[ESR of adsorbed chlorophyll a]

Oxygen and water vapour effect on the dark ESR signal of chlorophyll a adsorbed on silica in vacuum has been studied. It has been determined that when O2 and H2O are absent chlorophyll a adsorption is accompanied by an increase of the paramagnetic states portion (PSP) of the pigment. The PSP is increasing when oxygen is admitted. Water vapour does not increase the PSP of the adsorbed chlorophyll a. It has been supposed that the ESR signal of chlorophyll a in solution belongs to the chlorophyll-oxygen complex and that any strengthening of the pi-pi-interactions results in intensification of the ESR signal. It has been concluded that the ESR signal of adsorbed chlorophyll a belongs to the charge transfer complex of the molecular oxygen and the chlorophyll aggregate with strong pi-pi-interactions.