Comparison of the steady-state FTIR absorption spectra of coumarin-102 (C-102) in tetrachloroethylene with added aniline of various concentrations, in neat aniline and in neat N,N-dimethylaniline (DMA), indicates formation of a hydrogen-bonded complex between C-102 and aniline in solution. Subpicosecond time-resolved infrared absorption spectroscopy has been applied to study the dynamics of the hydrogen-bond following photoexcitation of C-102 chromophore in a C-102-aniline hydrogen-bonded complex. Upon photoexcitation at 400 nm, the hydrogen bond between C-102 and aniline breaks within 250 fs. Reformation of hydrogenbond between the excited C-102 molecule and aniline takes place within about 30 ps. Biexponential temporal dynamics monitored at CdO stretching vibration (1736-1742 cm -1 ) in neat aniline, which is a strongly structured solvent due to formation of intermolecular hydrogen bonds, reveals the biphasic solvation dynamics of aniline with solvation times 0.6 and 7.2 ps. These time constants have been assigned to nondiffusive and diffusive structural reorganization of the solvent. † Part of the special issue "Charles S. Parmenter Festschrift".

Transient absorption spectroscopy has been used to study sub-picosecond energy transfer processes in isolated photosystem II (PS II) reaction centers. As reported previously [Durrant, J. R.; et al. Proc. Natl. Acad. Sci. U.S.A. 1992 , 89, 11632−11636], using long wavelength (694 nm) excitation, spectral evolution of the isotropic Q y band bleach/stimulated emission is dominated by energy transfer processes with a 100 ± 50 fs time constant. In contrast, depolarization of this signal occurs with a time constant of 400 ± 30 fs, from an initial anisotropy of ∼0.4 to a value of ∼0.15 at 1.5 ps. This decay of the anisotropy is attributed to energy transfer between at least two degenerate states contributing to reaction center absorption circa 680 nm, with these states having approximately orthogonal transition dipoles. The transient anisotropy barely changes between 1.5 and 60 ps, indicating that under these excitation conditions equilibration of the excitation energy between reaction center excited states occurs on a sub-picosecond time scale. Transient data collected for pheophytin Q x absorption bands indicate that pheophytin molecules are included in the 100 fs equilibration process. These results are discussed in the context of the PS II multimer model and are shown to be in good agreement with this model.

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CooA from Rhodospirillum rubrum is a transcriptional activator in which a heme prosthetic group acts as a CO sensor and regulates the activity of the protein.In this study, the electronic relaxation of the heme, and the concurrent recombination between ligands and the heme at ϳ280 K were examined in an effort to understand the environment around the heme and the dynamics of the ligands. Upon photoexcitation of the reduced CooA at 400 nm, electronic relaxation of the heme occurred with time constants of 0.8 and 1.7 ps. The ligand rebinding was substantially completed with a time constant of 6.5 ps, followed by a slow relaxation process with a time constant of 173 ps. In the case of CO-bound CooA, relaxation of the excited heme occurred with two time constants, 1.1 and 2.4 ps, which were largely similar to those with reduced CooA. The subsequent CO recombination process was remarkably fast compared with that of other CO-bound heme proteins. It was well described as a biphasic geminate recombination process with time constants of 78 ps (60%) and 386 ps (30%). About 10% of the excited heme remained unligated at 1.9 ns. The dynamics of rebinding of CO thus will help us to understand how the physiologically relevant diatomic molecule approaches the heme binding site in CooA with picosecond resolution.CooA, a transcriptional activator from Rhodospirillum rubrum, is a heme protein that acts as a CO sensor in vivo by binding . CooA is the first example of a transcriptional regulator containing heme as a prosthetic group (1). Only CObound CooA activates transcription of the genes for the key CO-oxidizing enzymes (2-6). Although the ferrous heme in CooA is in a six-coordinate form (3, 4), one of the heme axial ligands is replaced by exogenous CO upon the binding of CO (3,5,7), which triggers the conformational change in CooA required for specific binding to the target DNA (2,3,5,6,8).Though CO has been widely used as a probe to study the biochemical and biophysical properties of heme proteins, it has been thought to have no physiological role. CooA is the first example of a heme protein in which CO has a physiological function. Analysis of the dynamics of binding and escape of the ligand in heme proteins provides information on the intrinsic reactivity of the site for heme iron biding with the ligand, and how the reactivity and the pathway of the ligand are controlled by the protein. Observation of the motion of ligands such as O 2 , NO, and CO within heme proteins is facilitated by the simple photodissociation of diatom-heme protein complexes (9 -11). The dynamics of geminate rebinding, escaping, and bimolecular rebinding of ligands can be studied by various spectroscopic methods over a wide time range.Flash photolysis studies on CO-bound CooA will provide some useful information on the mechanisms of CO sensing by the heme and information on the regulation of CooA activity by CO. Measurement of transient absorption in the Soret band region on a tens of nanosecond or longer time scale has recently been carried out in studi...

The rate of the electron transfer reaction from the reduced primary electron acceptor chlorophyll a (A0 -) to the secondary acceptor quinone (Q) was measured by picosecond−nanosecond laser spectroscopy at 280 K in the photosynthetic reaction center (RC) complex of plant photosystem I (PS I). The free energy change (ΔG 0) of the reaction was varied between −1.1 and +0.2 eV by the reconstitution of 13 different quinone/quinonoid compounds after the extraction of the intrinsic phylloquinone. Phylloquinone and its natural analog menaquinone, both of which show a ΔG 0 value of −0.34 eV, gave the highest rate constant (k) of (23 ps)-1. Analysis of log k versus ΔG 0 plot according to the quantum mechanical electron transfer theory gave the total reorganization energy (λtotal) of 0.30 eV and the electronic coupling (V) of 14 cm-1. The natural system is shown to be highly optimized to give a ΔG 0= −λtotal condition. The λtotal value is smaller and the V value is larger than those estimated in the corresponding reaction between the reduced primary acceptor bacteriopheophytin (H-) and the secondary acceptor ubiquinone (QA) in the purple bacterial RC complex. It is concluded that the A0 -Q → A0Q- reaction in the PS I RC occurs in protein environments, which give a low dielectric property, with a shorter electron transfer distance compared to the reaction between H and QA.

Deuterium isotope effect on the solvation dynamics is observed in the system of aniline (AN) as a solvent for the first time by the dynamic Stokes shift method. Perdeuterated AN (AN–d7) or amino deuterated AN (AN–d2) shows slower solvation dynamics than normal AN. Deuterium effect on the solvation of N,N–dimethylaniline (DMA) is also studied and there is no isotope effect on the solvation process. The differences between AN and DMA are proposed to be related to the presence and absence of the intermolecular hydrogen bondings.