An attempt at quantitation of sharp light-induced electron paramagnetic resonance signal in photosynthetic materials

Quantitative determinations were made of the number of spins responsible for the sharp, fast decaying EPR signal induced by light in photosynthetic material. The species resposible for the observed EPR signal is present at the level of approx. 1 per 100 molecules of chlorophyll. Values ranging between 1 and 4 were obtained for the ratio between free radicals and molecules of photoconverter P700.

In view of the difficulties and errors inherent in the quantitation procedures involved, this finding does not exclude the possibility that the oxidized of P700 is the species responsible for the observed EPR signal, as the correlation of several properties of the radical and P700 had previously suggested. The presence of radical species other than or in addition to P700⁺ is, however, also compatible with the results.