Identification of the Chromophores in Prussian blue

Prussian blue was the world's first synthetic dye. Its structural, optical and magnetic properties have led to many applications in technology and medicine, and provide paradigms for understanding coordination polymers, framework materials and mixed-valence compounds. The intense red absorption of Prussian blue that characterises chemical and physical properties critical to many of these applications is now shown to arise from localised intervalence charge transfer transitions within two chromophoric variants (ligand isomers) of an idealised "dimer" fragment {(NC)5FeII}(mu-CN){FeIII(NC)3(H2O)2}. This fragment is only available in modern interpretations of the material's crystal structure, with the traditional motif {(NC)5FeII}(mu-CN){FeIII(NC)5} shown not to facilitate visible absorption. Essential to the analysis is the demonstration, obtained independently using absorption and magnetic circular dichroism spectroscopies, that spectra of Prussian blues are strongly influenced by particle size and (subsequent) light scattering. These interpretations are guided and supported by density functional theory calculations (CAM-B3LYP), supplemented by coupled cluster and Bethe-Salpeter spectral simulations, as well as electron paramagnetic resonance spectroscopy of Prussian blue and a model molecular dimeric ion [Fe2(CN)11]6-

The deep red state of photosystem II in Cyanidioschyzon merolae

We identified and characterised the deep red state (DRS), an optically-absorbing charge transfer state of PSII, which lies at lower energy than P680, in the red algae Cyanidioschyzon merolae by means of low temperature absorption and magnetic circular dichroism spectroscopies. The photoactive DRS has been previously studied in PSII of the higher plant Spinacia oleracea, and in the cyanobacterium Thermosynechococcus vulcanus. We found the DRS in PSII of C. merolae has similar spectral properties. Treatment of PSII with dithionite leads to reduction of cytochrome (cyt) b559 and the PsbV-based cyt c550 as well as the disassembly of the oxygen-evolving complex. Whereas the overall visible absorption spectrum of PSII was little affected, the DRS absorption in the reduced sample was no longer seen. This bleaching of the DRS is discussed in terms of a corresponding lack of a DRS feature in D1D2/cyt b559 reaction centre preparations of PSII.Australian Research Council through grants DP110104565 and DP 15010313

The primary donor of far-red photosystem II: ChlD1 or PD2?

Far-red light (FRL) Photosystem II (PSII) isolated from Chroococcidiopsis thermalis is studied using parallel analyses of low-temperature absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectroscopies in conjunction with fluorescence measurements. This extends earlier studies (Nurnberg et al 2018 Science 360 (2018) 1210-1213). We confirm that the chlorophyll absorbing at 726 nm is the primary electron donor. At 1.8 K efficient photochemistry occurs when exciting at 726 nm and shorter wavelengths; but not at wavelengths longer than 726 nm. The 726 nm absorption peak exhibits a 21 ± 4 cm-1 electrochromic shift due to formation of the semiquinone anion, QA-. Modelling indicates that no other FRL pigment is located among the 6 central reaction center chlorins: PD1, PD2 ChlD1, ChlD2, PheoD1 and PheoD2. Two of these chlorins, ChlD1 and PD2, are located at a distance and orientation relative to QA- so as to account for the observed electrochromic shift. Previously, ChlD1 was taken as the most likely candidate for the primary donor based on spectroscopy, sequence analysis and mechanistic arguments. Here, a more detailed comparison of the spectroscopic data with exciton modelling of the electrochromic pattern indicates that PD2 is at least as likely as ChlD1 to be responsible for the 726 nm absorption. The correspondence in sign and magnitude of the CD observed at 726 nm with that predicted from modelling favors PD2 as the primary donor. The pros and cons of PD2 vs ChlD1 as the location of the FRL-primary donor are discussed.We recognize the support of the Australian Research Councilthrough grants DP110104565 and DP150103137 (EK), FT140100834(NC). This work was supported by BBSRC grants BB/L011506/1 andBB/R001383/1 (AWR, AF and DN

