Light at the End of the Funnel: Fluorescence-Detected Two-Dimensional Electronic Spectroscopy to Probe Photosynthesis in Bacteria

Abstract

Photosynthesis relies on solar energy absorption by the light-harvesting complexes and ultrafast energy transfer to the reaction centers, where the excitation is converted into a stable charge-separated state. This process happens with a high quantum efficiency ∼1. The mechanisms underlying the ultrafast energy and charge transfer in photosynthesis have been the subject of numerous spectroscopic studies. In this thesis, I focus on fluorescence-detected two-dimensional electronic spectroscopy (F2DES) which offers high sensitivity and the ability to be combined with a microscope, an added advantage to its coherently-detected counterpart, C-2DES. Fundamental differences between C-2DES and F-2DES techniques lead to strik- ingly different looking spectra. Currently, F-2DES is not very well-understood as compared to C-2DES and warrants more investigation. The light-harvesting 2 complex in photosynthetic bacteria is a well characterized pigment-protein complex and makes for an ideal model system to understand the potential of F-2DES for studying multichromophoric systems. In this thesis, we compare F-2DES and C-2DES by studying the energy transfer dynamics in purple bacterial LH2 using the two methods. From our experiments, we determine that C-2DES is clearly a better choice than the F-2DES to study energy transfer dynamics in large systems. This is because in case of F-2DES, an increasing number of chromophores in the system reduces the relative weights of pathways that can reveal energy transfer signatures to the ones that are present as a large background in the signal. We also present preliminary F-2DES simulations on LH2 to support our findings. In addition, we report preliminary F2DES measurements to probe the energy transfer in a newly discovered phototrophic bacterial species Gemmatimonas phototrophica. We point out that the F-2DES measurements may benefit from lower acquisition times, which would help achieve higher averaging. To this end, we present a rapid-scanning F-2DES methodology based on continuous time-delay scanning and digital lock-in acquisition. In addition, a broadband method capable of improving the bandwidth accessed with our current F-2DES measurements is demonstrated.