Spin-correlated radical pairs in photosynthetic reaction centers: role of anisotropic hyperfine interaction as revealed by computational modeling

Assembly of galvinoxyl doped in polymer-fullerene photovoltaic blends

Galvinoxyl (Gx) is a stable free radical used as a dopant in active layers of organic solar cells. Here, the nanoscale arrangement of Gx molecules in a composite of the PCDTBT polymer and modified C60 fullerene, PCBM, was studied using a two-pulse electron spin echo (ESE) technique. The results show that the Gx molecules assemble into clusters, which can be described by the model of 8 molecules on the surface of a sphere with a radius of 2.0 nm. Such a structure can arise due to the octahedral packing of 6 PCBM molecules surrounded by 8 Gx molecules. ESE decays also indicate that these clusters repel each other, forming a quasi-regular nanostructure in the matrix. The Gx concentration of 2 wt% at which clusters appear correlates with the literature data on the Gx-induced enhancement of photocurrent, which provides structural insight into the possible molecular mechanism of this enhancement.

OOP-ESEEM Spectroscopy: Accuracies of Distances of Spin-Correlated Radical Pairs in Biomolecules

In addition to the commonly used electron–electron double resonance (ELDOR) technique, there are several other electron paramagnetic resonance (EPR) methods by which structure information can be obtained by exploiting the dipolar coupling between two radicals based on its characteristic r⁻³ dependence. In this contribution, we explore the potential of out-of-phase-electron-spin echo envelope modulation (OOP-ESEEM) spectroscopy to collect accurate distance information in photo-sensitive (bio) molecules. Although the method has already been applied to spin-correlated radical pairs in several classes of light-active proteins, the accuracy of the information obtained has not yet been extensively evaluated. To do this in a system-independent fashion, OOP-ESEEM time traces simulated with different values of the dipolar and exchange couplings were generated and analyzed in a best-possible way. Excellent agreement between calculated and numerically fitted values over a wide range of distances (between 15 and 45 Å) was obtained. Furthermore, the limitations of the method and the dependence on various experimental parameters could be evaluated.

Pulsed EPR spectroscopy on short-lived intermediates in Photosystem I

The application of pulsed electron paramagnetic resonance spectroscopy on short-lived intermediates in Photosystem I is reviewed. The spin polarization in light-induced radical pairs gives rise to a phase shifted 'out-of-phase' electron spin echo signal. This echo signal shows a prominent modulation of its intensity as a function of the spacing between the two microwave pulses. Its modulation frequency is determined by the electron-electron spin couplings within the radical pair. Thereby, the measurement of the dipolar coupling gives direct information about the spin-spin distance and can therefore be used to determine cofactor distances with high precision. Application of this technique to the radical pair P(*+)(700)A(*-)(1) in Photosystem I is discussed. Moreover, if oriented samples (e.g. single crystals) are used, the angular dependence of the dipolar coupling can be used to derive the orientation of the axis connecting donor and acceptor with respect to an external (crystal) axes system. Using out-of-phase electron spin echo envelope modulation spectroscopy, the localization of the secondary acceptor quinone A(1) has become possible.

Recruitment of a foreign quinone into the A(1) site of photosystem I. II. Structural and functional characterization of phylloquinone biosynthetic pathway mutants by electron paramagnetic resonance and electron-nuclear double resonance spectroscopy

Electron paramagnetic resonance (EPR) and electron-nuclear double resonance studies of the photosystem (PS) I quinone acceptor, A(1), in phylloquinone biosynthetic pathway mutants are described. Room temperature continuous wave EPR measurements at X-band of whole cells of menA and menB interruption mutants show a transient reduction and oxidation of an organic radical with a g-value and anisotropy characteristic of a quinone. In PS I complexes, the continuous wave EPR spectrum of the photoaccumulated Q(-) radical, measured at Q-band, and the electron spin-polarized transient EPR spectra of the radical pair P700(+) Q(-), measured at X-, Q-, and W-bands, show three prominent features: (i) Q(-) has a larger g-anisotropy than native phylloquinone, (ii) Q(-) does not display the prominent methyl hyperfine couplings attributed to the 2-methyl group of phylloquinone, and (iii) the orientation of Q(-) in the A(1) site as derived from the spin polarization is that of native phylloquinone in the wild type. Electron spin echo modulation experiments on P700(+) Q(-) show that the dipolar coupling in the radical pair is the same as in native PS I, i.e. the distance between P700(+) and Q(-) (25.3 +/- 0.3 A) is the same as between P700(+) and A(1)(-) in the wild type. Pulsed electron-nuclear double resonance studies show two sets of resolved spectral features with nearly axially symmetric hyperfine couplings. They are tentatively assigned to the two methyl groups of the recruited plastoquinone-9, and their difference indicates a strong inequivalence among the two groups when in the A(1) site. These results show that Q (i) functions in accepting an electron from A(0)(-) and in passing the electron forward to the iron-sulfur clusters, (ii) occupies the A(1) site with an orientation similar to that of phylloquinone in the wild type, and (iii) has spectroscopic properties consistent with its identity as plastoquinone-9.