Marion C. Thurnauer

A general model of electron spin polarization arising from the interactions within radical pairs

We present a general description of electron spin polarization observed for interacting radical pairs. Unlike previous treatments, both chemically induced dynamic electron polarization and correlated radical pair polarization are included for ST0 mixing. A key feature of this model is that the members of the pair can remain interacting throughout the time sequence of electron spin polarization. Explanations of the time‐dependent evolution of the electron spin polarization are presented using the density matrix and its associated vector diagram. A new vector method is formulated for calculating and visualizing chemically induced electron spin polarization. The approach is based on the vector, Δρ(t) which represents the displacement of the density vector ρ(t) from its direction at the time of its ‘‘birth.’’ In addition, these electron spin polarization phenomena are described in spectroscopic terms as well as in the usual mathematical manner.

The chlorophyll triplet state as a probe of structure and function in photosynthesis

In this review, we have tried to emphasize work on the triplet state of chlorophyll ( 3 Chl) in photosynthetic and related systems which has appeared since previous reviews were published, and to demonstrate the impact that the newly available crystal structure has had on many of these studies

X-ray absorption reveals surface structure of titanium dioxide nanoparticles.

Chemistry Division, Argonne National Laboratory, 9700 S. Cass A venue, Argonne, Illinois 60439, U.S.A. E-mail: Ichen@anlchm.chm.anL gov Fax: (630)-252-9289 peak intensities and peak positions. Their results indicated that as the coordination number of the Ti site increases from 4 to 6, the pre-edge intensity decreases and the energy of the peak increases by approximately 0.9 eV for each additional coordinating oxygen atom, starting from around 4969.6 eV for the tetrahedral Ti site. In this paper, we present our recent XAFS study at the Ti K-edge on TiO2 nanoparticles of 19 ]~ diameter without and with ascorbic acid. Based on our experimental results, as well as the results by Farges et a1.(1997), surface distortion of the Ti sites in small nanoparticles and its implication to the surface binding and photocatalytic activities will be discussed.

Electron spin echo envelope modulation due to exchange and dipolar interactions in a spin-correlated radical pair

Abstract The effects of exchange and electron—electron dipolar interactions on electron spin echo envelope modulation and free induction decay in a photo-induced spin-correlated radical pair are examined. Analytical expressions are derived for FID and spin echo signals as a two-dimensional function of the time intervals between the laser pulse and the microwave pulses. It is predicted that these interactions can induce deep modulation or oscillation on the out-of-phase spin echo envelope whereas the in-phase echo vanishes. The modulation frequencies provide useful information for systems with small exchange and dipolar interactions even if the hyperfine inhomogeneity often produces unresolved spectra in cw EPR.

Transient EPR of light-induced radical pairs in plant photosystem I: observation of quantum beats

Abstract Electron spin polarization of light-induced radical pairs in plant photosystem I is studied by transient EPR following pulsed laser excitation. The time evolution of the transverse magnetization is monitored at various static and microwave magnetic fields. Quantum beat oscillations are observed at early times after light excitation of fully deuterated whole algae Synechococcus lividus . Model calculations for the time profiles, based on the correlated radical pair mechanism, provide information on the spin—spin coupling and lifetime of the secondary radical pair.

The ordering of the zero field triplet spin sublevels in the chlorophylls. A magnetophotoselection study

Abstract The ordering of the zero field triplet spin sublevels in several monomeric chlrophylls has been determined by magneto- photoselection techniques. For all the chlorophylls which we have examined D > 0. The sign of E changes for the chlorophylls with a chlorin ring structure (i.e. chlorophyll a and related molecules) compared to those with a tetrahydroporphin ring structure (i.e., bacteriochlorophyll a and related molecules).

