Optical spectroscopy of light-harvesting systems

This work deals with the exploration of strategies for simulating lineshapes of optical spectroscopies of photosynthetic pigments and complexes. The first part of the thesis shows the effectiveness of a single tool, the spectral density, for reproducing high-resolution vibronic structure of single pigments. This tool can be applied to the simulation of both linear and nonlinear spectroscopies, such as absorption, fluorescence, Resonance Raman and two-dimensional optical spectroscopies. The second part of the thesis extends this formalism to multichromophoric systems. Exciton mixing, spectral densities of chromophores, the use of multiscale modelling and the inclusion of disorder effects, together with a suitable level of open quantum systems theory, are successfully applied to the study of optical spectra and energy transfer dynamics of a selected light-harvesting complex-Peridinin Chlorophyll Protein

Photosynthesis | Light-harvesting complex I and II - Pigments and proteins

Light-harvesting complexes are a family of membrane-embedded pigment-binding proteins that work as antennas for the photosystems I and II of plants and algae. Here we provide an overview of their properties, introducing their protein and pigment components and discussing their functional and structural association with the cores to form the photosystem I and II supercomplexes. Finally, we highlight the importance of their structural flexibility for carrying out specific functions

Far-red absorption and light-use efficiency trade-offs in chlorophyll f photosynthesis

Plants and cyanobacteria use the chlorophylls embedded in their photosystems to absorb photons and perform charge separation, the first step of converting solar energy to chemical energy. While oxygenic photosynthesis is primarily based on chlorophyll a photochemistry, which is powered by red light, a few cyanobacterial species can harness less energetic photons when growing in far-red light. Acclimatization to far-red light involves the incorporation of a small number of molecules of red-shifted chlorophyll f in the photosystems, whereas the most abundant pigment remains chlorophyll a. Due to its different energetics, chlorophyll f is expected to alter the excited-state dynamics of the photosynthetic units and, ultimately, their performances. Here we combined time-resolved fluorescence measurements on intact cells and isolated complexes to show that chlorophyll f insertion slows down the overall energy trapping in both photosystems. While this marginally affects the efficiency of photosystem I, it substantially decreases that of photosystem II. Nevertheless, we show that despite the lower energy output, the insertion of red-shifted chlorophylls in the photosystems remains advantageous in environments that are enriched in far-red light and therefore represents a viable strategy for extending the photosynthetically active spectrum in other organisms, including plants. However, careful design of the new photosynthetic units will be required to preserve their efficiency

The photosynthetic apparatus of the CAM plant Tillandsia flabellate and its response to water deficit

CAM plants are superior to C3 plants in drought resistance because of their peculiar photosynthesis pathway and morphological features. While those aspects have been studied for decades, little is known about the photosynthetic machinery of CAM plants. Here, we used a combination of biochemical and biophysical methods to study the photosynthetic apparatus of Tillandsia flabellate, an obligatory CAM plant. Most of the Photosystems super- and sub-complexes have properties very similar to those of Arabidopsis, with the main difference that in Tillandsia PSI-LHCI complexes bind extra LHCI. Functional measurements show that the PSI/PSII ratio is rather low compared to other plants and that the antenna size of both PSI and PSII is small. Upon 30-day water deficiency, the composition of the photosystems does not change significantly, PSII efficiency remains high and no Photosystem II photoinhibition was detected despite a reduction of non-photochemical quenching (NPQ)

Understanding the Relation between Structural and Spectral Properties of Light-Harvesting Complex II

[Image: see text] Light-harvesting complex II (LHCII) is a pigment–protein complex present in higher plants and green algae. LHCII represents the main site of light absorption, and its role is to transfer the excitation energy toward the photosynthetic reaction centers, where primary energy conversion reactions take place. The optical properties of LHCII are known to depend on protein conformation. However, the relation between the structural and spectroscopic properties of the pigments is not fully understood yet. In this respect, previous classical molecular dynamics simulations of LHCII in a model membrane [Sci. Rep.2015, 5, 1–10] have shown that the configuration and excitonic coupling of a chlorophyll (Chl) dimer functioning as the main terminal emitter of the complex are particularly sensitive to conformational changes. Here, we use quantum chemistry calculations to investigate in greater detail the effect of pigment–pigment interactions on the excited-state landscape. While most previous studies have used a local picture in which electrons are localized on single pigments, here we achieve a more accurate description of the Chl dimer by adopting a supramolecular picture where time-dependent density functional theory is applied to the whole system at once. Our results show that specific dimer configurations characterized by shorter inter-pigment distances can result in a sizable intensity decrease (up to 36%) of the Chl absorption bands in the visible spectral region. Such a decrease can be predicted only when accounting for Chl–Chl charge-transfer excitations, which is possible using the above-mentioned supramolecular approach. The charge-transfer character of the excitations is quantified by two types of analyses: one focusing on the composition of the excitations and the other directly on the observable total absorption intensities

Light-harvesting complexes access analogue emissive states in different environments

