Structure revision of scytonemin imine and its relationship to scytonemin chromism and cyanobacterial adaptability.

Scytonemin, a UV-protective pigment produced by cyanobacteria, is essential for microbial survival under extreme solar radiation. Recent studies suggest its structural analog, scytonemin imine, may serve as a biosynthetic marker for cyanobacteria exposed to intense light. Here, we present a structural revision, revealing scytonemin imine as a cyclic hydropyrrolo[2,3-b]indole, rather than the previously proposed primary imine. This reassignment is supported by 1D and 2D NMR, Q-TOF mass spectrometry, confirmatory synthesis, and DFT calculations. Our synthesis demonstrates that scytonemin converts to scytonemin imine under mild conditions-ammonia and acetone exposure-suggesting the imine adduct is likely an artifact of isolation. However, our findings also indicate that this artifact may reveal a previously unrecognized in-vivo state of scytonemin, which is released upon condensation with acetone. This reactivity uncovers a new chromism within the scytonemin scaffold, supporting the idea that biogenic scytonemin analogs may filter visible light to regulate photosynthesis and protect against ROS-mediated photodamage during high light exposure or desiccation. This chromatic transformation highlights scytonemin's structural adaptability, offering insight into its role as a protective pigment in ancient and modern cyanobacteria and its relevance for understanding microbial adaptation to extreme environments.