Single-molecule spectroscopy on pigment-protein complexes

Abstract

Single-molecule spectroscopy is used to study pigment-protein interactions relevant in photoreceptor proteins as well as in photosynthetic light- harvesting antennas. The photoreceptor protein studied is phytochrome; a light-sensitive molecular switch which is reversibly triggered by red and far- red light, respectively. A photo-induced conformational change of the protein- bound pigment initiates a signal cascade, thereby controlling growth and other developmental processes in plants, bacteria and fungi. For phytochromes an intrinsic heterogeneity was proposed based on ensemble spectroscopy as well as computational analysis. Single-molecule spectroscopy is applied to directly characterize the phytochrome heterogeneity. An inter-molecular heterogeneity is confirmed, and moreover, a dynamic intra-complex heterogeneity is revealed. The high-resolution spectra obtained at the low-temperature conditions of 1.4 K contain vibrational information of individual chromophores. The narrow line structures from different molecules were analyzed by a pattern recognition technique, developed for single-molecule spectra. Additionally, fluorescence line narrowing spectroscopy is applied on phytochromes. Strong heterogeneity between different bacterial phytochrome species is observed. Photosystem I (PSI) is a large pigment-protein complex essential in photosynthesis. It harbors a large light-harvesting system composed of about 300 chlorophyll molecules. A small number of low energy chlorophyll states contribute to the fluorescence emission of PSI. These states are characterized by single- molecule spectroscopy at 1.4 K. Single-emitter profiles are resolved. State- specific spectral dynamics are observed, and an additional yet unknown fluorescent state is found. In the further parts of this thesis, PSI is employed as a model system to study excitation energy transfer characteristics between pigments in photosynthetic light-harvesting systems, as well as to analyze the source for spectral diffusion and in the last part to characterize plasmonic effects on multi-FRET coupled systems.