The Excited-State Dynamics of Phycocyanobilin in Dependence on the Excitation Wavelength

• Published in: Chemphyschem 2004-08, Vol.5 (8), p.1171-1177
• Main Authors: Dietzek, Benjamin, Maksimenka, Raman, Hermann, Gudrun, Kiefer, Wolfgang, Popp, Jürgen, Schmitt, Michael
• Format: Article
• Language: English
• Subjects: Bacterial Proteins - chemistry; Chemistry; chromophores; Exact sciences and technology; General and physical chemistry; Kinetics; Lasers; Models, Chemical; Photochemistry; Phycobilins; Phycocyanin - isolation & purification; Phycocyanin - radiation effects; phycocyanobilin; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); Protein Conformation; Pyrroles - isolation & purification; Pyrroles - radiation effects; Spirulina; Tetrapyrroles; time-resolved spectroscopy
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.200400056

The primary light‐induced processes of phycocyanobilin were studied by means of transient‐grating spectroscopy, whereby the excitation wavelength was varied over the spectral region of the ground‐state absorption. On the basis of the results obtained, both the rate of the photoreaction in phycocyanobilin and the ratio of the decay of different excited‐state species via two decay channels depend on the excitation wavelength. Furthermore, the formation of the photoreaction product is also dependent on the pump color. These data support a recently established model for the primary photoprocesses in phycocyanobilin. In addition, phycocyanobilin protonated at the basic pyrrolenine‐type nitrogen atom was included in the transient absorption study. The decay behavior was found to be almost unchanged when compared with the unprotonated form, and this suggests that protonation of the tetrapyrrole ring structure has no effect on the overall photochemistry. The open‐chain tetrapyrrole chromophore phycocyanobilin (PCB) is of importance as a model substance for a series of biological chromophores. In solution, three different PCB species, PCBA, PCBB, and PCBC, coexist in thermal equilibrium (see picture). By applying femtosecond‐resolved transient grating and transient absorption spectroscopy, a recent kinetic model suggested for the excited‐state processes in PCB was verified.