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Abstract

Pheophytin a (Pheo) in Photosystem II reaction centres was exchanged for 13(1)-deoxo-13(1)-hydroxy-pheophytin a (13(1)-OH-Pheo). The absorption bands of 13(1)-OH-Pheo are blue-shifted and well separated from those of Pheo. Two kinds of modified reaction centre preparations can be obtained by applying the exchange procedure once (RC(1x)) or twice (RC(2x)). HPLC analysis and Pheo Q(X) absorption at 543 nm show that in RC(1x) about 50% of Pheo is replaced and in RC(2x) about 75%. Otherwise, the pigment and protein composition are not modified. Fluorescence emission and excitation spectra show quantitative excitation transfer from the new pigment to the emitting chlorophylls. Photoaccumulation of Pheo(-) is unmodified in RC(1x) and decreased only in RC(2x), suggesting that the first exchange replaces the inactive and the second the active Pheo. Comparing the effects of the first and the second replacement on the absorption spectrum at 6 K did not reveal substantial spectral differences between the active and inactive Pheo. In both cases, the absorption changes in the Q(Y) region can be interpreted as a combination of a blue shift of a transition at 684 nm, a partial decoupling of chlorophylls absorbing at 680 nm and a disappearance of Pheo absorption in the 676-680 nm region. No absorption decrease is observed at 670 nm for RC(1x) or RC(2x), showing that neither of the two reaction centre pheophytins contributes substantially to the absorption at this wavelength.

Site Energies of Active and Inactive Pheophytins in the Reaction Center of Photosystem II from Chlamydomonas reinhardtii

“…It is also unlikely that both pheophytins contribute to the absorption near 676-681 nm region as proposed in Refs. 7,33,41,42 In contrast, we have demonstrated that the Q x -/Q y -region site-energies of Pheo D1 and Pheo D2 are most likely at ~545/~680 nm and ~541.5/~670 nm, respectively, in good agreement with our previous assignment. 26,34 These values should be used to model excitonic structure and excitation energy transfer dynamics of the PSII RC.…”

“…Here we only note that the ratio of the nonresonant TBHB spectrum (I = 100 mW) to NPHB (saturated persistent hole) in spinach (see Figure 3) is about 3.5, while the same ratio in C. reinhardtii (obtained under identical conditions; see Figure 3) is about 5.5. Interestingly, the ratio of ~3.6 was also observed in destabilized P680-type RCs 7,41 , which suggested that both pheophytins have their Q y absorption maxima at 676-680 nm (at 6 K), are inconsistent with data shown above. In addition, the blue shift of both the Q x and Q y transitions of Pheo D1 in destabilized RCs (see Figure S1; Supporting Information) is consistent with resonance Raman spectroscopy data 69 , where it was suggested that Pheo D1 in the PSII RC is likely H-bonded (as BPheo L in the BRC 70,71 ), with the glutamine residue D1-Gln130 being the likely homologue of the glutamic acid residue L-104 in BRC.…”

Computer simulation of topological defects around a colloidal particle or droplet dispersed in a nematic host

“…Cubic periodic boundary conditions were employed in all the simulations. To facilitate the analysis, the position of the droplet was fixed in the centre of the simulation box, and a global constraint was applied to fix the orientation of the director along the z axis [27].…”

Low-temperature spectroscopy of fully active PSII cores. Comparisons with CP43, CP47, D1/D2/cyt b559 fragments

“…By subtracting scaled spectra of the two preparations, it is evident that the accessory chl a removed upon forming the 5-chl a preparation absorbs near 670 nm and has no MCD 'deficit'. The observation of a pronounced MCD 'deficit' in D1/D2/cyt b 559 preparations, peaking where bleaches are characteristically seen [18,19], points to the utility of this method as a P680 fingerprint. An extended theoretical base for the MCD of coupled pigments [20] may assist in more detailed assignments of pigments in fully active PSII.…”