Ensemble and single-molecule spectroscopy demonstrates that both emission and absorption of peridinin−chlorophyll−protein photosynthetic antennae can be largely enhanced through plasmonic interactions. We find up to 18-fold increase of the chlorophyll fluorescence for complexes placed near a silver metal layer. This enhancement, which leaves no measurable effects on the protein structure, is observed when exciting either chlorophyll or carotenoid and is attributed predominantly to an increase of the excitation rate in the antenna. The enhancement mechanism comes from plasmon-induced amplification of electromagnetic fields inside the complex. This result is an important step toward applying plasmonic nanostructures for controlling the optical response of complex biomolecules and improving the design and functioning of artificial light-harvesting systems.Strong enhancement of electromagnetic fields generated through plasmon resonances in metal films and particles has recently stimulated a considerable interest in diverse research fields such as optical spectroscopy, cell imaging, quantum information processing, nanophotonics, and biosensors. [1][2][3][4][5] This versatility results from a dramatic influence that plasmons impose on the absorption and emission properties of nearby located dipoles, for example, semiconductor nanocrystals and nanowires 6-12 or dye molecules. [13][14][15][16][17][18] Optical response of an emitter coupled to a plasmonic structure depends upon spatial arrangement as well as spectral characteristics of a studied system. Remarkable progress has been made in on-demand design of metal nanostructures, which is essential for tuning the resonance frequency and thus the coupling strength. 13,14,19 Complementary efforts focused on developing advanced experiments to study dipoles placed in the vicinity of a metal nanoparticle have shed light on the interplay between radiative and nonradiative processes in these systems. 16,18 This very relation determines whether the fluorescence is enhanced [9][10][11]16 or quenched due to the dominating role of nonradiative energy transfer from the dipole to the metal. 15,18 Metal-enhanced fluorescence (MEF) has been observed for many hybrid systems that include nanocrystals on corrugated metal surfaces, 10,11 dye molecules coupled to metal nanoparticles, 18 and nanocrystal-nanoparticle bioconjugates. 8 In all these cases, very stable and highly fluorescing emitters have been selected. It would be, however, highly desirable to apply MEF to weakly fluorescing systems such as DNA, 20 carbon nanotubes 21 or, yet experimentally unexplored in this context, light-harvesting complexes. These latter proteinpigment systems, which contain chlorophyll (Chl) and carotenoid molecules embedded in a protein matrix, participate in the photosynthesis process by collecting sunlight energy and transferring it to reactions centers. The presence of fluorescing Chls and the protein, separated by a few nanometers, renders light-harvesting complexes ideal for studying the prot...

Reactive oxygen species (ROS) are continuously generated as a by-product of normal aerobic metabolism. Elevated ROS formation leads to potential damage of biological structures and is implicated in various diseases. Astaxanthin, a xanthophyll carotenoid, is a secondary metabolite responsible for the red-orange color of a number of marine animals and microorganisms. There is mounting evidence that astaxanthin has powerful antioxidant, anti-inflammatory, and antiapoptotic activities. Hence, its consumption can result in various health benefits, with potential for therapeutic application. Astaxanthin contains both a hydroxyl and a keto group, and this unique structure plays important roles in neutralizing ROS. The molecule quenches harmful singlet oxygen, scavenges peroxyl and hydroxyl radicals and converts them into more stable compounds, prevents the formation of free radicals, and inhibits the autoxidation chain reaction. It also acts as a metal chelator and converts metal prooxidants into harmless molecules. However, like many other carotenoids, astaxanthin is affected by the environmental conditions, e.g., pH, heat, or exposure to light. It is hence susceptible to structural modification, i.e., via isomerization, aggregation, or esterification, which alters its physiochemical properties. Here, we provide a concise overview of the distribution of astaxanthin in tissues, and astaxanthin structures, and their role in tackling singlet oxygen and free radicals. We highlight the effect of structural modification of astaxanthin molecules on the bioavailability and biological activity. These studies suggested that astaxanthin would be a promising dietary supplement for health applications.

