Femtosecond time‐resolved spectroscopy on biological photoreceptor chromophores

Abstract

This paper reviews our results on femtosecond timeresolved spectroscopy (transient absorption, transient-grating and fluorescence spectroscopy) to study the photophysics and photochemistry of the two very important biological photoreceptor chromophores phycocyanobilin (PCB) and protochlorophyllide a (PChla). The compound PCB serves as a model chromophore for the photoreceptor phytochrome. By means of transient-grating spectroscopy where the excitation wavelength was varied over the spectral region of the S0 → S1-absorption the ultrafast processes were studied upon excitation with varying excess energy delivered to the system. On the basis of the results obtained, both the rate of the photoreaction in PCB and the rate of the decay of different excited-state species via different decay channels depend on the excitation wavelength. Furthermore, transient absorption experiments illuminating the excited-state dynamics of PChla, a porphyrin-like compound and, as substrate of the NADPH/protochlorophyllide oxidoreductase (POR), a precursor of the chlorophyll biosynthesis are presented. In addition to pump-energy-dependent measurements performed with PChla dissolved in methanol, the excited-state dynamics of PChla was interrogated in different solvents that were chosen to mimic different environmental conditions. In addition to the femtosecond time-resolved absorption experiments the picosecond time-resolved fluorescence of the system was studied. The transient absorption and time-resolved fluorescence data allow suggesting a detailed model for the excited-state relaxation of PChla describing the excited-state processes in terms of The model suggested to account for the excited-state relaxation processes in protochlorophyllide a upon photoexcitation is presented a branching of the initially excited state population into a reactive and nonreactive path. Thus, the excited-state potential energy surface exhibits two distinct S1 and Sx minima separated from the Franck-Condon region along two most likely orthogonal reaction coordinates. Finally, the model derived is related to models suggested to account for the reduction of PChla to chlorophyllide a within the natural enzymatic environment of POR.

Protochlorophyllide a: A Comprehensive Photophysical Picture

“…Moreover, an intermediate rise of the excited-state absorption between 670 and 730 nm is clearly seen (Figure 5 A), which, as mentioned before, confirms the tens-of-ps time constant for the formation of the S ICT state (Scheme 1). [14,24] With the identification of the ns component, the experiments presented here corroborate the 3.5 ns lifetime for the thermally equilibrated S 1 excited state. In addition, due to an improved signal-to-noise ratio and data acquisition over a longer delay time, they enable to evaluate the component spectrum associated with the fluorescent S 1 excited state (Figures 5 C,D).…”

Dynamics of charge separation in the excited-state chemistry of protochlorophyllide

“…In n-hexane only the decay of the initial anisotropy signal can be observed, whereas in methanol the anisotropy rises up to $20 ps and then declines. This remarkable difference in the anisotropy kinetics confirms the strong effect of the solvent polarity on the excited-state dynamics of PChlide as was seen recently [16,18].…”

“…1) [15][16][17][18]27]. According to this model a main branching into a reactive and nonreactive pathway occurs in the excited-state of PChlide.…”

“…In particular, the question of how specific molecular motions within the enzyme's active site may influence the catalytic reaction can be addressed. Following this strategy we have extensively studied the excited-state processes of PChlide in a number of solvents with different polarity, viscosity, and protic properties [15][16][17][18]. On the basis of these results the initially excited Franck-Condon state population undergoes decay into two different relaxation channels, a reactive and a nonreactive one (Fig.…”

“…From the studies undertaken till now, it appears that the photoproduct represents an intramolecular charge-transfer state (S ICT ) with the charge-transfer character being most probably due to the electron withdrawing effect of the carbonyl group within the cyclopentanone ring which is directly attached to the conjugated p-electron path of the porphyrin macrocycle in PChlide ( Fig. 1) [15][16][17][18][19]. It is conceivable that the charge separation within 0009-2614/$ -see front matter Ó 2010 Elsevier B.V. All rights reserved.…”