Oxygenic photosynthesis is the fundamental process by which sunlight energy is stored as chemical energy in organic compounds and oxygen is released in the atmosphere. It starts with the capture of a photon by one of the pigments embedded within one of the two photosystems, Photosystem I (PSI) or II (PSII). These photosystems are large assemblies of many pigments held together by the protein scaffold. The absorption of the photon brings the pigment to an electronic excited state. The excitation energy is then transferred from pigment-to-pigment to the reaction center (RC) of the photosystem, where it is used to perform charge separation (CS). The pigment-to-pigment energy transfer within photosynthetic complexes occurs on a very fast, femtosecond (fs, 10^(-15) second) to picosecond (ps, 10^(-12) second) timescale, which ensures that the photosystems are extremely efficient in using the energy for charge separation. In this thesis, aspects of the light-harvesting of photosynthetic pigment-protein complexes were investigated. The spectroscopic properties (absorption, emission) and energy-transfer processes were studied with a variety of different techniques, including advanced ultrafast time-resolved spectroscopic methods (two-dimensional electronic spectroscopy (2DES) and time-resolved fluorescence spectroscopy). In these time-resolved experiments, the complexes are excited with ultrashort (fs temporal width) pulses of light, after which the optical response (photon-echo, fluorescence) is monitored in time. By measuring these signals, excitation energy transfer (EET) and energy trapping within these complexes can be determined. Oxygenic photosynthesis is mainly powered by visible light in the 400–700 nm range. Expanding the absorption range to 750 nm would result in 19% more photons available for photosynthesis [Chen, M. & Blankenship, R. E. (2011) Trends Plant Sci., 16, 427–431]. Moreover, improved far-red light-harvesting can be advantageous in shaded environments. For these reasons extention of the absorption beyond the 400–700 nm range is an important approach in the global aim to improve crop productivity to meet the increasing global demands for food production. This thesis focuses on the far-red light-harvesting properties of PSI from higher plants and cyanobacteria. The aim is to understand underlying aspects that are important for far-red light (FRL, 700–800 nm) absorption and EET within PSI. These aspects can be useful to enhance the far-red light-harvesting in photosynthesis of other organism, such as plants. The chapters of this thesis contain several insights that can be generally important in the goal to enhance the far-red light-harvesting abilities of photosynthetic complexes. The integration of new low-energy states, such as from long-wavelength Chlorophylls (Chls) or new Chl a red form states, is a viable strategy to enhance the absorption of far-red light of these complexes. However, additional alterations to the light-harvesting mechanism may be required to obtain a highly efficient complex with optimally enhanced far-red light absorption. Notably, as shown by the investigated natural (light-harvesting complex I and FRL-specific PSI complex) and artificial (Chl f-containing hybrid PSI) complexes the protein scaffold is a determining factor in optimization of the far-red light-harvesting properties of photosynthetic complexes. Conclusively, in this thesis we provide several important lessons for the aim to effectively enhance the far-red light-harvesting capacity of other photosynthetic organisms.
|Award date||3 Feb 2021|
|Place of Publication||s.l.|
|Publication status||Published - 3 Feb 2021|
- Excitation energy transfer
- Photosystem I
- Two-dimensional electronic spectroscopy
- Ultrafast Spectroscopy