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.
Photodamage of pheophytin a (pheo a) in the isolated photosystem Ⅱ (PSⅡ) reaction center D1/D2/Cyt b559 complex from spinach has been investigated by high performance liquid chromatographic method in detail. The results showed that: (1) There is one pheo a molecule which is not associated with the primary photochemistry in the PSⅡ reaction center complex. It may be considered that there are two different electron transfer branches in the PSⅡ reaction center just as in the purple bacterium photosynthetic reaction center. (2) The damaged pheo a may be attributed to the one bonding to the D2 protein comparing the D2 subunit in the PSⅡ reaction center with M subunit in the purple bacterium photosynthetic reaction center. (3) A possible arrangement model of redox cofactors in the PSⅡ reaction center was proposed based on our experiment.
Photodamage of some pigments in the isolated photosystem Ⅱ (PS Ⅱ) reaction center D1/D2/Cyt b559 complex from spinach has been investigated by means of high performance liquid chromatography. The light induced damage of pheophytin a (pheo a) in the complex was observed for the first time. The content of pheo a decreased about 47% by illumination, suggesting only one of the two pheo a molecules in the PSⅡ reaction center complex was damaged. No damage of β carotene was found.
