Photosynthesis is one of the fascinating fields of current interdisciplinary research. It seems miraculous that nature, in the process of evolution, has managed to bring about the process of photosynthesis. The first step involves a charge separation at the reaction centers, which proceeds with 100% quantum yield from the photoexcited singlet state of the bacteriochlorophyll donor, despite the fact that the wasteful deactivation of the electron into the ground state should be highly favored. Biomimetic model compounds (that is, those which resemble the pigments nature has developed) have been constructed from porphyrins and quinones. These model systems have allowed the study of the factors contributing to the highly efficient charge separation. This report focuses on recent developments in the study of electron transfer in porphyrinoquinones. Some of the results of these investigations may not be not fully understood and are often the subject of controversial discussions.

Partially deuterated and various substituted flavin and thiaflavin model compounds have been synthesized. For the first time, high-resolution H, D, and 14N ENDOR and TRIPLE resonance experiments in fluid solutions have been performed on the paramagnetic derivatives of these compounds. Additionally, valuable information has been obtained about hyperfine anisotropies and molecular structures from ENDOR in rigid matrices. Solid matrix ENDOR studies of native flavoenzymes, namely, "Old Yellow Enzyme" (NADPH dehydrogenase), two flavodoxins, and a methanol oxidase are reported. The ENDOR matrix signals of the various flavoproteins are different in intensity, suggesting that the microenvironments are remarkably different. Applicabilities and limitations of the ENDOR technique in the studies of flavins and flavoenzymes are discussed.

Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time-domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet- and probably singlet-initiated ET routes can concurrently be detected, and the free energy of the charge-separated state is estimated from the spectral dynamics.

Intramolecular electron transfer (ET) in three photosynthetic model systems, oriented in liquid crystals (LCs), was monitored by continuous wave time-resolved electron paramagnetic resonance (CW-TREPR) spectroscopy: (1) zinc porphyrin (ZnTPP) linked via an amide spacer to a lumiflavin (PaF); (2) ZnTPP linked to a benzoquinone via a phenyl spacer in the para (p-PpQ); and (3) in the meta (m-PpQ) positions. The anisotropic liquid crystalline environment makes the ET products detectable over a wide range of temperatures, Le., 210 I T I 330 K. Under such experimental conditions the ET rates are reduced quite dramatically into the solvent controlled adiabatic regime. The spectral line shape differences reflect the effect of the molecular architecture, namely, the relative orientation of the donor-acceptor as well as the spacer moiety. These differences in molecular structures are manifested by the TREPR spectra through the magnitude of the spinspin coupling (J) and the dipolar interaction (D), thus leading to different electron spin polarization mechanisms.

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