Towards a structure-based exciton Hamiltonian for the CP29 antenna of photosystem II

The exciton Hamiltonian pertaining to the first excited states of chlorophyll (Chl) a and b pigments in the minor light-harvesting complex CP29 of plant photosystem II is determined based on the recent crystal structure at 2.8 Å resolution applying a combined quantum chemical/electrostatic approach as used earlier for the major light-harvesting complex LHCII. Two electrostatic methods for the calculation of the local transition energies (site energies), referred to as the Poisson–Boltzmann/quantum chemical (PBQC) and charge density coupling (CDC) method, which differ in the way the polarizable environment of the pigments is described, are compared and found to yield comparable results, when tested against fits of measured optical spectra (linear absorption, linear dichroism, circular dichroism, and fluorescence). The crystal structure shows a Chl a/b ratio of 2.25, whereas a ratio between 2.25 and 3.0 can be estimated from the simulation of experimental spectra. Thus, it is possible that up to one Chl b is lost in CP29 samples. The lowest site energy is found to be located at Chl a604 close to neoxanthin. This assignment is confirmed by the simulation of wild-type-minus-mutant difference spectra of reconstituted CP29, where a tyrosine residue next to Chl a604 is modified in the mutant. Nonetheless, the terminal emitter domain (TED), i.e. the pigments contributing mostly to the lowest exciton state, is found at the Chl a611–a612–a615 trimer due to strong excitonic coupling between these pigments, with the largest contributions from Chls a611 and a612. A major difference between CP29 and LHCII is that Chl a610 is not the energy sink in CP29, which is presumably to a large extent due to the replacement of a lysine residue with alanine close to the TED

Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment

Anisotropic circular dichroism (ACD) spectroscopy of macroscopically aligned molecules reveals additional information about their excited states that is lost in the CD of randomly oriented solutions. ACD spectra of light-harvesting complex II (LHCII)-the main peripheral antenna of photosystem II in plants-in oriented lipid bilayers were recorded from the far-UV to the visible wavelength region. ACD spectra show a drastically enhanced magnitude and level of detail compared to the isotropic CD spectra, resolving a greater number of bands and weak optical transitions. Exciton calculations show that the spectral features in the chlorophyll Q y region are well-reproduced by an existing Hamiltonian for LHCII, providing further evidence for the identity of energy sinks at chlorophylls a603 and a610 in the stromal layer and chlorophylls a604 and a613 in the lumina] layer. We propose ACD spectroscopy to be a valuable tool linking the three-dimensional structure and the photophysical properties of pigment-protein complexes

Theory of Isotropic and Anisotropic Circular Dichroism Spectra of Pigment-Protein Complexes

