Abstract:Phytochromes are biological photoswitches
that interconvert between
two parent states (Pr and Pfr). The transformation is initiated by
photoisomerization of the tetrapyrrole chromophore, followed by a
sequence of chromophore and protein structural changes. In the last
step, a phytochrome-specific peptide segment (tongue) undergoes a
secondary structure change, which in prokaryotic phytochromes is associated
with the (de)activation of the output module. The focus of this work
is the Pfr-to-Pr photoconversion o… Show more
“…Considering also the protonation state and processes, which play a crucial role in the photoconversion of phytochromes [20][21][22][23][24][25][26][27] and significantly change the spectral properties of the PCB molecule, Göller and co-workers, via semiempirical AM1 calculations, found three possible all-Z ground state species (in accordance with further experimental results, see below), which are depicted in Fig. 2 [18,28].…”
Deactivation processes of photoexcited (λex = 580 nm) phycocyanobilin (PCB) in methanol were investigated by means of UV/Vis and mid-IR femtosecond (fs) transient absorption (TA) as well as static fluorescence spectroscopy, supported by density-functional-theory calculations of three relevant ground state conformers, PCBA, PCBB and PCBC, their relative electronic state energies and normal mode vibrational analysis. UV/Vis fs-TA reveals time constants of 2.0, 18 and 67 ps, describing decay of PCBB*, of PCBA* and thermal re-equilibration of PCBA, PCBB and PCBC, respectively, in line with the model by Dietzek et al. (Chem Phys Lett 515:163, 2011) and predecessors. Significant substantiation and extension of this model is achieved first via mid-IR fs-TA, i.e. identification of molecular structures and their dynamics, with time constants of 2.6, 21 and 40 ps, respectively. Second, transient IR continuum absorption (CA) is observed in the region above 1755 cm−1 (CA1) and between 1550 and 1450 cm−1 (CA2), indicative for the IR absorption of highly polarizable protons in hydrogen bonding networks (X–H…Y). This allows to characterize chromophore protonation/deprotonation processes, associated with the electronic and structural dynamics, on a molecular level. The PCB photocycle is suggested to be closed via a long living (> 1 ns), PCBC-like (i.e. deprotonated), fluorescent species.
“…Considering also the protonation state and processes, which play a crucial role in the photoconversion of phytochromes [20][21][22][23][24][25][26][27] and significantly change the spectral properties of the PCB molecule, Göller and co-workers, via semiempirical AM1 calculations, found three possible all-Z ground state species (in accordance with further experimental results, see below), which are depicted in Fig. 2 [18,28].…”
Deactivation processes of photoexcited (λex = 580 nm) phycocyanobilin (PCB) in methanol were investigated by means of UV/Vis and mid-IR femtosecond (fs) transient absorption (TA) as well as static fluorescence spectroscopy, supported by density-functional-theory calculations of three relevant ground state conformers, PCBA, PCBB and PCBC, their relative electronic state energies and normal mode vibrational analysis. UV/Vis fs-TA reveals time constants of 2.0, 18 and 67 ps, describing decay of PCBB*, of PCBA* and thermal re-equilibration of PCBA, PCBB and PCBC, respectively, in line with the model by Dietzek et al. (Chem Phys Lett 515:163, 2011) and predecessors. Significant substantiation and extension of this model is achieved first via mid-IR fs-TA, i.e. identification of molecular structures and their dynamics, with time constants of 2.6, 21 and 40 ps, respectively. Second, transient IR continuum absorption (CA) is observed in the region above 1755 cm−1 (CA1) and between 1550 and 1450 cm−1 (CA2), indicative for the IR absorption of highly polarizable protons in hydrogen bonding networks (X–H…Y). This allows to characterize chromophore protonation/deprotonation processes, associated with the electronic and structural dynamics, on a molecular level. The PCB photocycle is suggested to be closed via a long living (> 1 ns), PCBC-like (i.e. deprotonated), fluorescent species.
“… Fig. 1 Photocycle of bacterial phytochromes as derived from spectroscopic data 28 . In Agp2, Pfr is the stable dark state.…”
Section: Mainmentioning
confidence: 99%
“…Together with vibrational spectroscopy studies 25 – 28 , it was thus possible to identify key structural elements in the chromophore binding pocket that are essential for communicating the light-induced structural changes of the chromophore to the tongue segment of the protein 29 . These are, inter alia, residues Y165, R211 and His278 and the propionic side chains B (prop B ) and C (prop C ) 17 , 25 , 27 , 28 , 30 . Specifically, prop C plays a remarkable role as it is protonated in Pfr but deprotonates with the decay of the Meta-F as a prerequisite for the structural conversion of the tongue.…”
The biological function of phytochromes is triggered by an ultrafast photoisomerization of the tetrapyrrole chromophore biliverdin between two rings denoted C and D. The mechanism by which this process induces extended structural changes of the protein is unclear. Here we report ultrafast proton-coupled photoisomerization upon excitation of the parent state (Pfr) of bacteriophytochrome Agp2. Transient deprotonation of the chromophore’s pyrrole ring D or ring C into a hydrogen-bonded water cluster, revealed by a broad continuum infrared band, is triggered by electronic excitation, coherent oscillations and the sudden electric-field change in the excited state. Subsequently, a dominant fraction of the excited population relaxes back to the Pfr state, while ~35% follows the forward reaction to the photoproduct. A combination of quantum mechanics/molecular mechanics calculations and ultrafast visible and infrared spectroscopies demonstrates how proton-coupled dynamics in the excited state of Pfr leads to a restructured hydrogen-bond environment of early Lumi-F, which is interpreted as a trigger for downstream protein structural changes.
“…Proton release is observed to take place at the step from Meta-F to Pr state, when the photoreceptor becomes activated (72). Recently, it has been demonstrated that the pscC deprotonation of BV chromophore is essential for triggering a secondary structure change (73). In the case of prototypical phytochromes, it has been spectroscopically observed that one of the inner pyrrole rings transiently loses a proton during the transition from Meta-R to Pfr states (67,74).…”
Section: Pka Calculations Of Phytochromes With Poisson-boltzmann Elecmentioning
This perspective article highlights the challenges in the theoretical description of photoreceptor proteins using multiscale modeling, as discussed at the CECAM workshop in Tel Aviv, Israel. The participants have identified grand challenges and discussed the development of new tools to address them. Recent progress in understanding representative proteins such as green fluorescent protein, photoactive yellow protein, phytochrome, and rhodopsin is presented, along with methodological developments.
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