PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Magnetic field dependent singlet oxygen production in a biological system. Yan Liu^*,1, Ruth Edge¹, Kevin Henbest², Christiane Timmel², Peter Hore², Peter Gast¹, ¹ Department of Biophysics, Leiden, The Netherlands² Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford, UK

ABSTRACT- The yield of singlet oxygen (¹_g, ¹O₂) photosensitized by bacterial photosynthetic reaction centers, and the ensuing oxidative damage to the protein complex and its associated cofactors, are shown to be magnetic field-dependent. ¹O₂ formed by flash photolysis of the carotenoidless mutant of Rhodobacter sphaeroides (R-26) is detected via its phosphorescence at 1270 nm. The ¹O₂ yield drops by 50% for magnetic fields of 20-100 mT and by 10% for a field of 1 mT. The photobleaching of the 800 nm absorption band of the accessory bacteriochlorophylls, resulting from ¹O₂ attack on the reaction centre, is about 45% smaller in a field of 15 mT than it is in the absence of an applied field. The origin of the magnetic field effect—the Radical Pair Mechanism—and the conditions under which the ¹O₂ yield might be increased by an applied magnetic field are both discussed. We believe this to be the first clear demonstration that a (modified) biological system, in which the Radical Pair Mechanism is known to operate, can generate toxic products in amounts that depend on the presence of a weak applied magnetic field. By applying the technique of ESR spin trapping we plan to extend these measurements to high magnetic fields (> 1T) where an increase in the singlet oxygen production is expected.

KEY WORDS: singlet oxygen, bacterial reaction centre, magnetic field effect, photodegradation