The light-harvesting complexes (LHCs) of plants can regulate the level of excitation in the photosynthetic membrane under fluctuating light by switching between different functional states with distinct fluorescence properties. One of the most fascinating yet obscure aspects of this regulation is how the vast conformational landscape of LHCs is modulated in different environments. Indeed, while in isolated antennae the highly fluorescent light-harvesting conformation dominates, LHC aggregates display strong fluorescence quenching, representing therefore a model system for the process of energy dissipation developed by plants to avoid photodamage in high light. This marked difference between the isolated and oligomeric conditions has led to the widespread belief that aggregation is the trigger for the photoprotective state of LHCs. Here, a detailed analysis of time-resolved fluorescence experiments performed on aggregates of CP29 - a minor LHC of plants - provides new insights into the heterogeneity of emissive states of this antenna. A comparison with the data on isolated CP29 reveals that, though aggregation can stabilize short-lived conformations to a certain extent, the massive quenching upon protein clustering is mainly achieved by energetic connectivity between complexes that maintain the same long-lived and dissipative states accessed in the isolated form. Our results also explain the typical far-red enhancement in the emission of antenna oligomers in terms of a sub-population of long-lived redshifted complexes competing with quenched complexes in the energy trapping. Finally, the role of selected chlorophylls in shaping the conformational landscape of the antenna is also addressed by studying mutants lacking specific pigments

Design principles of solar light harvesting in plants:Functional architecture of the monomeric antenna CP29

In plants and green algae, light-harvesting complexes (LHCs) are a large family of chlorophyll binding proteins functioning as antennae, collecting solar photons and transferring the absorbed energy to the photosynthetic reaction centers, where light to chemical energy conversion begins. Although LHCs are all highly homologous in their structure and display a variety of common features, each complex finds a specific location and task in the energy transport. One example is CP29, which occupies a pivotal position in Photosystem II, bridging the peripheral antennae to the core. The design principles behind this specificity, however, are still unclear. Here, a synergetic approach combining steady-state and ultrafast spectroscopy, mutational analysis and structure-based exciton modeling allows uncovering the energy landscape of the chlorophylls bound to this complex. We found that, although displaying an overall highly conserved exciton structure very similar to that of other LHCs, CP29 possesses an additional terminal emitter domain. The simultaneous presence of two low energy sites facing the peripheral antennae and the core, allows CP29 to efficiently work as a conduit in the energy flux. Our results show that the LHCs share a common solid architecture but have finely tuned their structure to carry out specific functions

Understanding the Relation between Structural and Spectral Properties of Light-Harvesting Complex II

[Image: see text] Light-harvesting complex II (LHCII) is a pigment–protein complex present in higher plants and green algae. LHCII represents the main site of light absorption, and its role is to transfer the excitation energy toward the photosynthetic reaction centers, where primary energy conversion reactions take place. The optical properties of LHCII are known to depend on protein conformation. However, the relation between the structural and spectroscopic properties of the pigments is not fully understood yet. In this respect, previous classical molecular dynamics simulations of LHCII in a model membrane [Sci. Rep.2015, 5, 1–10] have shown that the configuration and excitonic coupling of a chlorophyll (Chl) dimer functioning as the main terminal emitter of the complex are particularly sensitive to conformational changes. Here, we use quantum chemistry calculations to investigate in greater detail the effect of pigment–pigment interactions on the excited-state landscape. While most previous studies have used a local picture in which electrons are localized on single pigments, here we achieve a more accurate description of the Chl dimer by adopting a supramolecular picture where time-dependent density functional theory is applied to the whole system at once. Our results show that specific dimer configurations characterized by shorter inter-pigment distances can result in a sizable intensity decrease (up to 36%) of the Chl absorption bands in the visible spectral region. Such a decrease can be predicted only when accounting for Chl–Chl charge-transfer excitations, which is possible using the above-mentioned supramolecular approach. The charge-transfer character of the excitations is quantified by two types of analyses: one focusing on the composition of the excitations and the other directly on the observable total absorption intensities

Silent Contained Rupture of an Inflammatory IgG4-Related Abdominal Aortic Aneurysm

This report describes a case of a 66-year-old male patient with accidental diagnosis of chronic contained rupture of an aortic aneurysm. Surgery was performed through a median laparotomy. A thick periaortic tissue with fibrosis and lymphnodes covered the AAA. Immunohistochemical examination of the aneurismatic aortic wall revealed intense positivity for inflammatory markers and a large number of immunoglobulin G4 (IgG4) positive cells. The postoperative course was uneventful and patient was discharged in the fifth postoperative day. Patient was then followed periodically at the outpatient rheumatologic clinic. No adverse events occurred during 3 and 6 months follow up. Conclusion: Identification of IgG4-inflammatory aneurysms as an expression of the IgG4-related systemic disease is essential both for clinical follow up and surgical and pharmacological treatment considering the possibility of aneurysm rupture and the involvement of other organs

Uncovering the interactions driving carotenoid binding in light-harvesting complexes

Carotenoids are essential constituents of plant light-harvesting complexes (LHCs), being involved in protein stability, light harvesting, and photoprotection. Unlike chlorophylls, whose binding to LHCs is known to require coordination of the central magnesium, carotenoid binding relies on weaker intermolecular interactions (such as hydrogen bonds and van der Waals forces), whose character is far more elusive. Here we addressed the key interactions responsible for carotenoid binding to LHCs by combining molecular dynamics simulations and polarizable quantum mechanics/molecular mechanics calculations on the major LHC, LHCII. We found that carotenoid binding is mainly stabilized by van der Waals interactions with the surrounding chlorophyll macrocycles rather than by hydrogen bonds to the protein, the latter being more labile than predicted from structural data. Furthermore, the interaction network in the binding pockets is relatively insensitive to the chemical structure of the embedded carotenoid. Our results are consistent with a number of experimental data and challenge the role played by specific interactions in the assembly of pigment-protein complexes