UC Berkeley

Photosynthetic organisms have evolved over billions of years to efficiently convert solar energy into storable chemical energy. Light is absorbed by antenna complexes and then that energy is transferred to a reaction center wherein charge separation occurs – the foundation of the chemical reactions in photosynthesis. This process is the only terawatt energy conversion process on the planet, but the energy conversion is limited to the nanoscale – the spatial scale of the energy transfer occurring in the photosynthetic apparatus. Despite decades of study, there are still mysteries in the light harvesting and energy transfer processes that underlie solar energy conversion in photosynthetic organisms. This dissertation presents advanced spectroscopic tools for exploring the energetic dynamics of light harvesting in photosynthetic pigment-protein complexes.

Chapter 1 introduces the photosynthetic light-harvesting process and several pigment-protein complexes found in a broad spectrum of photosynthetic organisms – bacterial light harvesting 2(LH2), land plant photosystem II (PSII), and the phycocyanobilin complex 645 (PC645) in cryptophyte algae. Chapter 1 will also introduce advanced spectroscopic techniques that provide new insights into the function of these complex systems – Quantum Light Spectroscopy (QLS), Two-Dimensional Electronic Vibrational Spectroscopy (2DEV), as well as visible-pump IR-probe spectroscopy.

Chapter 2 explores the initial light absorption event in LH2. The goal of the study was to observe a single photon absorption event, the start of all photosynthetic processes. Determining the mechanism behind this initial step could lead to further insight in how photosynthesis functions so effectively. The complexity of studying a complicated system at the single photon level necessitated the development of a new technique: Quantum Light Spectroscopy (QLS). The QLS system uses an entangled photon pair in a heralded photon set up – one photon interacts with the system, producing a fluorescent photon after absorption, and the other photon acts as a herald to incoming signal. By using this heralded setup, we eliminate extraneous photons from the environment. We have shown the first experimental evidence of true single photon absorption in a photosynthetic system – rather than a statistical average of single photon absorption. This experiment serves as a proof of concept for this technique, opening the door to future studies at the single photon level – including those of low light energy harvesting in solar cells.

In Chapter 3, we break down photosystem II (PSII), a multi-subunit protein complex, into its main constituents and use a bottom-up approach to study how each component works together to transfer energy to the reaction center (RC). This energy transfer process is remarkably efficient and resistant to damage, highlighting the importance of understanding the design principles one step at a time. We thus study the PSII Core Complex, PSII Extended Core, and PSII Supercomplex – each of these contains increasing numbers of components and thus the complexity of the potential energy transfer pathways. Two-Dimensional Electronic-Vibrational Spectroscopy (2DEV) is used due to its advantages of high resolution in both the excitation (visible) and detection (mid-IR) axes. This enables the mapping of the energy transfer dynamics on to the physical space – specifically, the environment-specific vibrational structure enables the dynamics to be connected to specific pigments in the protein scaffold of PSII. Global analysis is performed on excitation frequency slices of the 2DEV spectra to gain insight into the pathways and timescales of the energy transfer from the peripheral antenna to the core of PSII.

In Chapter 4, we seek to describe the energy flow throughout the PC645 complex in cryptophyte algae. Contrary to the relatively flat energy landscape of the previously studied systems (LH2 and PSII) which are limited to a multitude of similarly absorbing pigments, PC645 is composed of pigments which absorb very different frequencies. Notably, this absorption is in the green region where PSII does not absorb strongly. However, PC645 needs to down convert that energy from higher frequencies to the lower frequencies absorbed by its own chlorophyll containing photosystems (an energy gap of nearly 1500 cm-1). We use visible pump-IR probe spectroscopy at specific excitation frequencies to study the dynamics of the energy transfer across the pigments in this pigment protein complex. 2DEV spectra at specific waiting times are used to probe the vibrational frequencies of the PC645 pigments within in the pigment-protein complex. This is the first study of this kind on PC645. Global analysis in combination with the vibrational structure of the pigments is used to determine the timescales and pigments involved in the energy transfer within the system.

This dissertation highlights the complex molecular architectures of photosynthetic light harvesting that enable life on Earth – from bacteria to land plants to algae. I describe complex spectroscopic techniques as well as novel approaches that enable the study of these systems. Examining the fundamentals of photosynthesis – particularly across diverse photosynthetic light harvesting systems as shown here – can provide new insight into energy harvesting processes and lead to higher efficiency in our agriculture and energy harvesting needs.