The radical pair mechanism is used to elucidate how applied magnetic fields that are weaker in strength than typical hyperfine interactions can influence the yields and kinetics of recombination reactions of free radicals in solution. The so-called low field effect is shown to arise from coherent superpositions of degenerate electron-nuclear spin states in a spin-correlated radical pair in zero field. A weak applied magnetic field causes these (zero-quantum) coherences to oscillate, leading to coherent interconversion of singlet and triplet electronic states of the radical pair and hence changes in the yields of recombination products and of the free radicals that escape into solution. For singlet geminate radical pairs, the low field effect leads to a boost in the concentration of free radicals, which may be relevant in the context of in vivo biological effects of electromagnetic fields. Using analytical approaches in limiting cases, the maximum possible low field effects are calculated for a variety of ...

Effects of weak magnetic fields on free radical recombination reactions

SVD-based quantification of magnetic resonance signals

Photophysics of photosynthesis. Structure and spectroscopy of reaction centers of purple bacteria

Preface. Part One: Optical Methods. 1. Developments in Classical Optical Spectroscopy J. Amesz. 2. Linear and Circular Dichroism G. Garab. 3. Fluorescence K. Sauer, M. Debreczeny. 4. Ultrafast Spectroscopy of Photosynthetic Systems R. Jimenez, G.R. Fleming. 5. Data Analysis of Time-Resolved Measurements A.R. Holzwarth. 6. Photosynthetic Thermoluminescence as a Simple Probe of Photosystem II Electron Transport Y. Inoue. 7. Accumulated Photon Echo Measurements of Excited State Dynamics in Pigment-Protein Complexes T.J. Aartsma, R.J.W. Louwe, P. Schellenberg. 8. Spectral Hole Burning: Methods and Applications to Photosynthesis N. Raja, S. Reddy, G.J. Small. 9. Infrared and Fourier-Transform Infrared Spectroscopy W. Mantele. 10. Resonance Raman Studies in Photosynthesis - Chlorophyll and Carotenoid Molecules B. Robert. 11. Stark Spectroscopy of Photosynthetic Systems S.G. Boxer. 12. The Photoacoustic Method in Photosynthesis - Monitoring and Analysis of Phenomena which Lead to Pressure Changes Following Light Excitation S. Malkin. Part Two: Magnetic Resonance. 13. Magnetic Resonance: an Introduction A.J. Hoff. 14. Time-Resolved Electron Paramagnetic Resonance Spectroscopy - Principles and Applications H. Levanon. 15. Electron Spin Echo Methods in Photosynthesis Research R.D. Britt. 16. ENDOR Spectroscopy W. Lubitz, F. Lendzian. 17. Optically Detected Magnetic Resonance (ODMR) of Triplet States in Photosynthesis A.J. Hoff. 18. MagicAngle Spinning Nuclear Magnetic Resonance of Photosynthetic Components H.J.M. de Groot. Part Three: Structure and Oxygen. 19. Structure Determination of Proteins by X-Ray Diffraction M. Schiffer. 20. Electron Microscopy E.J. Boekema, M. Roegner. 21. X-Ray Absorption Spectroscopy:Determination of Transition Metal Site Structures in Photosynthesis V.K. Yachandra, M.P. Klein. 22. Moessbauer Spectroscopy P.G. Debrunner. 23. Characterization of Photosynthetic Supramolecular Assemblies Using Small Angle Neutron Scattering D.M. Tiede, P. Thiyagarajan. 24. Measurements of Photosynthetic Oxygen Evolution H.J. van Gorkom, P. Gast.

Biophysical techniques in photosynthesis

Improved algorithm for noniterative time-domain model fitting to exponentially damped magnetic resonance signals.

An alternative to fourier transform spectral analysis with improved resolution

Using e.s.e. (electron spin-echo), RYDMR (reaction-yield-detected magnetic resonance) and e.s.r. (electron spin resonance) in protein single crystals, we have investigated the cation, the triplet and the initial radical pair associated with the process of photoinduced charge separation in photosynthesis. The initial charge separation of bacterial photosynthesis occurs in a few picoseconds within in a reaction-centre protein containing eight electron-transfer components, namely four bacterial chlorophylls, two bacteriopheophytins and two quinones. Two bacteriochlorophylls, BChl2, form the primary donor and one bacteriopheophytin serves as the primary acceptor. E.s.e. determination of the anisotropic 15N hyperfine interactions have shown that the primary donor cation resides symmetrically in two of the four BChls in the form of a special pair of bacterial chlorophylls, BChl+2. In direct contrast, our e.s.r. studies on the triplet state of the primary donor in single crystals of reaction centres suggest that the triplet state resides asymmetrically in BChl2, where the triplet is highly asymmetrical in R. viridis but is much more symmetrical (C2) in R. sphaeroides. RYDMR studies of the initial radical pair formed indicate that the extra bacteriochlorophyll molecule, BChlB, that is between the special pair donor cation and the primary acceptor bacteriopheophytin anion is not involved in a discrete electron-transfer step in bacterial reaction centres. By combining the results of our magnetic-resonance experiments with X-ray structural information, the following description emerges: (1) the ground-state, primary donor is a supermolecule dimer with approximately C2 symmetry; (2) the bridging BChlB molecule probably functions as a superexchange site for rapid transfer of electrons from the primary donor to the primary acceptor but with negligible back reaction; (3) the special pair, BChl2, is lower in energy than the bridging molecule, BChlB, such that the initial radical pair is formed via super-exchange between the distant (10 A edge-to-edge) special pair BChl2 and the bacteriopheophytin.

Magnetic resonance of ultrafast chemical reactions

The electron spin echo modulation spectrum of organic radicals in randomly oriented solids is analyzed in detail for axially symmetric hyperfine tensors. Numerical calculations of the modulation spectra are compared with experimental results from the radical cations of 15N labeled bacteriochlorophyll a and the primary donor of photosynthetic bacterium Rhodospirillum rubrum. Determination of hyperfine coupling constants from experimental spectra in the frequency domain is complicated by the anisotropy of modulation intensity, the effects of second and higher order hyperfine interactions, overlapping spectra from several nuclei, multiple quantum coherences among hyperfine levels, and the spectrometer deadtime. Nonlinear least‐squares analysis of the time domain data [Tang et al., J. Chem. Phys. 83, 4197 (1985)] is the only reliable method for extracting coupling constants from randomly oriented organic radicals.

Analysis of electron spin echo modulation in randomly oriented solids: 15N modulation of radical cations of bacteriochlorophyll a and the primary donor of photosynthetic bacterium Rhodospirillum rubrum

A novel analysis of EPR measurements on polyacetylene is demonstrated by the analysis of the conventional line shape in time domain. Quantitative results of the hyperfine-coupling constant, the on-chain diffusion rate, and the off-chain hopping rate were extracted by nonlinear curve fitting to the time-domain signals, and they are consistent with the soliton model of polyacetylene.

C. P. Lin

Papers

An alternative to fourier transform spectral analysis with improved resolution

Magnetic resonance of ultrafast chemical reactions

Analysis of electron spin echo modulation in randomly oriented solids: 15N modulation of radical cations of bacteriochlorophyll a and the primary donor of photosynthetic bacterium Rhodospirillum rubrum

Time-domain analysis of EPR measurements of polyacetylene and soliton diffusion

Electron spin echo study of iodine doped trans-polyacetylene