ANALYSIS OF THE COMPLEX BAND SPECTRUM OF P700 BASED ON PHOTOSELECTION STUDIES WITH PHOTOSYSTEM I PARTICLES

Abstract

— The linear dichroism of the flash induced absorption changes of immobilized photosystem I reaction center particles from spinach chloroplasts has been investigated by means of the photoselection technique. Under flash excitation of predominantly β-carotenes at 500 nm, the photoinduced linear dichroism of the absorption changes has been measured in the spectral region from 620–720 nm. The most prominent feature in the spectrum of the dichroic ratio is the symmetrical pole at 687 nm. In parallel to all photoselection experiments, we recorded the light induced absorption changes of P700 under saturating flash excitation. A Gaussian deconvolution of this well-known difference spectrum of P700 has been attempted taking the additional features of the linear dichroism spectrum into account. From 670-720 nm, the best fit for both spectra was obtained by the following components: 1 a disappearing wide band at 695.5 nm (σ= 200.0 cm−1) attributed to the reduced special pair of chlorophyll a (Ch1-a); 2 an appearing narrower band at 690 nm (σ= 120.0 cm−1) with half the oscillatory strength of the former tentatively attributed to the non-oxidized moiety in the special pair; and 3 a bathochromic bandshift centered at 688.4 nm attributed to the local electrochromic response of certain antennae Ch1-a molecules close to the primary electron donor. The linear dichroism gives no evidence for any substructure within the absorption band of the reduced special pair.