Probing the Fucoxanthin-Chlorophyll a/c-Binding Proteins (FCPs) of the Marine Diatom Fragilariopsis sp. by Resonance Raman Spectroscopy.

We report resonance Raman spectra of the light-harvesting fucoxanthin-chlorophyll a/c-binding proteins (FCPs) of marine diatom Fragilariopsis sp. The Raman shifts in the 15N-isotope-enriched diatom provide the first spectroscopic evidence for the characterization of the Ca-N marker bands and, thus, of the penta- and hexacoordinated states of chlorophylls a/c in the FCPs. Under 405 and 442 nm Raman excitations, all of the marker bands of Chl a/c are observed and the isotope-based assignments provide new information concerning the structure of Chls a/c in the FCPs and their interactions with the protein environment. Therefore, the Raman spectrum at 405 nm originates from the π-π* transitions of Chl a/c and not from a different, non π-π* electronic transition, as previously reported (BBA Bioenergetics, 2010, 1797, 1647-1656). Based on the 15N isotope shifts of the Ca-N and in conjunction with other marker bands, two distinct conformations of five- and six-coordinated Chl a and Chl c are observed. In addition, two keto carbonyls were observed at 1679 (strong H-bonded) and 1691 cm-1 (weak H-bonded) in both the 405 and 442 nm Raman spectra, respectively. Collectively, the results provide solid evidence of the nature of the vibrational modes of the active Chl a/c photosynthetic pigments in the FCPs.

Reversible thermally induced spin crossover in the myoglobin-nitrito adduct directly monitored by resonance Raman spectroscopy.

Myoglobin has been demonstrated to function as a nitrite reductase to produce nitric oxide during hypoxia. One of the most intriguing aspects of the myoglobin/nitrite interactions revealed so far is the unusual O-binding mode of nitrite to the ferric heme iron, although conflicting data have been reported for the electronic structure of this complex also raising the possibility of linkage isomerism. In this work, we applied resonance Raman spectroscopy in a temperature-dependent approach to investigate the binding of nitrite to ferric myoglobin and the properties of the formed adduct from ambient to low temperatures (293 K to 153 K). At ambient temperature the high spin state of the ferric heme Fe-O-N[double bond, length as m-dash]O species is present and upon decreasing the temperature the low spin state is populated, demonstrating that a thermally-induced spin crossover phenomenon takes place analogous to what has been observed in many transition metal complexes. The observed spin crossover is fully reversible and is not due to linkage isomerism, since the O-binding mode is retained upon the spin transition. The role of the heme pocket environment in controlling the nitrite binding mode and spin transition is discussed.

Microstructure-driven annihilation effects and dispersive excited state dynamics in solid-state films of a model sensitizer for photon energy up-conversion applications.

Bimolecular processes involving exciton spin-state interactions gain attention for their deployment as wavelength-shifting tools. Particularly triplet-triplet annihilation induced photon energy up-conversion (TTA-UC) holds promise to enhance the performance of solar cell and photodetection technologies. Despite the progress noted, a correlation between the solid-state microstructure of photoactuating TTA-UC organic composites and their photophysical properties is missing. This lack of knowledge impedes the effective integration of functional TTA-UC interlayers as ancillary components in operating devices. We here investigate a solution-processed model green-to-blue TTA-UC binary composite. Solid-state films of a 9,10 diphenyl anthracene (DPA) blue-emitting activator blended with a (2,3,7,8,12,13,17,18-octaethyl-porphyrinato) PtII (PtOEP) green-absorbing sensitizer are prepared with a range of compositions and examined by a set of complementary characterization techniques. Grazing incidence X-ray diffractometry (GIXRD) measurements identify three PtOEP composition regions wherein the DPA:PtOEP composite microstructure varies due to changes in the packing motifs of the DPA and PtOEP phases. In Region 1 (≤2 wt%) DPA is semicrystalline and PtOEP is amorphous, in Region 2 (between 2 and 10 wt%) both DPA and PtOEP phases are amorphous, and in Region 3 (≥10 wt%) DPA remains amorphous and PtOEP is semicrystalline. GIXRD further reveals the metastable DPA-ß polymorph species as the dominant DPA phase in Region 1. Composition dependent UV-vis and FT-IR measurements identify physical PtOEP dimers, irrespective of the structural order in the PtOEP phase. Time-gated photoluminescence (PL) spectroscopy and scanning electron microscopy imaging confirm the presence of PtOEP aggregates, even after dispersing DPA:PtOEP in amorphous poly(styrene). When arrested in Regions 1 and 2, DPA:PtOEP exhibits delayed PtOEP fluorescence at 580 nm that follows a power-law decay on the ns time scale. The origin of PtOEP delayed fluorescence is unraveled by temperature- and fluence-dependent PL experiments. Triplet PtOEP excitations undergo dispersive diffusion and enable TTA reactions that activate the first singlet-excited (S1) PtOEP state. The effect is reproduced when PtOEP is mixed with a poly(fluorene-2-octyl) (PFO) derivative. Transient absorption measurements on PFO:PtOEP films find that selective PtOEP photoexcitation activates the S1 of PFO within ∼100 fs through an up-converted 3(d, d*) PtII-centered state.

