Chemical Synthesis of Scytonemin and Structural Derivatives for Elucidating Photoprotective Mechanisms in Cyanobacteria
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Authors
Larson, Garrett Evan
Issue Date
2025
Type
Dissertation
Language
en_US
Keywords
Chemistry , Natural Products , Organic Chemistry , Organic Synthesis
Alternative Title
Abstract
Photoprotection is a vital process for organisms to survive, especially in extreme environments. Cyanobacteria are organisms that rely on photoprotection to protect themselves from the intense UV radiation they are exposed to. One of the main pigments involved in cyanobacterial photoprotection is scytonemin, which has been shown to be an integral part of photoprotection in many different species of cyanobacteria. It has been hypothesized to provide protection for organisms that predate ozone layer formation due to its ability to detect UV-C, it also blocks 85-90% of UV-A and B, providing protection for current atmospheric conditions as well. The unique structure of scytonemin allows it to be protected and adapt to extreme environments where cyanobacteria live. Mechanistic studies of the key UV-pigment have not been widely explored resulting in very little knowledge about how scytonemin functions as a UV-protectant in for cyanobacteria and related organisms. One major limitation to these studies is the small quantities available through isolation from the natural source. A practical chemical synthesis would enable photophysical and related studies. I developed a novel and practical synthesis of scytonemin, which is described in chapter 2. This synthesis was more concise and started from economical feasible starting materials compared to previously reported syntheses. Other attempts toward the synthesis of scytonemin are included in Chapter 5, which describe alternative routes to scytonemin along with alternative reactions towards optimizing the key transformations. The chemical synthesis of scytonemin also enabled the synthesis of the scytonemin imine, which is an analogue of scytonemin. Scytonemin imine was reported in 2013 by Grant and Louda and is proposed to play an important role in cyanobacteria growing in high light environments. However, many structural ambiguities were reported in the originally proposed structure and through reisolation, DFT calculation of NMR spectra, and confirmatory synthesis, we verified that the structure was indeed a cyclic imine as opposed to the previously determined primary imine moiety. During the synthesis and characterization of the imine, it was also discovered that scytonemin imine demonstrated significant solvatochromic properties through our initial proton NMR analysis. Chapter 4 provides further detail that demonstrates that scytonemin imine forms a quinoid tautomer in polar solvents. This is a newly described structural feature of this chromophore that can have significant implications in both the photochemistry and redox properties of this scaffold. It also reveals the potential environmentally driven chromism that can be tied to adaptability to the wide range of pressures that these organisms can resist. This synthesis enabled further examination into the role of scytonemin in the photoprotection of cyanobacteria throughout the geologic time scale. The synthesis of the scytonemin imine was integral to the structural revision and led to the observation of the solvatochromic properties of the scytonemin imine. These characteristics add to the complex story of scytonemin and scytonemin analogues along with their role in the photoprotection of cyanobacteria.