Alan R. McIntosh

Flash photolysis electron spin resonance studies of the electron acceptor species at low temperatures in photosystem I of spinach subchloroplast particles.

The light-induced electron spin resonance signals of Photosystem I spinach subchloroplast particles have been studied at approximately 6 degrees K. Using the technique of flash photolysis-electron spin resonance with actinic illumination at 647 nm, a kinetic analysis of the previously observed bound ferredoxin ESR signals was carried out. Signal I (P700+) exhibits a partial light-reversible behavior at 6 degrees K so it was expected that if the bound ferredoxin is the primary acceptor of Photosystem I, it should also exhibit a partial reversible behavior. However, none of the bound ferredoxin ESR signals showed any such light reversible behavior. A search to wider fields revealed two components which did exhibit the expected kinetic behavior. These components are very broad (about 80 G) and are centered at g equals to 1.75 and g equals to 2.07. These two components exhibit the expected characteristics of the primary electron acceptor. A model is presented to account for the reversible and irreversible photochemical changes in Photosystem I. The possible identity of the primary acceptor responsible for these two new components, is discussed in terms of the available information. The primary acceptor may be an iron-sulfur protein, but not of the type characteristic of the bound or water-soluble ferredoxins found so far in chloroplasts.

Synthesis of a model compound for the photosynthetic electron transfer

Abstract In order to elucidate the role of special pair chlorophyll in photosynthetic electron transfer, a new model compound 1, where two etioporphyrins are covalently tied in a face-to-face orientation and further linked to a quinone with a polymethylene chain, was synthesized.

CIDEP observations in photosystem I of green plant and algal photosynthesis

Abstract Flash photolysis experiments with electron paramagnetic resonance detection were carried out between 10 K and 300 K on samples of green plant and algal species. Chemically induced dynamic electron polarization was evident for the signals observed in the g = 2.0 region for 100 KHz modulated detection and also for a system with no magnetic field modulation. The light reversible signals decaying in about 1 ms at low temperatures are interpreted as arising from photosystem I of the green plant and algal samples. Evidence is presented which indicates that the origin of the electron spin polarization is the well established radical-pair mechanism.

Electron spin resonance spectrum of species "X" which may function as the primary electron acceptor in photosystem I of green plant photosynthesis.

Purified Photosystem I particles from spinach when reduced with 10 mM dithionite at pH 9 exhibited a 50% light reversible-ESR Signall (P-700+) at about 10 K. It was possible to show by signal-averaging techniques that a light-reversible ESR spectrum concomitant with the reversible Single 1 can be observed with approximate principal g factors at g = 2.07, g = 1.86 and g = 1.75.

A study of chemically induced dynamic electron polarization (CIDEP) in Photosystem I of whole algal cells at ambient temperatures

Abstract Time-resolved electron paramagnetic resonance (EPR) studies were carried out at room temperature and at 273 K on whole-cell samples of the photosynthetic algae: Anacystis nidulans and Scenedesmus obliquus , the latter being 97% deuterated from the growing medium. These photosynthetic organisms show greatly enhanced EPR signals which result from the generation of nonequilibrium spin populations, a phenomenon known as chemically induced dynamic electron polarization (CIDEP). We report magnetic-field profiles of the early transient signals of Photosystem I which are very similar to those observed at low temperatures. The results suggest that one or more early reduced electron acceptors in Photosystem I are being observed at ambient physiological temperatures.

The utilization of time-resolved dielectric loss to probe the role of the surface in heterogeneous photochemistry

The measurement of time-resolved (or transient) dielectric loss in semiconductors using an e.p.r. spectrometer interfaced to a flash-photolysis system is discussed. In particular, it is demonstrated that aqueous dispersions of pigments can readily be studied using this technique. In this way, the system can be monitored in the presence of donors and acceptors where dynamic interfacial redox reactions are occurring. A flow system was utilized to avoid setting problems as well as providing the potential for the acquisition of first-flash kinetic data. Finally, surface modified pigments are contrasted to their corresponding starting materials.