Ultrafast polarized fluorescence measurements on monomeric and self-associated melittin

The anisotropic and magic-angle fluorescence decay of the single tryptophan (Trp) residue of melittin, a bee venom peptide, was investigated by time-resolved fluorescence anisotropy using a streak camera setup. The peptide was dissolved either in distilled water or in Hepes/NaOH buffer containing low (10 mM) or high (2 M) concentrations of NaCl, the latter resulting in tetramerized melittin. For melittin in distilled water and low NaCl concentration, two anisotropy decay times were found in the order of similar to50 and similar to800 picoseconds, reflecting, local and overall peptide dynamics, respectively, and for tetramerized melittin, two anisotropy decay times of similar to200 and similar to5500 picoseconds were found. Decay-associated spectra of the isotropic fluorescence decay show three time components in the range of similar to20 picoseconds, similar to500 picoseconds, and similar to3500 picoseconds, respectively. The relative amplitudes of the latter two change upon the self-association of melittin. This change can be explained by the existence of different rotamers of Trp in melittin, of which one is more favored in the melittin tetramer than in the melittin monomer

The low-energy forms of photosystem I light-harvesting complexes: Spectroscopic properties and pigment-pigment interaction characteristics

In this work the spectroscopic properties of the special low-energy absorption bands of the outer antenna complexes of higher plant Photosystem I have been investigated by means of low-temperature absorption, fluorescence, and fluorescence line-narrowing experiments. It was found that the red-most absorption bands of Lhca3, Lhca4, and Lhca1-4 peak, respectively, at 704, 708, and 709 nm and are responsible for 725-, 733-, and 732-nm fluorescence emission bands. These bands are more red shifted compared to "normal" chlorophyll a (Chl a) bands present in light-harvesting complexes. The low-energy forms are characterized by a very large bandwidth (400-450 c

Pigment Organization and Energy Transfer Dynamics in Isolated Photosystem I (PSI) Complexes from Arabidopsis thaliana Depleted of the PSI-G, PSI-K, PSI-L, or PSI-N Subunit

AbstractGreen plant photosystem I (PSI) consists of at least 18 different protein subunits. The roles of some of these protein subunits are not well known, in particular those that do not occur in the well characterized PSI complexes from cyanobacteria. We investigated the spectroscopic properties and excited-state dynamics of isolated PSI-200 particles from wild-type and mutant Arabidopsis thaliana plants devoid of the PSI-G, PSI-K, PSI-L, or PSI-N subunit. Pigment analysis and a comparison of the 5K absorption spectra of the various particles suggests that the PSI-L and PSI-H subunits together bind approximately five chlorophyll a molecules with absorption maxima near 688 and 667nm, that the PSI-G subunit binds approximately two red-shifted β-carotene molecules, that PSI-200 particles without PSI-K lack a part of the peripheral antenna, and that the PSI-N subunit does not bind pigments. Measurements of fluorescence decay kinetics at room temperature with picosecond time resolution revealed lifetimes of ∼0.6, 5, 15, 50, 120, and 5000ps in all particles. The 5- and 15-ps phases could, at least in part, be attributed to the excitation equilibration between bulk and red chlorophyll forms, though the 15-ps phase also contains a contribution from trapping by charge separation. The 50- and 120-ps phases predominantly reflect trapping by charge separation. We suggest that contributions from the core antenna dominate the 15-ps trapping phase, that those from the peripheral antenna proteins Lhca2 and Lhca3 dominate the 50-ps phase, and that those from Lhca1 and Lhca4 dominate the 120-ps phase. In the PSI-200 particles without PSI-K or PSI-G protein, more excitations are trapped in the 15-ps phase and less in 50- and 120-ps phases, which is in agreement with the notion that these subunits are involved in the interaction between the core and peripheral antenna proteins

Power Play: Grotesque Ornament And The Art Of Political Persuasion In Early Modern France

This study aimed to survey the use of grotesque ornament in France from the sixteenth to the eighteenth centuries, and assessed its meanings in both public and private spaces. After collecting a large number of examples of grotesque images and objects, three central themes were developed to guide further research. These themes were Appropriation, wherein the motif's historical resonance was important to the development of royal and noble legitimacy, and a symbol of power; Physical Exuberance, which took into account both the materiality of the design of grotesques and their reflection of political ideals, and lastly Visual Play, which considered how artists were using grotesques, as well as their flexibility in meaning. Each successive chapter explored how these themes operated in relation to specific examples. The Literature Review was developed in order to explore four aspects of the scholarly material currently available for the study of grotesques. First, it aimed to situate grotesques within the larger framework of new works in the field of ornament. It then began to consider how ancient works were received in sixteenth century France, and what they could offer readers about the uses and meanings of grotesque ornament. This involved a re-reading of Vitruvius, Horace, Qunitilian and Lucretius, in order to understand the ancient concepton of ornament (specifically grotesque ornament) for both its civic and rhetorical properties, and its reception in France. The review concluded with a synopsis of recent scholarship on grotesque imagery, largely from the field of decorative arts. The meanings of grotesque ornament were then explored in a chapter that aimed to give a general overview of grotesque ornament in France during the early modern period, and that expanded on the themes developed for this study. Further evidence was culled from contractual language in original documents from the period, and from a considertation of how the materiality of grotesque images might alter meaning. These ideas were then investigated through three central case studies. The first case study, centered on the printmaker, Juste de Juste, who worked at the First School of Fontainebleau in the 1540s, provided the first major study of his work. The chapter considered an artist's role within the context of Fontainebleau, the network of artists that disseminated ideas through it, and how artistic processes converged with new scientific endeavors, specifically anatomy. The case study posited that primtmaking was an essentially experimental practice for many of these artists, and that for Juste de Juste, a way to express his own identity. The second case study provided the first in-depth survey of grotesque ornament on the facades of houses in Toulouse, France. The city allowed for the examination of the civic character of the motif, as well as its relationship to forms developed at Fontainebleau. Grotesques were adapted to localized building traditions, and were made to display wealth and power in the cityscape. The next chapter on a series of grotesques painted by Simon Vouet for Anne of Austria in the 1640s similarly took up the exploration of power through ornamental display, but rather in the context of an interior space within the Palais Royal. The study found a variation in the historical appropriation of the motif, and exposed the role of female agency. It also expanded the discussion of siting royal authority in specific places, and how the dissemination of prints was important to establishing the imagery associated with that authority. These case studies were followed by an epilogue that discussed the continued use of grotesque ornament well into the eighteenth century, especially through the work of artists such as Watteau, and how the motif's flexibility allowed for its use in both Rococo and Neoclassical contexts. The epilogue alludes to the expansion of the marketplace, where the mass consumption of ornament was evident, and how this stimulated the global development of an exportable French style

