Femtosecond dynamics in the lactim tautomer of phycocyanobilin: a long-wavelength absorbing model compound for the phytochrome chromophore

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.