Circular dichroism of the peripheral chlorophylls in photosystem II reaction centers revealed by electrochemical oxidation

Abstract

Visible absorption spectra and circular dichroism (CD) of the red absorption band of isolated photosystem II reaction centers were measured at room temperature during progressive bleaching by electrochemical oxidation, in comparison with aerobic photochemical destruction, and with anaerobic photooxidation in the presence of the artificial electron acceptor silicomolybdate. Initially, selective bleaching of peripheral chlorophylls absorbing at 672 nm was obtained by electrochemical oxidation at +0.9 V, whereas little selectivity was observed at higher potentials. Illumination in the presence of silicomolybdate did not cause a bleaching but a spectral broadening of the 672-nm band was observed, apparently in response to the oxidation of carotene. The 672-nm absorption band is shown to exhibit a positive CD, which accounts for the 674-nm shoulder in CD spectra at low temperature. The origin of this CD is discussed in view of the observation that all CD disappears with the 680-nm absorption band during aerobic photodestruction.

Transfer of Frequency-Dependent Polarizabilities: A Tool To Simulate Absorption and Circular Dichroism Molecular Spectra

“…Finally, we compare the TDC and TFDP CD and absorption spectra generated for the photosystem I-derived 96-mer in Figure . We provide this as an example of a potential application only; however, it is noteworthy that the theoretical spectra are compatible with an earlier experimental observation in similar systems. − For illustration, we replot the experimental absorption and ECD spinach photosystem spectra from ref in Figure . The simplified model used in the present study qualitatively corresponds to the principle experimental features (i.e., the dominance of the Soret; calculated at 380 nm, experimentally around 420 nm), Q (580/680 nm) porphyrin absorption bands, and a relatively large dissymmetry factor (ratio of CD to absorption) within 10 –2 to 10 –3 , although a more detailed comparison is not relevant at this stage.…”

“…Absorption ( A ) and ECD (Δ A ) experimental spectra of a spinach photosystem according to ref . The ECD signal in the Soret region (<400 nm) may be obscured by the large absorption signal.…”

Band Shape Heterogeneity of the Low-Energy Chlorophylls of CP29: Absence of Mixed Binding Sites and Excitonic Interactions

“…However, intense optical activity may be induced in monomers due to structural deformation of the molecule, in this case induced by the protein. There are several suggestions in the literature for an intense "intrinsic" CD signal at specific chl binding sites in the D1/D2/cyt b-559 complex (38) and for a redshifted antenna chlorophyll, absorbing at 679 nm, in the alga Pleurochloris meiringensis (39,40). In the present context it may be significant that, in LHCII, it is in fact the A2-CHL612 which has been suggested to display the greatest waving deformation of the porphyrin ring structure, with respect to the chlorin plane (41).…”

Reaction Center of Photosystem II with No Peripheral Pigments in D2 Allows Secondary Electron Transfer in D1

“…This means that the transient spectra recorded at long delays portray the recovery of the ground state of P 680 due to electron transfer from Car D1 to P 680 +• ; as suggested by Loll and co-workers (13,14), the positive hole will be shared between Car D1 and Chlz D1 . If the broad band in the 700-1000 nm region is indeed a superposition of Car D1 +• and Chlz D1 +• , the negative peak around 680 nm in a long-delay spectrum (∆@d, d g 1) must be a superposition of the bleaching signals of P 680 and Chlz D1 , which is believed to have its peak absorbance at about 672 nm at room temperature (32).…”

“…+• ]/[Car D1 +• ] is approximately 1.7. To consolidate the above estimate through an analysis of the bleaching signals around 680 nm in Figure 4, we assumed first that the absorption coefficients of P 680 +• and Chlz D1 +• are negligible in this spectral region and, second, that the absorption spectrum of Chlz D1 , which is believed to absorb maximally at 672 nm (32), can be approximated by redshifting the spectrum of Chla in an organic solvent. Each member of the set ∆@d was modeled, in the 550-700 nm region, as a superposition of ∆A(λ; 5 µs) and Z 1 (λ) ≡ C(λ s), where C(λ) is the absorption spectrum of Chla in 95% acetone and s (i.e., the magnitude of the wavelength shift) is chosen so as to bring the peak absorbance of Z 1 (λ) at 672 nm.…”