The majot red form in Spirulina trimers (C709) is the low energy band of an excitonic dimer

Abstract

The strongly red shifted chlorophyll antenna forms associated with photosystem I are intriguing both from the standpoint of their biological function and the physicochemical interactions which give rise to them. Due to their uncommonly strong electron/phonon coupling and charge transfer characteristics it is often suggested that they are brought about by strong Coulombic interactions with other chlorophylls leading to the formation of excitonic dimers. The accompanying excitonic splitting would then give rise to a low energy, red shifted, absorption band. In the present study direct evidence for this suggestion is presented. Selective destruction of the strongly dichroic red shifted chlorophyll state (CD) and absorption maxima at 709nm and extinguishing near 725nm) in photosystem I trimers from Spirulina by either high intensity illumination (photobleaching) or incubation with low concentrations of Triton X-100 is accompanied by changes in the circular dichroism spectrum of the same amplitude and of opposite sign at 677nm. The difference spectra band shapes are strongly suggestive of a dimeric chlorophyll structure with excitonic bands peaking at these two wavelengths. In the assumption that the monomer absorption bands are of similar energy the interaction energy is in excess of 300cm-1. The observation that the red CD tail extinguishes near 725nm at room temperature while the absorption spectrum does so at 750nm is discussed in termsof the low temperature CD spectrum.