Absorption changes of P-700 reversible in milliseconds at low temperature in Triton-solubilized Photosystem I particles

Abstract

Triton-solubilized Photosystem I particles from spinach chloroplasts exhibit largely reversible P-700 absorption changes over the temperature range from 4.2 K to room temperature. For anaerobic samples treated with dithionite and neutral red at pH 10 and illuminated during cooling, a brief (1 μs) saturating flash produces absorption changes in the long wavelength region that decay in 0.95 ± 0.2 ms from 4.2 to 50 K. Above 80 K a faster (100 ± 30 μs) component dominates in the decay process, but this disappears again above about 180 K. The major decay at temperatures above 200 K occurs in about 1 ms. The difference spectrum of these absorption changes between 500 and 900 nm closely resembles that of P-700. Using ascorbate and 2,6-dichlorophenolindophenol as the reducing system with a sample of Photosystem I particles cooled in darkness to 4.2 K, a fully reversible signal is seen upon both the first and subsequent flashes. The decay time in this case is 0.9 ± 0.3 ms.