Wavelength-dependent photoproduct formation of phycocyanobilin in solution – Indications for competing reaction pathways

Abstract

Phycocyanobilin (PCB), an open-chain tetrapyrrole, makes up – slightly modified – the chromophore in phytochromes, the importance of which triggered model studies on the photochemistry of PCB in solution. We use pump-wavelength dependent femtosecond transient absorption spectroscopy including the near-IR region for probing to gain new insight into the photoreaction. The data reveal the coexistence of three ground state species: The dominant species PCB A undergoes photoreaction into PCB B and PCB C indicating a branching of the initial reaction path leading to formation of PCB B and PCB C. On this basis the photoreaction involves fast structural rearrangements within the tetrapyrrolic macrocycle, which are followed by slower rate-limiting changes in the protonation state of the pyrrolenine/pyrrole rings.