Evidence for reversible light-dependent transitions in the photosynthetic pigments of diatoms.

Marine diatoms contribute to oxygenic photosynthesis and carbon fixation and handle large changes under variable light intensity on a regular basis. The unique light-harvesting apparatus of diatoms are the fucoxanthin-chlorophyll a/c-binding proteins (FCPs). Here, we show the enhancement of chlorophyll a/c (Chl a/c), fucoxanthin (Fx), and diadinoxanthin (Dd) marker bands in the Raman spectra of the centric diatom T. pseudonana, which allows distinction of the pigment content in the cells grown under low- (LL) and high-light (HL) intensity at room temperature. Reversible LL-HL dependent conformations of Chl c, characteristic of two conformations of the porphyrin macrocycle, and the presence of five- and six-coordinated Chl a/c with weak axial ligands are observed in the Raman data. Under HL the energy transfer from Chl c to Chl a is reduced and that from the red-shifted Fxs is minimal. Therefore, Chl c and the blue-shifted Fxs are the only contributors to the energy transfer pathways under HL and the blue- to red-shifted Fxs energy transfer pathway characteristic of the LL is inactive. The results indicate that T. pseudonana can redirect its function from light harvesting to energy-quenching state, and reversibly to light-harvesting upon subsequent illumination to LL by reproducing the red-shifted Fxs and decrease the number of Dds. The LL to HL reversible transitions are accompanied by structural modifications of Chl a/c and the lack of the red-shifted Fxs.

Application of double-pulse laser-induced breakdown spectroscopy (DP-LIBS), Fourier transform infrared micro-spectroscopy and Raman microscopy for the characterization of copper-sulfides.

The combined application of the structure sensitive techniques Fourier transform infrared µ-spectroscopy and Raman microscopy in conjunction with different approaches of laser-induced breakdown spectroscopy (LIBS) including the two-color double pulse (DP-LIBS) have been applied towards the characterization of whole ore copper-sulfide minerals. Discrete information from the surface of the whole ore minerals that lead to the establishment of infrared marker bands and from the surface of bioleached samples that allow the monitoring of jarosite and biofilm formation are provided by FTIR mapping experiments. Raman data can provide information related to the type of the mineral and of the secondary minerals formed on the surface of the ore. Of the four different LIBS approaches applied towards the characterization of the composition of the whole ore minerals, the DP-LIBS shows the highest sensitivity with increasing signals for both the Fe and Cu metals in the whole ore samples.

Photoreduction of carotenoids in the aerobic anoxygenic photoheterotrophs probed by real time Raman spectroscopy.

