Femtosecond Dynamics in the Lactim Tautomer of Phycocyanobilin: A Long-Wavelength Absorbing Model Compound for the Phytochrome Chromophore

• Published in: Chemphyschem 2014-12, Vol.15 (17), p.3824-3831
• Main Authors: Singer, Patrick, Fey, Sonja, Göller, Andreas H., Hermann, Gudrun, Diller, Rolf
• Format: Article
• Language: English
• Subjects: Chemistry; Exact sciences and technology; femtochemistry; General and physical chemistry; lactam/lactim tautomerization; Photochemistry; Physical chemistry of induced reactions (with radiations, particles and ultrasonics); phytochromes; quantum chemistry; Spectrum analysis
• ISSN: 1439-4235; 1439-7641
• DOI: 10.1002/cphc.201402383

Transient UV/Vis absorption spectroscopy is used to study the primary dynamics of the ring‐A methyl imino ether of phycocyanobilin (PCB‐AIE), which was shown to mimic the far‐red absorbance of the Pfr chromophore in phytochromes (R. Micura, K. Grubmayr, Bioorg. Med. Chem. Lett.­ 1994, 4, 2517–2522). After excitation at 615 nm, the excited electronic state is found to decay with τ1=0.4 ps followed by electronic ground‐state relaxation with τ2=1.2 and τ3=6.7 ps. Compared with phycocyanobilin (PCB), the initial kinetics of PCB‐AIE is much faster. Thus, the lactim structure of PCB‐AIE seems to be a suitable model that could not only explain the bathochromic shift in the ground‐state absorption but also the short reaction of the Pfr as compared to the Pr chromophore in phytochrome. In addition, the equivalence of ring‐A and ring‐D lactim tautomers with respect to a red‐shifted absorbance relative to the lactam tautomers is demonstrated by semiempirical calculations. Fast and furious: Steady‐state absorption properties and ultrafast photoinduced dynamics observed in the ring‐A methyl imino ether of phycocyanobilin (PCB‐AIE) and phycocyanobilin (PCB) are related to the bathochromic shift and primary photoresponse found in the red and far‐red absorbing form of canonical phytochromes. The experimental results are supported by semiempirical calculations.