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Pump–Dump–Probe and Pump–Repump–Probe Ultrafast Spectroscopy Resolves Cross Section of an Early Ground State Intermediate and Stimulated Emission in the Photoreactions of the Pr Ground State of the Cyanobacterial Phytochrome Cph1

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Title: Pump–Dump–Probe and Pump–Repump–Probe Ultrafast Spectroscopy Resolves Cross Section of an Early Ground State Intermediate and Stimulated Emission in the Photoreactions of the Pr Ground State of the Cyanobacterial Phytochrome Cph1
Authors: Fitzpatrick, Ann E.1, Lincoln, Craig N.1, van Wilderen, Luuk J. G. W.1, van Thor, Jasper J.1
Source: Journal of Physical Chemistry B. Jan2012, Vol. 116 Issue 3, p1077-1088. 12p.
Subjects: Intermediates (Chemistry), Cross-sectional method, Ground state (Quantum mechanics), Chemical reactions, Cyanobacteria, Phytochromes
Abstract: The primary photoreactions of the red absorbing ground state (Pr) of the cyanobacterial phytochrome Cph1 from SynechocystisPCC 6803 involve C15C16 Z–Ephotoisomerization of its phycocyanobilin chromophore. The first observable product intermediate in pump–probe measurements of the photocycle, “Lumi-R”, is formed with picosecond kinetics and involves excited state decay reactions that have 3 and 14 ps time constants. Here, we have studied the photochemical formation of the Lumi-R intermediate using multipulse picosecond visible spectroscopy. Pump–dump–probe (PDP) and pump–repump–probe (PRP) experiments were carried out by employing two femtosecond visible pulses with 1, 14, and 160 ps delays, together with a broadband dispersive visible probe. The time delays between the two excitation pulses have been selected to allow interaction with the dominant (3 and 14 ps) kinetic phases of Lumi-R formation. The frequency dependence of the PDP and PRP amplitudes was investigated at 620, 640, 660, and 680 nm, covering excited state absorption (λmax= 620 nm), ground state absorption (λmax= 660 nm), and stimulated emission (λmax= 680 nm) cross sections. Experimental double difference transient absorbance signals (ΔΔOD), from the PDP and PRP measurements, required corrections to remove contributions from ground state repumping. The sensitivity of the resulting ΔΔOD signals was systematically investigated for possible connectivity schemes and photochemical parameters. When applying a homogeneous (sequentially decaying) connectivity scheme in both the 3 and 14 ps kinetic phases, evidence for repumping of an intermediate that has an electronic ground state configuration (GSI) is taken from the dump-induced S1 formation with 620, 640, and 660 nm wavelengths and 1 and 14 ps repump delays. Evidence for repumping a GSI is also seen, for the same excitation wavelengths, when imposing a target connectivity scheme proposed in the literature for the 1 ps repump delay. In contrast, using a 680 nm dump pulse, ground state formation is observed for all models examined. The ΔΔOD signals were dominated by stimulated emission, at both 1 and 14 ps delays for the longer wavelength excitation. The GSI, which is revealed by the PRP measurements and not resolved from pump–probe measurements, is found to be directly formed from the excited state of Pr, and its formation is considered using heterogeneous, homogeneous, and target models to globally fit the data. [ABSTRACT FROM AUTHOR]
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Abstract:The primary photoreactions of the red absorbing ground state (Pr) of the cyanobacterial phytochrome Cph1 from SynechocystisPCC 6803 involve C15C16 Z–Ephotoisomerization of its phycocyanobilin chromophore. The first observable product intermediate in pump–probe measurements of the photocycle, “Lumi-R”, is formed with picosecond kinetics and involves excited state decay reactions that have 3 and 14 ps time constants. Here, we have studied the photochemical formation of the Lumi-R intermediate using multipulse picosecond visible spectroscopy. Pump–dump–probe (PDP) and pump–repump–probe (PRP) experiments were carried out by employing two femtosecond visible pulses with 1, 14, and 160 ps delays, together with a broadband dispersive visible probe. The time delays between the two excitation pulses have been selected to allow interaction with the dominant (3 and 14 ps) kinetic phases of Lumi-R formation. The frequency dependence of the PDP and PRP amplitudes was investigated at 620, 640, 660, and 680 nm, covering excited state absorption (λmax= 620 nm), ground state absorption (λmax= 660 nm), and stimulated emission (λmax= 680 nm) cross sections. Experimental double difference transient absorbance signals (ΔΔOD), from the PDP and PRP measurements, required corrections to remove contributions from ground state repumping. The sensitivity of the resulting ΔΔOD signals was systematically investigated for possible connectivity schemes and photochemical parameters. When applying a homogeneous (sequentially decaying) connectivity scheme in both the 3 and 14 ps kinetic phases, evidence for repumping of an intermediate that has an electronic ground state configuration (GSI) is taken from the dump-induced S1 formation with 620, 640, and 660 nm wavelengths and 1 and 14 ps repump delays. Evidence for repumping a GSI is also seen, for the same excitation wavelengths, when imposing a target connectivity scheme proposed in the literature for the 1 ps repump delay. In contrast, using a 680 nm dump pulse, ground state formation is observed for all models examined. The ΔΔOD signals were dominated by stimulated emission, at both 1 and 14 ps delays for the longer wavelength excitation. The GSI, which is revealed by the PRP measurements and not resolved from pump–probe measurements, is found to be directly formed from the excited state of Pr, and its formation is considered using heterogeneous, homogeneous, and target models to globally fit the data. [ABSTRACT FROM AUTHOR]
ISSN:15206106
DOI:10.1021/jp206298n