Chapter 5 Electron paramagnetic resonance in photosynthesis

Electron magnetic resonance in heterogeneous photocatalysis research

The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.

Twenty years of biophysics of photosynthesis in Padova, Italy (1984-2005): a tale of two brothers

This paper tells the history of two brothers, almost a generation apart in age, who met again, after having followed different academic paths, to introduce biophysical research in photosynthesis at the University of Padova. The development of two research groups, one in the Chemistry Department, the other in the Biology Department led to a comprehensive interdisciplinary group across academic barriers. The group of Giovanni Giacometti developed in Physical Chemistry, during the years before his retirement, with some roots which can be traced to the famous Linus Pauling school of the mid 1950s, and made possible, by the work of many students (especially Donatella Carbonera and Marilena Di Valentin) and of an older associate (Giancarlo Agostini). The group participated quite actively with a number of European and American laboratories in the application of physical techniques, especially Electron Spin Resonance (EPR) associated with Optical Spectroscopy (Optically Detected Magnetic Resonance; ODMR), and contributed to the development of the understanding of the structure-function relationships in photosynthetic membrane complexes, stimulated by the determination of the X-ray structure of the purple photosynthetic reaction center in the mid 1980s ( J. Deisenhofer, H. Michel, R. Huber and others). The younger brother of Giovanni, Giorgio Mario Giacometti, came to Padova after obtaining biochemical knowledge from the Rossi-Fanelli school in Rome, where Jeffries Wyman, Eraldo Antonini and Maurizio Brunori were the world masters of hemoglobin research. In Padova, together with a group of young scientists (at first Roberto Bassi and Roberto Barbato, now leaders of their own groups in Verona and in Alessandria respectively, followed soon by brilliant coworkers such as Fernanda Rigoni, Elisabetta Bergantino and more recently Ildikò Szabò and Paola Costantini), Giorgio approached more biochemical themes of oxygenic photosynthesis, such as purification and characterization of antenna chlorophyll-protein complexes, Photosystem II (PS II) particles and subunits, having always in mind structural and molecular problems at the level of the largest integrated particles, which are more difficult to investigate in detail by the spectroscopic techniques.

Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems

This review focuses on the recent advances in EPR spectroscopy as they are applied both to photoinduced electron transfer in the photosynthetic apparatus and to biomimetic systems. The review deals with time-resolved direct-detection cw and pulsed EPR and ENDOR methods, both at conventional bands [X-(9.5 GHz), K-(24 GHz), and Q-(35 GHz)(] and at high frequency bands (W-band, 95 GHz, and even higher frequency bands). EPR studies on photosynthetic and model systems in their doublet, triplet and radical pair states are surveyed, including their static and dynamic properties. APplications of time-resolved EPR in studying photoinduced electron and energy transfer in isotropic and anisotropic environments, and the concepts of electron spin polarization and magnetic field effects in photochemical reactions are also reviewed.

Long-wavelength absorbing antenna pigments and heterogeneous absorption bands concentrate excitons and increase absorption cross section

The light-harvesting apparatus of photosynthetic organisms is highly optimized with respect to efficient collection of excitation energy from photons of different wavelengths and with respect to a high quantum yield of the primary photochemistry. In many cases the primary donor is not an energetic trap as it absorbs hypsochromically compared to the most red-shifted antenna pigment present (long-wavelength antenna). The possible reasons for this as well as for the spectral heterogeneity which is generally found in antenna systems is examined on a theoretical basis using the approach of thermal equilibration of the excitation energy. The calculations show that long-wavelength antenna pigments and heterogeneous absorption bands lead to a concentration of excitons and an increased effective absorption cross section. The theoretically predicted trapping times agree remarkably well with experimental data from several organisms. It is shown that the kinetics of the energy transfer from a long-wavelength antenna pigment to a hypsochromically absorbing primary donor does not represent a major kinetic limitation. The development of long-wavelength antenna and spectrally heterogeneous absorption bands means an evolutionary advantage based on the chromatic adaptation of photosynthetic organelles to spectrally filtered light caused by self-absorption.

The carboxyl modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibits half of the high-affinity Mn-binding site in photosystem II membrane fragments

The diphenylcarbazide(DPC)/Mn2+ assay [Hsu, B.-D., Lee, J.-Y., & Pan, R.-L. (1987) Biochim. Biophys. Acta 890, 89-96] was used to assess the amount of the high-affinity Mn-binding site in manganese-depleted photosystem II (PS II) membrane fragments from spinach and Scenedesmus obliquus. The assay mechanism at high DPC concentration was shown to involve noncompetitive inhibition of only half of the control level of DPC donation to PS II by micromolar concentrations of Mn at pH 6.5 (i.e., one of two DPC donation sites is inhibited). At low DPC concentration both DPC and Mn2+ donate to PS II additively. Treatment with the carboxyl amino acid modifier 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide (EDC) inhibited half of the high-affinity Mn-binding site in spinach and Scenedesmus WT PS II membranes and all of the available site in Scenedesmus LF-1 mutant PS II membranes. A similar EDC concentration dependence was observed in all cases. Addition of 2 mM MnCl2 to the 10 mM EDC modification buffer provided complete protection for the Mn-binding site from modification. This protection was specific for Mn2+; six other divalent cations were ineffective. We conclude that EDC modifies that half of the high-affinity Mn-binding site that is insensitive to the histidine modifier diethyl pyrocarbonate (DEPC) [Seibert, M., Tamura, N., & Inoue, Y. (1989) Biochim. Biophys. Acta 974, 185-191] and directly affects ligands that bind Mn. The effects of EDC and DEPC that influence the high-affinity site are mutually exclusive and are specific to the lumenal side of the PS II membrane. Removal of the two more loosely bound of the four functional Mn from PS II membranes uncovers that part of the high-affinity site associated with carboxyl but not histidyl residues. We suggest that carboxyl residues on reaction center proteins are associated with half of the high-affinity Mn-binding site in PS II and are involved along with histidine residues in binding Mn functional in the O2-evolving process.

A guide to electron paramagnetic resonance spectroscopy of Photosystem II membranes

This guide is intended to aid in the detection and identification of paramagnetic species in Photosystem II membranes, by electron paramagnetic resonance spectroscopy. The spectral features and occurrence of each of the electron paramagnetic resonance signals from Photosystem II are discussed, in relation to the nature of the moiety giving rise to the signal and the role of that species in photosynthetic electron transport. Examples of most of the signals discussed are shown. The electron paramagnetic resonance signals produced by the cytochrome b6f and Photosystem I complexes, as well as the signals from other common contaminants, are also reviewed. Furthermore, references to seminal experiments on bacterial reaction centers are included. By reviewing both the spectroscopic and biochemical bases for the electron paramagnetic resonance signals of the cofactors that mediate photosynthetic electron transport, this paper provides an introduction to the use and interpretation of electron paramagnetic resonance spectroscopy in the study of Photosystem II.