Surface modification of TiO2 nanoparticles with bidentate ligands studied by EPR spectroscopy

Abstract The surface of 50 A TiO 2 nanoparticle colloids was modified in order to improve the kinetic and redox characteristics of this semiconductor. The surface was derivatized with different bidentate ligands (thiolactic, β-mercaptopropionic, mercaptoacetic acids, and alanine) and was investigated by electron paramagnetic resonance (EPR) and IR spectroscopies. Infrared spectroscopy suggests that at pH 4 these compounds bind to Ti(IV) surface atoms through the carboxyl group. However, when a thiol group is in the a position with respect to the carboxyl group, surface Ti(IV) atoms become chelated with both the carboxyl and thiol groups resulting in five-membered ring formation. This results in the formation of a charge transfer complex with an optical absorption threshold at 520 nm. Illumination at 77 K of TiO 2 colloids with surface chelated Ti(IV) atoms in the absence of electron scavengers leads to the formation of the carboxyl cation radical (trapped holes) and three distinct Ti(III) centers (trapped electrons) at 4.2 K. When the temperature is increased, the hole moves to the CH 3 group, which is the farthest from the colloid surface. In the presence of electron accepting species (Pb 2+ , Cd 2+ ) the signal for the trapped electron disappears at 250 K indicating electron transfer to the accepting species.

Time-resolved electron spin echo spectroscopy applied to the study of photosynthesis

A recent development in the research of photosynthesis has been the application of fast time-resolved electron paramagnetic resonance (EPR) [l-4]. The EPR method is particularly promising for systems which cannot readily be studied by optical techniques. The radicals which have been observed in photosynthetic systems by the magnetic resonance technique exhibit chemically-induced dynamic electron polarization (CIDEP) [5]. The mechanism which produces this spin polarization (a non-Boltzmann distribution of spin population) reflects the earlier fate of the radicals. The reported studies of photosynthetic systems by time resolved magnetic resonance have utilized what are now fairly standard EPR techniques [6]. They have been limited by lack of adequate time resolution, possible distortions or artifacts due to the use of magnetic field modulation [3,7], and the problem of positively identifying the observed radical species. We are applying time-resolved electron spin echo (ESE) spectroscopy, a pulsed form of EPR, to the study of photosynthetic systems. This new technique not only avoids many of the problems described above but also provides several new advantages. We report here on the first observation of spin-polarized signals in the blue-green alga Synechococcus Zividus using timeresolved ESE, and illustrate advantages of the technique.

Comparison of the electron spin polarized spectrum found in plant photosystem I and in iron-depleted bacterial reaction centers with time-resolved K-band EPR; evidence that the photosystem I acceptor A1 is a quinone

The suggestion that the electron acceptor A1 in plant photosystem I (PSI) is a quinone molecule is tested by comparisons with the bacterial photosystem. The electron spin polarized (ESP) EPR signal due to the oxidized donor and reduced quinone acceptor (P870+Q-) in iron-depleted bacterial reaction centers has similar spectral characteristics as the ESP EPR signal in PSI which is believed to be due to P700+A1-, the oxidized PSI donor and reduced A1. This is also true for better resolved spectra obtained at K-band (∼24 GHz). These same spectral characteristics can be simulated using a powder spectrum based on the known g-anisotropy of reduced quinones and with the same parameter set for Q- and A1-. The best resolution of the ESP EPR signal has been obtained for deuterated PSI particles at K-band. Simulation of the A1- contribution based on g-anisotropy yields the same parameters as for bacterial Q- (except for an overall shift in the anisotropic g-factors, which have previously been determined for Q-). These results provide evidence that A1 is a quinone molecule. The electron spin polarized signal of P700+ is part of the better resolved spectrum from the deuterated PSI particles. The nature of the P700+ ESP is not clear; however, it appears that it does not exhibit the polarization pattern required by mechanisms which have been used so far to explain the ESP in PSI.

Very high frequency EPR — 94 GHz instrument and applications to primary reaction centers from photosynthetic red bacteria and to other disordered systems

There are many advantages to carrying out electron paramagnetic resonance experiments at very high frequencies, either as part of a multifrequency strategy for solving problems or for special characteristics of high frequencies. These special characteristics include the potential for high point sensitivity, enhanced resolution, separation of similar species, altered sensitivity to motion, suppression of motional effects, and many others. This paper describes a three-millimeter-wavelength (W-band, 94 GHz) EPR spectrometer built for a multi-user facility and illustrates with some examples, most of them being disordered systems. One significant example is the oxidized primary reaction center, P865+, isolated from the red photosynthetic bacteriaRhodobacter Sphaeroides R-26. The W-band technique applied to both centers isolated from bacteria grown from either deuterated or ordinary growth media allows extraction of the fullg anisotropy in these centers and sets the stage for multifrequency EPR spectroscopy to yield a full analysis of the various contributions to linewidths in these systems.