Spectroscopy and control of ultrafast energy dynamics in natural light-harvesting complexes

Abstract

Coherent control and transient absorption spectroscopy techniques have been used widely in biophysics to manipulate and study ultrafast molecular dynamics such as excitation energy transfer, internal conversion and photoprotection mechanisms. In this thesis, we investigate how excitation energy transfer pathways originating from the S2 state of carotenoids in the light-harvesting complex of plants and green algae can be manipulated through coherent control; moreover, we investigate energy dissipation mechanisms in the nonameric fucoxanthinchlorophyll- a,c-binding protein of the centric diatom Cyclotella meneghiniana, using the transient absorption spectroscopy technique. By manipulating the temporal amplitude and phase of the excitation pulse (through the use of blind phase functions on a spatial light modulator) we were able to optimize the energy transfer channel over the internal conversion channel by a factor of 24% compared to the initial pulse. The optimized pulse exhibited a shape that consists of 7 sub-pulses with a separation time of 178 fs between every two consecutive sub-pulses, and a FWHMof 92 fs for each sub-pulse. We propose that the main mechanism responsible for the optimization is the enhancement of specific vibrational modes via impulsive stimulated Raman scattering in order to facilitate energy transfer. In order to investigate the energy dissipation mechanisms in the nonameric fucoxanthinchlorophyll- a,c-binding protein FCPb of the centric diatom Cyclotella meneghiniana, we performed transient absorption (pump-probe) spectroscopy on this complex. FCPb complexes in their unquenched state were compared with those in two types of quenching environments, namely aggregation induced quenching by detergent removal, and clustering via incorporation into liposomes. Through the application of global and target analysis, in combination with a fluorescence lifetime study and annihilation calculations, we were able to resolve two quenching channels for FCPb in both quenching environments. Both quenching channels involve chlorophyll-a pigments. The faster quenching channel operates on a timescale of tens of picoseconds and exhibits similar spectral signatures as the unquenched state. The slower quenching channel operates on a timescale of tens to hundreds of picoseconds, depending on the degree of quenching, and is characterized by enhanced population of low-energy states between 680 and 710 nm. These results indicate that FCPb is, in principle, able to function as a dissipater of excess energy and can do this in vitro even more efficiently than the homologous FCPa complex, which is the only complex involved in fast photoprotection in these organisms. This indicates that when a complex displays photoprotection-related spectral signatures in vitro it does not imply that the complex participates in photoprotection in vivo. We suggest that FCPa is favored over FCPb as the sole energy-regulating complex in diatoms because its composition can more easily establish the balance between light-harvesting and quenching required for efficient photoprotection.Thesis (PhD)--University of Pretoria, 2018.PhysicsPhDUnrestricte