Irreversible light-induced formation of P680+ and reduced cytochrome b559 in the D1-D2-Cyt b-559 complex at low temperature

In memory of Vladimir Anatolievich Shuvalov (1943-2022): an outstanding biophysicist

We present here a tribute to one of the foremost biophysicists of our time, Vladimir Anatolievich Shuvalov, who made important contributions in bioenergetics, especially on the primary steps of conversion of light energy into charge-separated states in both anoxygenic and oxygenic photosynthesis. For this, he and his research team exploited pico- and femtosecond transient absorption spectroscopy, photodichroism & circular dichroism spectroscopy, light-induced FTIR (Fourier-transform infrared) spectroscopy, and hole-burning spectroscopy. We remember him for his outstanding leadership and for being a wonderful mentor to many scientists in this area. Reminiscences by many [Suleyman Allakhverdiev (Russia); Robert Blankenship (USA); Richard Cogdell (UK); Arvi Freiberg (Estonia); Govindjee Govindjee (USA); Alexander Krasnovsky, jr, (Russia); William Parson (USA); Andrei Razjivin (Russia); Jian- Ren Shen (Japan); Sergei Shuvalov (Russia); Lyudmilla Vasilieva (Russia); and Andrei Yakovlev (Russia)] have included not only his wonderful personal character, but his outstanding scientific research.

Effect of dehydration on light-induced reactions in photosystem II: photoreactions of cytochrome b559

The effect of dehydration on the reaction pattern of photosystem II (PS II) has been studied by measuring and analyzing spectral changes induced by continuous wavelength illumination in films of untreated and hydroxylamine-washed PS II membrane fragments dehydrated to different levels. The obtained data revealed (i) the extent of light-induced formation of about one Q(A)(-*)per 230 chlorophylls (Chl) remains virtually invariant to dehydration down to the lowest values of relative humidity (6-8% RH); (ii) a decrease of the RH to 30% leads to severe blockage of the electron transfer from Q(A)(-*) to Q(B) and the progressive replacement of water oxidation by photooxidation of high potential (HP) cytochrome (Cyt) b559 in untreated PS II samples or accessory Chl and carotenoid (Car) molecules in samples with preoxidized Cyt b559; (iii) photooxidation of Cyt b559 is followed by its photoreduction, concomitant with photooxidation of Chl and Car; (iv) in dry samples with preoxidized Cyt b559, not more than a half of total Cyt b559 can be photochemically reduced, independent of the extent of Cyt b559 in the HP form; (v) at low RH values, Cyt b559 photoreduction in samples with preoxidized heme groups and photoaccumulation of Q(A)(-*) take place with biphasic kinetics with similar rate constants for both processes; (vi) Cyt b559 photoreduction in dry samples is DCMU insensitive, while the dark rereduction of photooxidized Cyt b559 is inhibited by DCMU; (vii) fast and slow kinetic phases of Cyt b559 photoreduction dramatically differ in their dependencies on the intensity of CW illumination and are associated with electron donation to Cyt b559 from Q(A)(-*) and pheophytin(-*), respectively. The pathways of light-induced electron transfer in PS II involving Cyt b559 are discussed.

Spectral and photochemical properties of borohydride-treated D1-D2-cytochrome b-559 complex of photosystem II

The D1-D2-cytochrome b-559 reaction center complex of photosystem II with an altered pigment composition was prepared from the original complex by treatment with sodium borohydride (BH4-). The absorption spectra of the modified and original complexes were compared to each other and to the spectra of purified chlorophyll a and pheophytin a (Pheo a) treated with BH4- in methanolic solution. The results of these comparisons are consistent with the presence in the modified complex of an irreversibly reduced Pheo a molecule, most likely 13(1)-deoxo-13(1)-hydroxy-Pheo a, replacing one of the two native Pheo a molecules present in the original complex. Similar to the original preparation, the modified complex was capable of a steady-state photoaccumulation of Pheo- and P680+. It is concluded that the pheophytin a molecule which undergoes borohydride reduction is not involved in the primary charge separation and seems to represent a previously postulated photochemically inactive Pheo a molecule. The Qy and Qx transitions of this molecule were determined to be located at 5 degrees C at 679.5-680 nm and 542 nm, respectively.