On the mutual orientation of pigments in Photosystem I particles from green plants

W. Junge, H. Schaffernicht, N. Nelson

Research output: Contribution to journalArticlepeer-review

Abstract

The mutual orientation of pigments in Photosystem I reaction centers from spinach is evaluated by polarized photochemistry. The photoinduced linear dichroism of the absorption changes of chlorophyll a1 at 701 nm is studied as function of the excitation wavelength. The Photosystem I reaction center particles contain about 100 and if depleted about 40 chlorophylls, respectively. To prevent their rapid Brownian rotation they were immobilized on DEAE-Sephadex. The excitation spectrum of the linear dichroism reveals a high degree of order between the long axis of β-carotene and the Qy transition moments of those chlorophyll a molecules absorbing at the red end of the spectrum. The latter are the most endangered ones for destructive oxidation via their triplet state. Hence, the location of β-carotene in close proximity to and in parallel with these chlorophylls seems to be most favourable for the protective role of β-carotene within the antennae system I. It is observed that the dichroic ratio of the absorption changes of chlorophyll a1 does not exceed a figure of 4 3, which characterizes a circularly degenerate system, even at far red excitation (724 nm). This will hit selectively those few chlorophyll a molecules with their peak absorption at about 700 nm (including the photooxidizable dimer). We conclude, if the dimer is the only species peaking at 700 nm then the two chlorophyll a within the dimer have their y-axes oriented perpendicular to each other. If there are some antennae in addition to the dimer, the y-axes of all chlorophyll-a peaking at 700 nm form a star which accounts for the circular degeneracy of absorption.

Original languageEnglish
Pages (from-to)73-85
Number of pages13
JournalBiochimica et Biophysica Acta - Bioenergetics
Volume462
Issue number1
DOIs
StatePublished - 12 Oct 1977
Externally publishedYes

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