Energy exchange between the chlorophyll antennae of monomeric subunits within the Photosystem I trimeric complex of the cyanobacterium Spirulina

The energy exchange between antenna chlorophylls of the monomeric subunits within a Photosystem I trimeric complex of the cyanobacterium Spirulina was studied, comparing the kinetics of the light-induced decrease of Photosyst em I fluorescence at 760 nm, emitted by the longwave chlorophyll absorbing at 735 nm and P700 photooxidation. Both kinetics have been measured at 77 K simultaneously using a fiberoptic system. The light-induced decrease of fl uorescence at 760 nm in trimers was faster than P700 oxidation, although th e decrease of fluorescence at 760 nm was ascribed to the quenching effect o f oxidized P700 that is a stronger quencher of fluorescence at 760 nm than P700 reduced [Shubin et al. (1995) J Photochem. Photobiol 27B: 153-160]. Th e proportionality of the reciprocal value of the half time for the fluoresc ence decrease at 760 nm in trimers to the light intensity indicates the pho tochemical nature of the fluorescence quenching. The nonlinear relationship between the variable part of fluorescence at 760 nm and the fraction of re duced P700 is considered to be a result of an energetic connectivity of the antennae of monomeric subunits within a trimer. When one P700 in PS I trim ers of Spirulina is oxidized, the energy from an antenna of monomeric subun it(s) with reduced P700 may migrate to a subunit with oxidized P700 and que nched there. This may explain the slower rate of P700 photooxidation in tri mers as compared with monomers. The analytical description of cooperativity processes in Photosystem I coincides well with the measured data. A model is presented describing the energetic interaction of chlorophyll antennae o f monomeric subunits within the trimer via the extreme long-wavelength chlo rophyll form. This intersubunit interaction may stimulate the dissipation o f excess energy as heat and, therefore, protect the pigment-protein complex against photodestruction.