Spectral hole burning: examples from photosynthesis

The optical spectra of photosynthetic pigment–protein complexes usually show broad absorption bands, often consisting of a number of overlapping, ‘hidden’ bands belonging to different species. Spectral hole burning is an ideal technique to unravel the optical and dynamic properties of such hidden species. Here, the principles of spectral hole burning (HB) and the experimental set-up used in its continuous wave (CW) and time-resolved versions are described. Examples from photosynthesis studied with hole burning, obtained in our laboratory, are then presented. These examples have been classified into three groups according to the parameters that were measured: (1) hole widths as a function of temperature, (2) hole widths as a function of delay time and (3) hole depths as a function of wavelength. Two examples from light-harvesting (LH) 2 complexes of purple bacteria are given within the first group: (a) the determination of energy-transfer times from the chromophores in the B800 ring to the B850 ring, and (b) optical dephasing in the B850 absorption band. One example from photosystem II (PSII) sub-core complexes of higher plants is given within the second group: it shows that the size of the complex determines the amount of spectral diffusion measured. Within the third group, two examples from (green) plants and purple bacteria have been chosen for: (a) the identification of ‘traps’ for energy transfer in PSII sub-core complexes of green plants, and (b) the uncovering of the lowest k = 0 exciton-state distribution within the B850 band of LH2 complexes of purple bacteria. The results prove the potential of spectral hole burning measurements for getting quantitative insight into dynamic processes in photosynthetic systems at low temperature, in particular, when individual bands are hidden within broad absorption bands. Because of its high-resolution wavelength selectivity, HB is a technique that is complementary to ultrafast pump–probe methods. In this review, we have provided an extensive bibliography for the benefit of scientists who plan to make use of this valuable technique in their future research

New Perspectives on Photosystem II Reaction Centres

25 Pags.- 9 Figs.- 3 Tabls. The definitive version is available at: https://www.publish.csiro.au/chWe apply the differential optical spectroscopy techniques of Circular Polarisation of Luminescence (CPL) and Magnetic CPL (MCPL) to the study of isolated reaction centres (RCs) of Photosystem II (PS II). The data and subsequent analysis provide insights into aspects of RC chromophore site energies, exciton couplings, and heterogeneities. CPL measurements are able to identify weak luminescence associated with the unbound chlorophyll-a (Chl-a) present in the sample. The overall sign and magnitude of the CPL observed relates well to the Circular Dichroism (CD) of the sample. Both CD and CPL are reasonably consistent with modelling of the RC exciton structure. The Magnetic CPL (MCPL) observed for the free Chl-a luminescence component in the RC samples is also easily understandable, but the MCPL seen near 680 nm at 1.8 K is anomalous, appearing to have a narrow, strongly negative component. A negative sign is inconsistent with MCPL of (exciton coupled) Qy states of either Chl-a or Pheo-a. We propose that this anomaly may arise as a result of luminescence from a transient excited state species created following photo-induced charge separation within the RC. A comparison of CD spectra and modelling of RC preparations having a different number of pigments suggests that the non-conservative nature of CD spectra observed is associated with the ‘special pair’ pigments PD1 and PD2.We recognise the support of the Australian Research Council through grants DP110104565 and DP150103137 (E.K.).Peer reviewe

Magneto-optic measurements of spectral holes in metallo-porphyrin derivatives in polymer matrices

Magneto-optic measurements on spectral holes in the Q band of zinc phthalocyanine and zinc tetrabenzoporphyrin are reported. Detailed modelling of the changes in hole shape in the magnetic field requires the inclusion of a broad distribution of crystal-field splittings and a Ham factor of 0.52 ± 0.03. Magnetic circular dichroism (MCD) spectra are also reported for spectral holes in zinc phthalocyanine and zinc tetrabenzoporphyrin. The wavelength dependence of the hole MCD gives insight into the distribution of the crystal-field split levels of the excited state. Modelling of the hole MCD shapes enables a number of factors contributing to the hole MCD shape to be identified

Electric-field-induced broadening of spectral holes in zinc phthalocyanine

The influence of an externally applied electric field on spectral holes burnt into the Q band of zinc phthalocyanine doped into polymethylmethacrylate has been investigated. The changes in hole shape as a function of electric field have been monitored and a linear Stark effect has been observed. The broadening has a strong dependence on the direction of polarisation of the burning and reading laser relative to the applied electric field. This dependence on polarisation indicates that not only is there a dipole induced in the plane of the chromophore by the host, but also that there are strong interactions with the chromophore perpendicular to the molecular plane. The magnitude of the Stark broadening depends strongly on the presence or absence of solvent in the sample