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Photochemical Reactions and Photoinduced Electron-Transfer Processes in Liquids, Frozen Solutions, and Proteins as Studied by Multifrequency Time-Resolved EPR Spectroscopy
- Source :
- ChemInform. 38
- Publication Year :
- 2007
- Publisher :
- Wiley, 2007.
-
Abstract
- In this overview, modern multifrequency EPR spectroscopy, in particular at high magnetic fields, is shown to provide detailed information about structure, motional dynamics, and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise photochemical reactions in liquid solution and light-initiated electron transfer processes both in biomimetic donor–acceptor model systems in frozen solution or liquid crystals and in natural photosynthetic-reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects. They contain valuable information about structure and dynamics of the transient reaction intermediates. Moreover, they are exploited for signal enhancement. Continuous-wave (cw) and pulsed versions of time-resolved high-field EPR spectroscopy, such as cw-transient-EPR (TREPR) and pulsed-electron-spin-echo (ESE) experiments, are compared with respect to their advantages and limitations for the specific system under study. For example, W-band (95-GHz) TREPR spectroscopy in conjunction with a continuous-flow system for light-generated short-lived transient spin-polarized radicals of organic photoinitiators in solution was performed with a time resolution of 10 ns. The increased Boltzmann polarization at high fields even allows detection of transient radicals without CIDEP effects. This enables one to determine initial radical polarization contributions as well as radical-addition reaction constants. Another example of the power of combined X-band and W-band TREPR spectroscopy is given for the complex electron-transfer and spin dynamics of covalently linked porphyrin–quinone as well as Watson–Crick base-paired porphyrin–dinitrobenzene donor–acceptor biomimetic model systems. Furthermore, W-band ESE experiments on the spin-correlated coupled radical pair in reaction centers of the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P865QA. The high orientation selectivity of high-field EPR provides single-crystal-like information even from disordered frozen-solution samples. The examples given demonstrate that high-field EPR adds substantially to the capability of ‘classical’ spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical systems, since transient intermediates can be observed in real time in their working states on biologically relevant time scales.
- Subjects :
- Chemistry
Radical
Organic Chemistry
Reaction intermediate
General Medicine
Photochemistry
Biochemistry
Catalysis
Photoinduced electron transfer
law.invention
Inorganic Chemistry
Paramagnetism
Electron transfer
Spin chemistry
law
Drug Discovery
Physical and Theoretical Chemistry
Spectroscopy
Electron paramagnetic resonance
Subjects
Details
- ISSN :
- 15222667 and 09317597
- Volume :
- 38
- Database :
- OpenAIRE
- Journal :
- ChemInform
- Accession number :
- edsair.doi.dedup.....5d56f11c8d18506ad6582ff64e31a7d2
- Full Text :
- https://doi.org/10.1002/chin.200703276