Electron transfer reactions in Photosystem II core complexes from Synechococcus at low temperature — difference spectrum of P680+QA−P680 QA at 77 K

AbstractAbsorbance difference spectroscopy has been used to study electron transfer reactions at low temperature in isolated Photosystem II complexes from Synechococcus, when the first quinone acceptor is in the oxidized form. (1) The flash-induced absorbance difference spectrum attributed to the formation of P680+QA− has been measured at 77 K between 300 nm and 900 nm. The The P680+QA−P680 QA difference spectrum in the QY region exhibits a marked temperature dependence. At 77 K the spectrum includes the main bleaching band at 675 nm, an absorbance increase at 681 nm, a smaller bleaching at 686 nm and a positive band at around 667 nm. The width of the main bleaching band is only ≈ 6 nm compared to about 15 nm of the 680 nm band at room temperature. It is proposed that a strong electrochromic red shift of an absorption band dominates the shape of the spectrum giving rise to an absorbance decrease at 675 nm and an absorbance increase at 681 nm. (2) While multiphasic decay kinetics of the secondary radical pair, P680+QA−, are found at room temperature, the decay becomes close to mono-exponential with an apparent half-life of 3 ms below 200 K. However, with a signal to noise ratio ≥ 50, two components with half-lives of ≈ 1.8 and ≈ 5 ms could clearly be resolved at 77 K. This biphasicity is attributed to frozen conformational states with a slightly different distance between P680 and QA. (3) In agreement with earlier work in the literature performed with PS II preparations of higher plants, we identified in PS II of Synechococcus three secondary donors oxidized with low quantum yield at 77 K: Cyt b-559, a carotenoid and a chlorophyll a characterized by a bleaching at 667 nm. Between 650 and 700 nm the light-induced absorbance difference spectra due to the oxidition of the secondary donor(s) and the reduction of QA exhibit also strong electrochromic band shifts. Predominant is a red-shift of an absorption band similar to that proposed to be present in the P680+QA−P680 QA difference spectrum. (4) On the basis of kinetic data, regarding all three secondary donors, we conclude that Car and Cyt b-559 donate electrons to P680+ in parallel pathways. As the rise of Car + formation is faster than the decay of P680+QA−, it is proposed that Car+ is rapidly rereduced by Chl down to an equilibrium level