The Aerobic anoxygenic phototrophic bacteria (AAPB) Roseobacter denitrificans and Roseobacter litoralis are widespread in the bacterioplankton community with a particular role in the marine carbon cycle. Measurements of carotenoids isolated from dark-grown cells indicated the presence of spheroidenone (SO, N = 11) and of 3,4 dihydrospheroidenone (N = 10) in the carotenoids isolated from illuminated cells. Time-dependent Raman 514 nm excitation experiments of R. denitrificans and R. litoralis cells grown under illumination demonstrated that v1 (C=C) of SO exhibits a time-dependent substantial frequency upshift relative to its frequency in the dark-grown cells, in a manner resembling shorting the conjugation length (N). We suggest that the irreversible dark-SO to light- 3,4 dihydrospheroidenone transition observed in the intact carotenoids of R. denitrificans and R. litoralis cells is an operative photoreduction strategy of SO containing AAPB that affects the energy transfer mechanism.

Extracellular electron uptake from carbon-based π electron surface-donors: oxidation of graphite sheets by Sulfobacillus thermosulfidooxidans probed by Raman and FTIR spectroscopies.

In this work we report Raman and FTIR evidence for extracellular electron uptake by Sulfobacillus thermosulfidooxidans from the solid phase carbon-based π-electron donor surface of graphite sheets. The primary step in the reaction is the intercalation of water on the surface of graphite followed by the formation of EPS and proceeds to form graphite oxide (GO) with a Raman I D/I G = 0.3 ratio which represents the highest defect content in the carbon lattice reported by bio-oxidation process. We propose and discuss a direct extracellular electron transfer mechanism via outer membrane redox proteins for the electron transfer.

Photosensitivity responses of Sagittula stellata probed by FTIR, fluorescence and Raman microspectroscopy.

Raman, fluorescence and FTIR experiments of prestine Sagittula stellata and Sagittula stellata-metal ion complexes grown in light and in dark were performed to probe the photosensitivity response of the cellular components in the marine bacterium. In the presence of Cu(ii) and Zn(ii) the frequency shifts of PO2 -, C-O-C and C-O-P vibrations indicate metal binding to nucleic acids, carbohydrates and polysaccharides. We assign the observed bands in the 514.1 nm Raman spectra of the prestine S. Stellata and of the extracted carotenoids to the C[double bond, length as m-dash]C and C-C stretching vibrations. The fluorescence excitation-emission matrix (EEM) of S. stellata in light, dark and in the presence of metal ions are reported and compared with the Raman and FTIR data. The novel ability of S. stellata although heterotrophic, to show light-dependent metal binding ability may be an important feature property that maintains a stable heterotroph-prototroph interaction and a dynamic system.

Multi-electron reduction of Wells-Dawson polyoxometalate films onto metallic, semiconducting and dielectric substrates.

The investigation of conditions allowing multi-electron reduction and reoxidation of polyoxometalate (POM) films onto solid substrates is considered an issue of critical importance for their successful incorporation in electronic devices, different types of sensors and catalytic systems. In the present paper, the rich multi-electron redox chemistry of films of Wells-Dawson ammonium salts, namely (NH4)6P2Mo18O62 and (NH4)6P2W18O62, on top of metallic (Al), semiconducting (ITO) and dielectric (SiO2) substrates under ambient conditions is investigated. The respective Keggin heteropolyacids, H3PMo12O40 and H3PW12O40, are also investigated for comparison. On Al substrates, the Wells-Dawson ammonium salts are found to be significantly more reduced (4-6e-) compared to the respective Keggin heteropolyacids (∼2e-), in accordance with their deeper lying lowest unoccupied molecular orbital (LUMO) level. Subsequent thermal treatment in air results in reoxidation of the initially highly reduced POM films. Similar behavior is found on ITO substrates, but in initially less reduced (2-4e-) Wells-Dawson POM films. On the other hand, on SiO2 substrates, the thermal reduction of (NH4)6P2Mo18O62 film is observed and attributed to the thermal oxidation of ammonium counterions by [P2Mo18O62]6- anions. Overall, the multi-electron reduction of Wells-Dawson ammonium salts onto metallic and semiconducting substrates (Al, ITO) is determined by the relative position of the LUMO level of POMs in relation to the Fermi level of the substrate (i.e. substrate work function) and affected in a synergistic way by the presence of ammonium counterions. In contrast, on dielectric substrates (SiO2) the reduction of Wells-Dawson POMs ((NH4)6P2Mo18O62) is attributed only to the oxidation of ammonium counterions.