Direct Evidence of an Excited-State Triplet Species upon Photoactivation of the Chlorophyll Precursor Protochlorophyllide

Abstract

The chlorophyll precursor protochlorophyllide (Pchlide), which is the substrate for the light-driven enzyme protochlorophyllide oxidoreductase, has unique excited-state properties that facilitate photocatalysis. Previous time-resolved spectroscopy measurements have implied that a long-lived triplet state is formed during the excited-state relaxation of Pchlide, although direct evidence of its existence is still lacking. Here we use time-resolved electron paramagnetic resonance (EPR) in combination with time-resolved absorption measurements at a range of temperatures (10-290 K), solvents, and oxygen concentrations to provide a detailed characterization of the triplet state of Pchlide. The triplet decays in a biphasic, oxygen-dependent manner, while the first reported EPR signature of a Pchlide triplet displays both emissive and absorptive features and an antisymmetric spectrum similar to other porphyrin triplet states. This work demonstrates that the Pchlide triplet is accessible to various cryogenic spectroscopic probes over a range of time scales and paves the way for understanding its potential role in catalysis.

Emission Properties and Ultrafast Carrier Dynamics of CsPbCl3 Perovskite Nanocrystals

“…Steady-state absorption spectra were taken using a PerkinElmer lambda-1050 spectrometer. A previously reported 19 Ti:sapphire amplifier system (Spectra Physics Solstice Ace) was used to generate 800 nm pulses at 1 kHz for the TA experiments. A portion of this beam was used to pump an optical parametric amplifier (Topas Prime) with an associated NIR−UV−vis unit to achieve the 100 fs pump pulses at 350 nm with a beam diameter of 360 μm.…”

Ultrafast Trap State-Mediated Electron Transfer for Quantum Dot Redox Sensing

“…For the transient absorption (TA) experiments, a Ti:sapphire amplifier system (Spectra Physics Solstice Ace) was used to generate 800 nm pulses at 1 kHz, as previously reported. 22 A portion of this beam was used to pump an optical parametric amplifier (OPA) (Topas Prime) with an associated NIR−UV−vis unit to achieve the pump pulses at 420, 450, and 480 nm (∼10 nm full width at half-maximum, 100 fs, 250 nJ). The rest of the OPA output was passed through a sapphire crystal to generate a white light continuum, which was used as the probe to record changes in absorption between 435 and 720 nm.…”

Photocatalysis as the ‘master switch’ of photomorphogenesis in early plant development

“…[85][86][87][88][89][90][91] This process is highly dependent on solvent polarity 85,86,90 and is attributed to a number of polar groups attached to Pchlide, including an electron-withdrawing carbonyl group on ring E. 91 The excited state of Pchlide is quenched through solvation of this intramolecular charge-transfer species, [85][86][87][88][89][90][91] and a triplet state is then formed on the nanosecond timescale with a quantum yield of approximately 30 %. [89][90][91][92] Excited state charge separation across Pchlide has also been confirmed computationally, and these calculations suggest that this leads to site-specific solvation of photoexcited Pchlide through strengthening of H-bonding interactions. 91,93 Photochemical studies of the mechanism in the ternary enzyme-substrate complex are challenging 74,76 but it appears that the dipolar character of excited state Pchlide is also important in LPOR photocatalysis (Figure 4).…”

“…Timeresolved spectroscopy has been used to investigate these excited state properties, and studies have shown that Pchlide is highly reactive. [85][86][87][88][89][90][91][92] Excitation brings about charge separation across Pchlide. [85][86][87][88][89][90][91] This process is highly dependent on solvent polarity 85,86,90 and is attributed to a number of polar groups attached to Pchlide, including an electron-withdrawing carbonyl group on ring E. 91 The excited state of Pchlide is quenched through solvation of this intramolecular charge-transfer species, [85][86][87][88][89][90][91] and a triplet state is then formed on the nanosecond timescale with a quantum yield of approximately 30 %.…”