Excited-State Structures and Dynamics of Light Harvesting Complexes in Photosynthetic Bacteria: How Novel Spectroscopy Unveils Design Principles in Photosynthesis

Natural photosynthesis is the process of converting solar light into chemical energy through a series of photophysical and biochemical processes. Photosynthesis in Nature is an ancient yet still vibrant process that fascinates scientists by its delicacy in both its necessary structural components and intricately coordinated biochemical processes. Light harvesting processes, which occurs in the first tens of femtoseconds (10^{−15} s) to tens of picoseconds (10^{−12} s), involve capturing energy from solar light and, through a series of photophysical events, transferring to the reaction centers where photochemical reactions take place. Throughout almost three-quarters of Earth’s history, Nature has evolved a set of design principles for optimizing photosynthetic light harvesting efficiency among different species dwelling in extremely different habitats. To this date, Nature still leaves us with a lot of questions on detail mechanisms and biological purposes of these design principles. In this dissertation, we seek to answer the questions regarding the design principles of excited state structures and dynamics of natural light-harvesting complexes inside photosynthetic organisms with a novel ultrafast spectroscopy. In particular, we seek to achieve this goal by studying two different light harvesting systems in two organisms, LH1 complex from purple bacterium Rhodobacter sphaeroides and phycobilisomes from cyanobacterium Synechococcus elongatus PCC 7942, with ultrafast two-dimensional electronic spectroscopy (2DES). We present some of the first results of 2DES in both LH1-only chromatophores and isolated phycobilisomes, highlighting their hidden excited state features and vibronic structures. We also seek to discuss their implications on designing efficient light harvesting machinery