Abstract:Linear dichroism (LD) spectroscopy is an important technique in the study of the orientation and organization of pigments in the photosynthetic membrane complexes in vivo and in vitro. In this work, the orientation of the pigments in the isolated photosystem Ⅱ (PSⅡ) sub-core reaction center complexes was analyzed and characterized by means of low temperature absorption and LD spectroscopy. The preparations containing different amounts of CP47 isolated from spinach (Spinacia oleracea L.) chloroplast were used in order to investigate the orientation of pigments in the PSⅡ sub-core CP47/D1/D2/Cyt b-559 (CP47/D1/D2) complexes. Chlorophyll a (Chl a) absorbing at 680 nm in CP47/D1/D2/Cyt b-559 complex showed an orientation of the Q y transition parallel to the membrane plane. It is proposed that there are two forms of β-carotene (β-Car) in CP47/D1/D2/Cyt b-559 complex, denoted as β-Car (Ⅰ)and β-Car (Ⅱ), with different orientations, β-Car (Ⅰ) at 470 and 505 nm is roughly parallel to the membrane plane, and β-Car (Ⅱ) at 460 and 490 nm seems to be perpendicular orientation. Upon the photoinhibitory experiment β-Car (Ⅱ) was found to be photosensitive and easily photodamaged. It also showed that the positive LD signal observed at 680 nm was quite complicated. This signal is tentatively attributed to P680 and some Chl a of antenna in CP47 protein based upon our measurements.