Stabilization of electron spin resonance probes for photosynthesis studies.

The major obstacle to the study of functional/structural interrelationships of spinach chloroplasts by using spin labels has been the rapid loss of the electron paramagnetic resonance (EPR) signals upon illumination with visible light. The present study demonstrates that the addition of ferredoxin and NADP+ in the presence of N-tris(hydroxymethyl)methylglycine (Tricine) buffer at pH 7.1 or higher mitigates the rapid loss of Biradical X [N,N'-bis(1-oxyl - 2,2,5,5 - tetramethylpyrroline-3-carboxy)-1,2-diaminoethane] and Monradical A (2,2,5,5-tetramethyl-3-carbamidpyrroline-1-oxyl). However, the 5-line EPR spectrum characteristic of Biradical X in aqueous solution was changed to a dominantly 3-line spectrum within a few minutes after illumination in the presence of ferredoxin and NADP+. Analysis of the double integration of the first derivative EPR spectrum revealed no decrease in Biradical X concentration for more than 30 min of illumination. Our data suggest that Biradical X attaches to some soluble macromolecule(s) and that illumination of chloroplasts promotes such an attachment.