The hemoprotein myoglobin is a model system to study protein dynamics. We used time-resolved serial femtosecond crystallography at an x-ray free-electron laser to resolve the ultrafast structural changes taking place in the carbonmonoxy myoglobin complex upon photolysis of the Fe-CO bond. Structural changes appear throughout the protein within 500 fs with the C-, F- and H-helices moving away from the heme and the E- and A-helices moving toward it. These collective movements are predicted by quantum mechanics/molecular mechanics simulations. Together with the observed oscillations of residues contacting the heme, the calculations support predictions that an immediate collective response of the protein takes place upon ligand dissociation due to coupling of vibrational modes of the heme to global modes of the protein
Light absorption of myoglobin triggers diatomic ligand photolysis and a spin crossover transition of iron(II) that initiate protein conformational change. The photolysis and spin crossover reactions happen concurrently on a femtosecond timescale. The microscopic origin of these reactions remains controversial. Here, we apply quantum wavepacket dynamics to elucidate the ultrafast photochemical mechanism for a heme–carbon monoxide (heme–CO) complex. We observe coherent oscillations of the Fe–CO bond distance with a period of 42 fs and an amplitude of ∼1 Å. These nuclear motions induce pronounced geometric reorganization, which makes the CO dissociation irreversible. The reaction is initially dominated by symmetry breaking vibrations inducing an electron transfer from porphyrin to iron. Subsequently, the wavepacket relaxes to the triplet manifold in ∼75 fs and to the quintet manifold in ∼430 fs. Our results highlight the central role of nuclear vibrations at the origin of the ultrafast photodynamics of organometallic complexes.
Photolabile protecting groups (PPGs or photocages) are increasingly subject to molecular design to meet requirements such as absorbance in the visible spectral range, high molar absorption coefficients, and high quantum yields of leaving group release. Improvements in these properties for the promising 3-diethylaminobenzyl (DEAMb) photocage, the photoactivity of which is based on the Zimmerman meta effect, are reported. Expansion of the aromatic system with a second aromatic ring resulted in improved spectral properties. A systematic trend relating the electronic (π-donor or acceptor) properties of the new aryl substituent and its position in the DEAMb ring to changes in the spectral properties could be observed. Conclusions from the experimental results were supported by computations obtained by using time-dependent DFT. A second generation of DEAMb-based photocages was designed. A rigid linker was introduced to ensure more efficient conjugation of the aromatic ring π systems by limiting rotational freedom. The resulting fluorenol (9-hydroxyfluorene)-based photocages had superior spectral properties to those of simple biphenyl systems. The best uncaging cross section achieved was 5320 m cm (ϵΦ ).
Combined spectroscopic and computational studies elucidate excited-state photocleavage in DEACM cages, explaining vastly different time scales for different leaving groups.
Here we report the design of a new coumarin-based photolabile protecting group with enhanced two-photon absorption. Two-photon excited fluorescence (TPEF), color-tuned ultrafast transient absorption spectroscopy and infrared (IR) measurements are employed to photochemically characterize the newly designed ATTO 390-DEACM-cargo triad. Increased two-photon cross-section values of the novel cage in comparison to the widely used protecting group DEACM ([7-(diethylamino)coumarin-4-yl]methyl) are extracted from TPEF experiments. Femtosecond pump-probe experiments reveal a fast intramolecular charge transfer, a finding that is confirmed by quantum chemical calculations. Uncaging of glutamate is monitored in IR measurements by photodecarboxylation of the carbamate linker between the photolabile protecting group and the glutamate, showing the full functionality of the novel two-photon activatable photocage.
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