The primary reactions in the protochlorophyll(ide) photoreduction as investigated by optical and ESR spectroscopy.

It has been found that at low temperatures (77K-153K) a long-lived (at these temperatures) singlet ESR signal induced by intensive light appears in etiolated leaves of plants and in model systems including both the monomeric and aggregated protochlorophyll.Comparison of the results of ESR, fluorescence and absorption spectra measurements made it possible to suggest that at the initial stages of the protochlorophyll(ide) photoreduction process at least two paramagnetic non-fluorescent intermediates are formed, one of which seems to be identical to the previously found intermediate with absorption maximum at 690 nm. On the strength of the obtained results a conclusion can be drawn that photoreduction of the semi-isolated double-c=c-bond of the chlorophyll precursor molecule in etiolated leaves and in model systems is actualized via at least two stages of free radicals formation. A scheme of the primary reactions of chlorophyllide biosynthesis has been proposed.