transverse and longitudinal field echoes are presented.     

Time-resolved electron paramagnetic resonance of radical pair intermediates in cryptochromes

Electron transfer plays a key role in many biological systems, including core complexes of photosynthesis and respiration. As this involves unpaired electron spins, electron paramagnetic resonance (EPR) is the method of choice to investigate such processes. Systems that show photo-induced charge separation and electron transfer are of particular interest, as here the processes can easily be synchronised to the experiment and therefore followed directly over its time course. One particular class of proteins, the cryptochromes, showing charge separation and in turn spin-correlated radical pairs upon excitation with blue light, have been investigated by time-resolved EPR spectroscopy in great detail and the results obtained so far are summarised in this contribution. Highlights include the first observation of spin-correlated radical pairs in these proteins, a fact with great impact on the proposed role as key part of a magnetic compass of migratory birds, as well as the assignment of the radical-pair partners and the unravelling of alternative and unexpected electron transfer pathways in these proteins, giving new insights into aspects of biological electron transfer itself.