Photosystem I of cyanobacteria contains different spectral pools of chlorophylls called red or long-wavelength chlorophylls that absorb at longer wavelengths than the primary electron donor P700. We measured the fluorescence spectra at the ensemble and the single-molecule level at low temperatures in the presence of oxidized and reduced P700. In accordance with the literature, it was observed that the fluorescence is quenched by P700(+). However, the efficiency of the fluorescence quenching by oxidized P700(+) was found to be extremely different for the various red states in PS I from different cyanobacteria. The emission of the longest-wavelength absorbing antenna state in PS I trimers from Thermosynechococcus elongatus (absorption maximum at 5K: ≈ 719nm; emission maximum at 5K: ≈ 740nm) was found to be strongly quenched by P700(+) similar to the reddest state in PS I trimers from Arthrospira platensis emitting at 760nm at 5K. The fluorescence of these red states is diminished by more than a factor of 10 in the presence of oxidized P700. For the first time, the emission of the reddest states in A. platensis and T. elongatus has been monitored using single-molecule fluorescence techniques.

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In this study we demonstrate the impact of temperature on the luminescence emission of plasmonic nanoparticles. We examine the optical properties of single gold nanorods (GNRs) in the temperature range 1.6–295 K by confocal microscopy. Decreasing temperature leads to a reduction of the full width at half-maximum (fwhm) of the luminescence spectra, thus we can conclude that the damping of the plasmonic oscillation is strongly reduced. The main contribution to the dephasing mechanism is electron–phonon scattering and we are able to determine its contribution to the dephasing using quantitative simulations, which can describe the temperature dependent dephasing of plasmonic nanoparticles.

Single-molecule spectroscopy at cryogenic temperatures was used to examine the impact of buffer solution, glycerol/buffer mixtures (25% and 66%), and poly(vinyl alcohol) (PVA) films on the conformation of photosystem I (PSI) from Thermosynechoccocus elongatus. PSI holds a number of chromophores embedded at different places within the protein complex that show distinguishable fluorescence at low temperatures. The fluorescence emission from individual complexes shows inter- and intracomplex heterogeneity depending on the solution wherein PSI was dissolved. Statistical evaluation of spectra of a large number of complexes shows that the fluorescence emission of some of these chromophores can be used as sensors for their local nanoenvironment and some as probe for the conformation of the whole protein complex. Preparation in glycerol/buffer mixtures yields a high homogeneity for all chromophores, indicating a more compact protein conformation with less structural variability. In buffer solution a distinct heterogeneity of the chromophores is observed. PSI complexes in PVA show highly heterogeneous spectra as well as a remarkable blue shift of the fluorescence emission, indicating a destabilization of the protein complex. Photosystem I prepared in PVA cannot be considered fully functional, and conclusions drawn from experiments with PSI in PVA films are of questionable value.

In this study we use a combination of absorption, fluorescence and low temperature single-molecule spectroscopy to elucidate the spectral properties, heterogeneities and dynamics of the chlorophyll a (Chla) molecules responsible for the fluorescence emission of photosystem II core complexes (PS II cc) from the cyanobacterium Thermosynechococcus elongatus. At the ensemble level, the absorption and fluorescence spectra show a temperature dependence similar to plant PS II. We report emission spectra of single PS II cc for the first time; the spectra are dominated by zero-phonon lines (ZPLs) in the range between 680 and 705nm. The single-molecule experiments show unambiguously that different emitters and not only the lowest energy trap contribute to the low temperature emission spectrum. The average emission spectrum obtained from more than hundred single complexes shows three main contributions that are in good agreement with the reported bands F685, F689 and F695. The intensity of F695 is found to be lower than in conventional ensemble spectroscopy. The reason for the deviation might be due to the accumulation of triplet states on the red-most chlorophylls (e.g. Chl29 in CP47) or on carotenoids close to these long-wavelength traps by the high excitation power used in the single-molecule experiments. The red-most emitter will not contribute to the fluorescence spectrum as long as it is in the triplet state. In addition, quenching of fluorescence by the triplet state may lead to a decrease of long-wavelength emission.