Excitation migration in fluctuating light-harvesting antenna systems

Complex multi-exponential fluorescence decay kinetics observed in various photosynthetic systems like photosystem II (PSII) have often been explained by the reversible quenching mechanism of the charge separation taking place in the reaction center (RC) of PSII. However, this description does not account for the intrinsic dynamic disorder of the light-harvesting proteins as well as their fluctuating dislocations within the antenna, which also facilitate the repair of RCs, state transitions, and the process of non-photochemical quenching. Since dynamic fluctuations result in varying connectivity between pigment–protein complexes, they can also lead to non-exponential excitation decay kinetics. Based on this presumption, we have recently proposed a simple conceptual model describing excitation diffusion in a continuous medium and accounting for possible variations of the excitation transfer pathways. In the current work, this model is further developed and then applied to describe fluorescence kinetics originating from very diverse antenna systems, ranging from PSII of various sizes to LHCII aggregates and even the entire thylakoid membrane. In all cases, complex multi-exponential fluorescence kinetics are perfectly reproduced on the entire relevant time scale without assuming any radical pair equilibration at the side of the excitation quencher, but using just a few parameters reflecting the mean excitation energy transfer rate as well as the overall average organization of the photosynthetic antenn

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A nation attains democratic consolidation when democratic ideals become permanently established within its society. An institutionalized political party system is a prerequisite for democratic consolidation and is based on stable rules of interparty competition, parties with stable social roots, and party organizations independent of individuals’ ambitions. Additionally, all actors must accord legitimacy to a party system in order for it to be institutionalized. This thesis focuses on the political party system in Haiti and how its lack of institutionalization has undermined the consolidation of democracy. It also examines the factors that are responsible for this lack of development. Such factors include the actions of Haitian elites, lack of social organization, and a lack of party development during the earliest stages of state building. The impact of United States foreign policies upon this nation’s party system are examined in order to discover which policies have assisted party system institutionalization and which have hindered its development. While the entirety of Haitian political history is reviewed, an emphasis is placed upon United States policy since the 1991 ouster of President Jean-Bertrand Aristide. In conclusion, this thesis recommends future United States policies designed to assist in the institutionalization of Haiti’s political party system

Excitation migration in fluctuating light-harvesting antenna systems

Complex multi-exponential fluorescence decay kinetics observed in various photosynthetic systems like photosystem II (PSII) have often been explained by the reversible quenching mechanism of the charge separation taking place in the reaction center (RC) of PSII. However, this description does not account for the intrinsic dynamic disorder of the light-harvesting proteins as well as their fluctuating dislocations within the antenna, which also facilitate the repair of RCs, state transitions, and the process of non-photochemical quenching. Since dynamic fluctuations result in varying connectivity between pigment–protein complexes, they can also lead to non-exponential excitation decay kinetics. Based on this presumption, we have recently proposed a simple conceptual model describing excitation diffusion in a continuous medium and accounting for possible variations of the excitation transfer pathways. In the current work, this model is further developed and then applied to describe fluorescence kinetics originating from very diverse antenna systems, ranging from PSII of various sizes to LHCII aggregates and even the entire thylakoid membrane. In all cases, complex multi-exponential fluorescence kinetics are perfectly reproduced on the entire relevant time scale without assuming any radical pair equilibration at the side of the excitation quencher, but using just a few parameters reflecting the mean excitation energy transfer rate as well as the overall average organization of the photosynthetic antenn

Excitation Migration, Quenching, and Regulation of Photosynthetic Light Harvesting in Photosystem II

Excitation energy transfer and quenching in LHCII aggregates is considered in terms of a coarse-grained model. The model assumes that the excitation energy transfer within a pigment-protein complex is much faster than the intercomplex excitation energy transfer, whereas the quenching ability is attributed to a specific pigment-protein complex responsible for the nonphotochemical quenching (NPQ). It is demonstrated that the pump-probe experimental data obtained at low excitation intensities for LHCII aggregates under NPQ conditions can be equally well explained at two limiting cases, either describing the excitation kinetics in the migration-limited or in the trap-limited regime. Thus, it is concluded that low excitation conditions do not allow one to unambiguously define the relationship between the mean times of excitation migration and trapping. However, this could be achieved by using high excitation conditions when exciton-exciton annihilation is dominant. In this case it was found that in the trap-limited regime the excitation kinetics in the aggregate should be almost insensitive to the excitation density, meaning that singlet-singlet annihilation has little effect on the NPQ decay kinetics, whereas in the migration-limited case there is a clear intensity dependence. In order to account for the random distribution of the NPQ-traps within the LHCII aggregates, excitation diffusion in a continuous medium with random static traps was considered. This description demonstrates a very good correspondence to the experimental fluorescence kinetics assuming a lamellar (quasi-3D) structure of the antenna characterized by the dimension d = 2.4 and therefore justifying the diffusion-limited approach on which the model is based. Using the coarse-grained model to describe the aggregate we estimate one NPQ-trap per 100 monomeric LHCII complexes. Finally we discuss the origin of the traps responsible for excitation quenching under NPQ conditions. © 2011 American Chemical Society