Photosynthese ist der Prozess den Pflanzen, Bakterien und Algen verwenden, um Sonnenlicht in chemische Energie umzuwandeln. Dadurch wird Leben auf der Erde möglich gemacht. Um die Mechanismen hinter diesem hocheffizienten Prozess zu verstehen, ist es erforderlich die Zusammenhänge zwischen Struktur und Funktion der Pigment-Protein Komplexe (PPK) zu untersuchen. Diese sogenannten Lichtsammelkomplexe werden zur Absorption der Lichtenergie der Sonne sowie zum Transport der Anregungsenergie verwendet. In dieser Doktorarbeit werden strukturbasierte Methoden zur Parametrisierung des Exziton-Hamiltonians der PPKs mit der Theorie der optischen Spektren, insbesondere des Zirkular-Dichroismus-(CD), verknüpft, um diese Struktur-Funktions-Zusammenhänge zu untersuchen. In der CD-Spektroskopie wird die Differenz in der Absorption von links und rechts zirkular polarisiertem Licht gemessen und das dadurch resultierende Spektrum ist besonders sensitiv auf die Lage der Pigmente des PPKs zueinander sowie auf die Energieniveaus der angeregten Zustände dieser Pigmente. Eine der Herausforderungen in der theoretischen Beschreibung von CD-Spektren ist, dass experimentelle Spektren von PPKs oft eine Nichtkonservativität im niederenergetischen Bereich des Spektrums zeigen, d.h. das Integral über das CD-Spektrum ungleich null ist. So zeigt zum Beispiel der CP29-Komplex des Photosystems II der grünen Pflanzen eine starke Nichtkonservativität im roten Bereich des CD-Spektrums, welche nicht mit der Standard Exzitonen-Theorie beschrieben werden kann. Um die Nichtkonservativität des CP29-Komplexes zu erklären, wird in dieser Arbeit der Einfluss der exzitonischen Kopplungen zwischen den niederenergetischen Qy-Übergängen der Chlorophylle und den S0-S2-Übergängen der Carotenoide sowie den hochenergetischen Übergängen der Chlorophylle, als auch den Beitrag des intrinsischen CDs der Chlorophylle untersucht. Die Stärke des Übergangsdipolsmoments der jeweiligen Übergänge wird mithilfe einer Kombination von quantenchemischen Rechnungen mit experimentellen Daten an isolierten Pigmenten in Lösung bestimmt. Um die dynamische Lokalisierung der exzitonischen Wellenfunktionen durch die Exziton-Schwingungs Kopplung zu berücksichtigen werden Exzitondomänen eingeführt und die quantenmechanische Mischung der hochenergetischen Zustände mit den Qy-Zuständen der Chlorophylle wird in erster Ordnung Störungstheorie beschrieben. Es wird gezeigt, dass der Großteil der Nichtkonservativität des CD-Spektrums von CP29 durch die exzitonische Kopplung zwischen den Chlorophyll-Qy-Übergängen und den hochenergetischen Chlorophyll- und Carotenoid-Übergängen erklärt werden kann. Seit kurzem ist es möglich, Zirkular-Dichroimus-Messungen an orientierten Pigment-Protein-Komplexen durchzuführen und so zusätzliche Informationen über diese PPKs zu erhalten. In dieser Arbeit wird eine Exzitonen-Theorie für die Berechnung dieser anisotropen Zirkular-Dichroimus-(ACD)-Spektren hergeleitet. Die ACD-Theorie wird auf den Baseplate-Komplex der grünen Schwefelbakterien angewendet und es wird gezeigt, dass nur durch die kombinierte Analyse von CD- und ACD-, mit linearem Absorptions- und Linear-Dichroismus-Spektrum eine eindeutige Parametrisierung des Exziton-Hamiltonians sowie eine eindeutige Bestimmung der Orientierung der Bacteriochlorophyll a Pigmente innerhalb der Dimere des Baseplate-Komplexes möglich ist. Es wird eine verfeinerte Struktur für die Dimere des Baseplate-Komplexes vorgeschlagen, in der die Pigmente um 30 innerhalb ihrer Pigmentebene rotiert sind gegenüber einer NMR / CD Analyse aus der Literatur. Durch ihre Einfachheit ist die in dieser Arbeit entwickelte Exzitonen-Theorie auch dafür geeignet, um Exziton-Hamiltonians von großen PPKs durch Berechnung von Spektren und Vergleich mit dem Experiment zu überprüfen. In dieser Arbeit wurden zu diesem Zweck die optischen Spektren des Lichtsammelkomplexes LHCII aus grünen Pflanzen mit verschiedenen Hamiltonians analysiert. Die niederenergetische Bande des ACD-Spektrums zeigt eine Vorzeichenumkehr im Vergleich zum CD Spektrum, die mit allen Hamiltonians reproduziert werden konnte. Die quantitative Analyse der optischen Spektren weißt darauf hin, dass die Energiesenken im LHCII durch die Chlorophylle a604 und a613 in der luminalen Schicht und durch die Chlorophylls a603 und a610 in der stromalen Schicht gebildet werden.Photosynthesis is the process used by plants, bacteria and algae to convert sun light into chemical energy providing the basis for life on earth. To understand the mechanisms behind the highly efficient photosynthetic processes it is necessary to understand the structure-function relationships in the pigment-protein complexes (PPCs) which nature uses to harvest and transport the excitation energy. In this thesis structure-based methods for the parametrization of the exciton Hamiltonian of the PPC are combined with theories of optical spectra, in particular circular dichroism (CD) spectra, to investigate these structure-function relationships. CD is the difference in absorption between left- and right circularly polarized light and the resulting spectrum is particularly sensitive to the mutual pigment geometry as well as to the excited-state energy levels of the involved pigments. One of the challenges in the theoretical description of CD spectra is that the experimental spectra of photosynthetic PPCs, e.g. the CP29 complex of plant photosystem II, often show a strong non-conservativity in the low-energy region of the spectrum, meaning that the integral area of the spectrum is non-zero, an effect that cannot be described by the standard exciton theory. To explain the measured non-conservativity of CP29 in the low-energy Qy spectral region we investigate the influence of the coupling between Qy transitions of chlorophylls (Chls) and the S0-S2 transitions of carotenoids (Cars) and high-energy transitions of the Chls and the contribution from the intrinsic CD of the Chls. The dipole strengths of these transitions has been estimated by using a combination of quantum chemistry results and experimental data on isolated pigments in solution. To take into account dynamic localization effects of the exciton wavefunctions due to the exciton-vibrational coupling, exciton domains are introduced, and a first-order perturbation theory is used for the mixing between the Qy and high-energy transitions. We find that the major part of the non-conservativity of the CD spectrum of the CP29 complex in PSII is due to the excitonic coupling between Chl Qy transitions and the high-energy Chl and Car transitions. Recently, circular dichroism measurements on oriented pigment-protein complexes have been reported, which give additional information about the PPC. In the present thesis a simple exciton theory for the description of these anisotropic circular dichroism (ACD) spectra is derived and applied to the baseplate complex of green sulfur bacteria. It is demonstrated that only the combined analysis of CD and ACD spectra together with the absorption and linear dichroism spectra allows for an unambiguous parametrization of the exciton Hamiltonian as well as to determine the orientation of the BChl a pigments in the dimers in the baseplate complex unambiguously. In this way a refined structure for the baseplate dimer is proposed, where the pigments are rotated by 30 in the pigment plane. Due to the additional information contained in the new ACD spectra, the derived exciton theory for ACD allows for a critical examination of exciton Hamiltonians. For this purpose the optical spectra of the major light-harvesting complex LHCII of green plants has been analysed, where a sign switch of the low-energy transition in the ACD spectrum with respect to the isotropic CD spectrum is observed. By using different Hamiltonians from the literature, the site energies of the Chls that form the energy sinks in these different models are investigated. We find further evidence that the energy sinks in LHCII are located at Chl a604 and a613 in the luminal layer and at Chl a603 and a610 in the stromal layer.eingereicht von DI Dominik LindorferUniversität Linz, Dissertation, 2019OeBB(VLID)385364

