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

Abstract

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.

The family of phytochrome-like photoreceptors: diverse, complex and multi-colored, but very useful

“…Given the difficulty to rationalize color-tuning in microbial and animal rhodopsins, which depend on the 'simpler' chromophore retinal, it is not surprising that theoretical approaches such as full-scale quantum chemical and QM/MM calculations on complex bilin chromophores in their protein environment are still in their infancy [49][50][51]. Here, progress is not only impeded by radiation effects on the chromophore during structural analyses [23 ,52], but also by intrinsic conformational heterogeneity of the P r state and [53][54][55][56] and to some extent also the P fr state [57].…”

Molecular Basis of Far-red Sensing in Cyanobacteriochrome

“…Several models have been proposed to produce a redshift to the far-red region based on spectroscopic and computational studies using model compounds (62)(63)(64)(65). Stanek and Grubmayr demonstrated that a deprotonated ring A lactim anion led to a significant redshift in the long-wavelength band (64).…”

“…Stanek and Grubmayr demonstrated that a deprotonated ring A lactim anion led to a significant redshift in the long-wavelength band (64). Diller and colleagues proposed that far-red absorption involves a D-ring lactim tautomer (65). Krois showed that a fully protonated bilin exhibited a redshift, which was further enhanced by bringing close two carbonyls in a constrained all-Z,syn model compound (62).…”

Spectroscopic Investigation on the Primary Photoreaction of Bathy Phytochrome Agp2‐Pr of Agrobacterium fabrum: Isomerization in a pH‐dependent H‐bond Network

“…Upon red‐light absorption the Pr chromophore undergoes a rapid Z / E isomerization at the C15 methine bridge, finally resulting in a ZZEssa stereochemistry of the Pfr chromophore . Both primary light induced reactions involve several excited‐state intermediates and evolve on a picosecond time scale, typically with a “slow” Pr reaction (30–100 ps or longer) and a “fast” Pfr reaction (a few ps) . The photoinduced Pr half‐cycle is characterized by the first electronic ground‐state intermediate lumi‐R (already in ZZE configuration) with red‐shifted absorption spectrum and consecutive, thermally driven reaction steps on the μs‐to‐ms time scale involving meta ‐Ra, meta ‐Rc and finally Pfr .…”

“…[14][15][16][17][18][19] Both primary light induced reactions involve several excited-state intermediates and evolve on ap icosecond time scale, typically with a" slow" Pr reaction (30-100 ps or longer) [20][21][22][23][24][25][26][27][28][29][30][31] and a" fast" Pfr reaction (a few ps). [20,21,23,30,[32][33][34] The photoinduced Pr half-cycle is characterizedb yt he first electronic ground-state intermediate lumi-R (already in ZZE configuration) with red-shifted absorption spectrum and consecutive, thermally driven reaction steps on the ms-to-ms time scale involving meta-Ra, meta-Rc and finally Pfr. [35,36] Previous Bathy phytochrome Agp2f rom Agrobacteriumf abrum exhibits an unusually low pK a = 7.6 in the Pr state in contrast to ap K a > 11 in the Pfr state, indicating ap H-dependent charge distribution and H-bond network in the Pr chromophoreb inding pocket around neutralp H. Here, we report on ultrafast UV/ Vis absorption spectroscopy of the primary Pr photoisomerization of Agp2 at pH 6a nd pH 9a nd upon H 2 O/D 2 Ob uffer exchange.…”