IMPROVED UV-VISIBLE SPECTRA OF THE S-TRANSITIONS IN THE PHOTOSYNTHETIC OXYGEN-EVOLVING SYSTEM

(1) The first part of this paper is devoted to methodological tests pertaining to the deconvolution of the spectra of the S-transitions from flash sequence data. (i) The spectrum and decay kinetics of 'inactive PS II centers' were analyzed, showing similarity with normal centers inhibited on the Q(B) site. When this contribution is subtracted, the change on the first flash agrees with estimate of the S1 --> S2 change derived from deconvolution. (ii) Preillumination procedures in PS II (BBY) particles, aimed at modulating the initial S0/S1 distribution are confirmed to express deactivation towards the S1 state, with no involvement of an 'S-1' state. (iii) Elimination of semiquinone binary oscillations in BBY's is achieved by allowing total reoxidation by DCBQ after each flash. (iv) A deconvolution treatment is described, using the difference between two sequences. This method allows a determination of the initial S0/S1 distributions that cross-checks the conclusions (ii). (v) Oscillations of the amplitude of the ms-phase of the 295 nm absorption changes are shown to differ significantly from those corresponding to the S-transitions. This phase is expected to reflect predominantly the O2 release reaction, with slight deviations due to other transitions. Deconvolution results are in satisfactory agreement with this prediction. (2) The upshot of this work is to present improved spectra of the S-transitions. The basic features that were previously established with different material and deconvolution method are confirmed: negligible UV change on S0 -> S1 and significantly different spectra for the two other transitions. A shoulder around 350 nm on the S1 --> S2 spectrum and a secondary peak in the same region for the S2 --> S3 spectrum are now resolved. The S0 --> S1 step causes an electrochromic shift in the blue region, with direction opposite to S1 --> S2. Interpretation of these results is discussed.