Mark G. Roelofs

The effect of large magnetic fields and the g-factor difference on the triplet population in photosynthetic reaction centers

Abstract The triplet population in reduced photosynthetic reaction centers is found to increase on application of large magnetic fields (1.5–14 KG) We conclude that the triplet is formed by charge recombination of spin-correlated radical ions with different g -factors. An appropriate theory for the field dependence gives the value δ g = 0.001.

Contributions of spin-spin interactions to the magnetic field dependence of the triplet quantum yield in photosynthetic reaction centers

Abstract Calculations are presented which show that dipolar coupling in the primary radical ion pair of quinone-depleted photosynthetic reaction centers substanually affects the magnetic field dependence of the triplet quantum yield, as does exchange coupling to the semiquinone-Fe(II) complex, when quinone is present. Inclusion of these interactions resolves significant discrepancies between theory and experiment.

Dependence of the yield of a radical-pair reaction in the solid state on orientation in a magnetic field

as is normally done, the spin relaxation rate would be expected to be much more rapid. A combination of dipolar coupling and a small amount of exchange coupling between the iron and copper could reasonably produce relaxation rates that broaden both signals past the point of experimental ~bservability.~’ The calculated magnetic moment (assuming the remaining iron and copper are both S = for such a spin system agrees with experimental values re- ported for the er~zyme~-~ at least as well as the antiferromag- netically coupled high-spin The major advantage of this intermediate-spin relaxation model is that it avoids the ne- cessity of requiring an exceptionally and possibly unreasonably large Fe-Cu coupling constant while at the same time adequately explaining both the experimental bulk susceptibility and EPR spectrum. In summary, the trimeric complex reported here exhibits some unusual and unexpected properties. The observed behavior of the model suggests a description for the spin behavior of the oxidase that is different from the explanation that is most generally ac- cepted. The relative merit of these two alternate explanations should now be subjected to an appropriate degree of critical evaluation based

The Mechanism of the Oscillating Air Oxidation of Benzaldehyde

During the oxygen or air oxidation of benzaldehyde catalyzed by cobalt and bromide several measureable parameters oscillate including: redox potential, absorbance at 620 nm, oxygen concentration, rate of benzaldehyde disappearance and free radical concentration. At 55–100°C in a 90/10 acetic acid/water mixture the oscillations will last for many hours[1]. A typical cycle is shown in Figure 1.