Photo-Induced Electron Spin Polarization in Chemical and Biological Reactions: Probing Structure and Dynamics of Transient Intermediates by Multifrequency EPR Spectroscopy

Abstract

In this minireview, modern multifrequency electron paramagnetic resonance (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 spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin–quinone donor–acceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $$ {\text{P}}_{865}^{ \cdot + } $$ $$ {\text{Q}}_{\text{A}}^{ \cdot - } $$ in frozen solution reaction centers from 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. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the QA binding site does not change under photoreduction of the quinone—in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of “classical” spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.