Rhodopseudomonas palustris belongs to the group of purple bacteria that have the ability to produce LH2 complexes with unusual absorption spectra when they are grown at low-light intensity. This ability is often related to the presence of multiple genes encoding the antenna apoproteins. Here we report, for the first time to our knowledge, direct evidence that individual low-light LH2 complexes have a heterogeneous alphabeta-apoprotein composition that modulates the site energies of Bchl a molecules, producing absorption bands at 800, 820, and 850 nm. The arrangement of the Bchl a molecules in the "tightly coupled ring" can be modeled by nine alphabeta-Bchls dimers, such that the Bchls bound to six alphabeta-pairs have B820-like site energies and the remaining Bchl a molecules have B850-like site energies. Furthermore, the experimental data can only be satisfactorily modeled when these six alphabeta-pairs with B820 Bchl a molecules are distributed such that the symmetry of the assembly is reduced to C(3). It is also clear from the measured single-molecule spectra that the energies of the electronically excited states in the mixed B820/850 ring are mainly influenced by diagonal disorder.

Single molecule spectroscopy experiments are reported for native peridinin-chlorophyll a-protein (PCP) complexes, and three reconstituted light-harvesting systems, where an N-terminal construct of native PCP from Amphidinium carterae has been reconstituted with chlorophyll (Chl) mixtures: with Chl a, with Chl b and with both Chl a and Chl b. Using laser excitation into peridinin (Per) absorption band we take advantage of sub-picosecond energy transfer from Per to Chl that is order of magnitude faster than the Förster energy transfer between the Chl molecules to independently populate each Chl in the complex. The results indicate that reconstituted PCP complexes contain only two Chl molecules, so that they are spectroscopically equivalent to monomers of native-trimeric-PCP and do not aggregate further. Through removal of ensemble averaging we are able to observe for single reconstituted PCP complexes two clear steps in fluorescence intensity timetraces attributed to subsequent bleaching of the two Chl molecules. Importantly, the bleaching of the first Chl affects neither the energy nor the intensity of the emission of the second one. Since in strongly interacting systems Chl is a very efficient quencher of the fluorescence, this behavior implies that the two fluorescing Chls within a PCP monomer interact very weakly with each other which makes it possible to independently monitor the fluorescence of each individual chromophore in the complex. We apply this property, which distinguishes PCP from other light-harvesting systems, to measure the distribution of the energy splitting between two chemically identical Chl a molecules contained in the PCP monomer that reaches 280 cm(-1). In agreement with this interpretation, stepwise bleaching of fluorescence is also observed for native PCP complexes, which contain six Chls. Most PCP complexes reconstituted with both Chl a and Chl b show two emission lines, whose wavelengths correspond to the fluorescence of Chl a and Chl b. This is a clear proof that these two different chromophores are present in a single PCP monomer. Single molecule fluorescence studies of PCP complexes, both native and artificially reconstituted with chlorophyll mixtures, provide new and detailed information necessary to fully understand the energy transfer in this unique light-harvesting system.

The objectives of the present study were to evaluate the chlorophyll content of green leafy vegetables found commercially and carry out a comparative investigation between in vivo and in vitro data. The chlorophyll of green leafy vegetable can be used as visible parameters of the quality of vegetables during storage, since it will be degraded gradually along with post-harvest senescence. Therefore, the development of reliable in vivo chlorophyll measurement should be advantageous rather than visual observation for the purpose of quality control and product sortation. Here, the existence of chlorophylls in ten green leafy vegetables were reported as SPAD values of a handheld SPAD-502 chlorophyll meter and % N of an Agriexpert CCN-6000 nitrogen meter (in vivo data), as well as total peak area data of HPLC measurement for chlorophyll a and b after exhaustive extraction using methanol (in vitro data). Both in vivo and in vitro measurement gave comparable grouping of vegetables with high and low content of chlorophyll. Moreover, correlation plots between SPAD values and total peak area of HPLC showed adequate linear correlation (R 2 > 0.7), revealing the potency of in vivo observation for the prediction of actual chlorophyll content in commercial leafy vegetables. SPAD values and % N presented strong linear relationship (R 2 > 0.9), in which SPAD-meter performed better detection at very low values. The calibration curve for each species of vegetable should be substantial to overcome the limiting factors of in vivo observation, such as leaf size, tissue thickness, and variation of chloroplast distribution.