ELECTRON-TRANSFER REACTIONS IN PHOTOSYSTEM-II CORE COMPLEXES FROM SYNECHOCOCCUS AT LOW-TEMPERATURE - DIFFERENCE SPECTRUM OF P680(-)() Q(A)()P680 Q(A) AT 77 K/

Absorbance difference spectroscopy has been used to study electron tra nsfer reactions at low temperature in isolated Photosystem II complexe s from Synechococcus, when the first quinone acceptor is in the oxidiz ed form. (1) The flash-induced absorbance difference spectrum attribut ed to the formation of P680(+)Q(A)(-) has been measured at 77 K betwee n 300 nm and 900 nm. The P680(+)Q(A)/P680Q(A) difference spectrum in t he Q(y) region exhibits a marked temperature dependence. At 77 K the s pectrum includes the main bleaching band at 675 nm, an absorbance incr ease at 681 nm, a smaller bleaching at 686 nm and a positive band at a round 667 nm. The width of the main bleaching band is only approximate to 6 nm compared to about 15 nm of the 680 nm band at room temperatur e. It is proposed that a strong electrochromic red shift of an absorpt ion band dominates the shape of the spectrum giving rise to an absorba nce decrease at 675 nm and an absorbance increase at 681 nm. (2) While multiphasic decay kinetics of the secondary radical pair, P680(+)Q(A) (-), are found at room temperature, the decay becomes close to mono-ex ponential with an apparent half-life of 3 ms below 200 K. However, wit h a signal to noise ratio greater than or equal to 50, two components with half-lives of approximate to 1.8 and approximate to 5 ms could cl early be resolved at 77 K. This biphasicity is attributed to frozen co nformational states with a slightly different distance between P680 an d Q(A). (3) In agreement: with earlier work in the literature performe d with PS II preparations of higher plants, we identified in PS II of Synechococcus three secondary donors oxidized with low quantum yield a t 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 absorba nce difference spectra due to the oxidition of the secondary donor(s) and the reduction of Q(A) exhibit also strong electrochromic band shif ts. Predominant is a red-shift of an absorption band similar to that p roposed to be present in the P680(+)Q(A)(-)/P680Q(A) difference spectr um. (4) On the basis of kinetic data, regarding all three secondary do nors, we conclude that Car and Cyt b-559 donate electrons to P680(+) i n parallel pathways. As the rise of Car(+) formation is faster than th e decay of P680(+)Q(A) Q(A)(-), it is proposed that Car(+) is rapidly rereduced by Chl down to an equilibrium level.