Theory of Anisotropic Circular Dichroism of Excitonically Coupled Systems: Application to the Baseplate of Green Sulfur Bacteria

A simple exciton theory for the description of anisotropic circular dichroism (ACD) spectra of multichromophoric systems is presented that is expected to be of general use for the analysis of structure–function relationships of molecular aggregates such as photosynthetic light-harvesting antennae. The theory is applied to the baseplate of green sulfur bacteria. It is demonstrated that only the combined analysis of ACD and circular dichroism (CD) spectra for the present baseplate bacteriochlorophyll (BChl) <i>a</i> dimer allows for an unambiguous determination of the parameters of the exciton Hamiltonian from experimental data. The analysis of experimental absorption and linear dichroism spectra suggests that either the NMR structure has to be refined or in addition to the dimers seen in the NMR structure and in the CD and ACD spectra, BChl <i>a</i> monomers are present in the baseplate carotenosome sample. A refined dimer structure is presented, explaining all four optical spectra

Anisotropic Circular Dichroism of Light-Harvesting Complex II in Oriented Lipid Bilayers: Theory Meets Experiment

Anisotropic circular dichroism (ACD) spectroscopy of macroscopically aligned molecules reveals additional information about their excited states that is lost in the CD of randomly oriented solutions. ACD spectra of light-harvesting complex II (LHCII)-the main peripheral antenna of photosystem II in plants-in oriented lipid bilayers were recorded from the far-UV to the visible wavelength region. ACD spectra show a drastically enhanced magnitude and level of detail compared to the isotropic CD spectra, resolving a greater number of bands and weak optical transitions. Exciton calculations show that the spectral features in the chlorophyll Q y region are well-reproduced by an existing Hamiltonian for LHCII, providing further evidence for the identity of energy sinks at chlorophylls a603 and a610 in the stromal layer and chlorophylls a604 and a613 in the lumina] layer. We propose ACD spectroscopy to be a valuable tool linking the three-dimensional structure and the photophysical properties of pigment-protein complexes