Spectroscopy and Imaging

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 free PCB. 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 forming 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.