Different crystal morphologies lead to slightly different conformations of light-harvesting complex II as monitored by variations of the intrinsic fluorescence lifetime

In 2005, it was found that the fluorescence of crystals of the major light-harvesting complex LHCII of green plants is significantly quenched when compared to the fluorescence of isolated LHCII (A. A. Pascal et al., Nature, 2005, 436, 134-137). The Raman spectrum of crystallized LHCII was also found to be different from that of isolated LHCII but very similar to that of aggregated LHCII, which has often been considered a good model system for studying nonphotochemical quenching (NPQ), the major protection mechanism of plants against photodamage in high light. It was proposed that in the crystal LHCII adopts a similar (quenching) conformation as during NPQ and indeed similar changes in the Raman spectrum were observed during NPQ in vivo (A. V. Ruban et al., Nature, 2007, 450, 575-579). We now compared the fluorescence of various types of crystals, differing in morphology and age. Each type gave rise to its own characteristic mono-exponential fluorescence lifetime, which was 5 to 10 times shorter than that of isolated LHCII. This indicates that fluorescence is not quenched by random impurities and packing defects (as proposed recently by T. Barros et al., EMBO Journal, 2009, 28, 298-306), but that LHCII adopts a particular structure in each crystal type, that leads to fluorescence quenching. Most interestingly, the extent of quenching appears to depend on the crystal morphology, indicating that also the crystal structure depends on this crystal morphology but at the moment no data are available to correlate the crystals' structural changes to changes in fluorescence lifetime

Spectroscopic Properties of a Self-Assembled Zinc Porphyrin Tetramer. II. Time-Resolved Fluorescence Spectroscopy.

Excited-state kinetics of complexes of a functionalized zinc tetraphenylporphyrin (ZnTPP) derivative, zinc mono(4-pyridyl)triphenylporphyrin (ZnPyP) in toluene and polystyrene/toluene mixtures have been investigated by time-resolved fluorescence spectroscopy. In addition to the ∼2.0 ns monomer fluorescence lifetime, a ≈ 1.5 ns component was found by applying global analysis to the time-resolved fluorescence decay. The 1.5 ns component is assigned to a cyclic porphyrin tetramer [Part I], with a ≈ 1 ns rotational correlation time at 10 °C. The initial fluorescence anisotropy of the monomer is found to be 0.1. In the tetramer an additional depolarization process occurs with a correlation time of ∼31 ps, resulting in a further decrease of the anisotropy from 0.1 to 0.025. This additional depolarization is ascribed to singlet energy transfer between the porphyrin units that constitute the tetramer. The intramolecular energy transfer processes have been simulated using the Monte Carlo method, yielding rate constants of (26 ± 4 ps

The native architecture of a photosynthetic membrane

In photosynthesis, the harvesting of solar energy and its subsequent conversion into a stable charge separation are dependent upon an interconnected macromolecular network of membrane-associated chlorophyll–protein complexes. Although the detailed structure of each complex has been determined, the size and organization of this network are unknown. Here we show the use of atomic force microscopy to directly reveal a native bacterial photosynthetic membrane. This first view of any multi-component membrane shows the relative positions and associations of the photosynthetic complexes and reveals crucial new features of the organization of the network: we found that the membrane is divided into specialized domains each with a different network organization and in which one type of complex predominates. Two types of organization were found for the peripheral light-harvesting LH2 complex. In the first, groups of 10–20 molecules of LH2 form light-capture domains that interconnect linear arrays of dimers of core reaction centre (RC)–light-harvesting 1 (RC–LH1–PufX) complexes; in the second they were found outside these arrays in larger clusters. The LH1 complex is ideally positioned to function as an energy collection hub, temporarily storing it before transfer to the RC where photochemistry occurs: the elegant economy of the photosynthetic membrane is demonstrated by the close packing of these linear arrays, which are often only separated by narrow 'energy conduits' of LH2 just two or three complexes wide