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

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 PCBA undergoes photoreaction into PCBB and PCBC indicating a branching of the initial reaction path leading to formation of PCBB and PCBC. 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.