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Modeling light-driven proton pumps in artificial photosynthetic reaction centers
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View: Figures


Image of FIG. 1.
FIG. 1.

The top figure presents the triad (donor , photosensitive part , and acceptor ) and the shuttle (Refs. 9 and 10). These are enclosed by color circles, which are schematically shown in the bottom figure. The tetra-arylporphyrin group acts as a photosensitive moiety (inside the green circle in the top structure). This is connected to both a naphthoquinone moiety fused to a norbornene system with a carboxylic acid group [which acts as an electron acceptor ] and to a carotenoid polyene [which acts as an electron donor ]. 2,5-diphenylbenzoquinone is the proton shuttle denoted by a pink hollow circle in the structure and by a solid pink circle in the cartoon.

Image of FIG. 2.
FIG. 2.

Schematic diagram of the light-induced proton pump across the lipid bilayer in a liposomic membrane. A molecular triad is symmetrically inserted in the lipid bilayer. The different stages in the proton pumping process are here denoted by (a), (b), (c), (d), (e), and (f). The two bluish vertical rectangles on both sides schematically represent two proton reservoirs with electrochemical potentials and . These two proton reservoirs correspond to the aqueous phases inside and outside of the liposome, respectively. The shuttle molecule is shown as a pink-colored oval and the protonated neutral shuttle is shown as a yellow oval. This shuttle freely diffuses in (d) (the black scribbled curves represent the thermal stochastic motion of the shuttle) across the membrane to transport a proton from the lower proton potential to the higher proton potential side of the membrane, where denotes the total potential difference between the two reservoirs.

Image of FIG. 3.
FIG. 3.

Energy diagram depicting the energy levels of states involved in an artificial photosynthetic reaction center before the diffusion of the shuttle to the -reservoir. (a)–(c) correspond to the stages (a), (b), and (c) in Fig. 2. The left and right panels represent electron and proton energy levels, respectively. The abbreviations , , , , and are the same as used in the text and in Fig. 1. Also, and represent the spatial coordinates of sites and , respectively. The thick brown arrows denote the path the electrons follow in this energy diagram, generating charge separation in (b) and shuttle charging and protonation in (c). Initially, light excites an electron from to and eventually to , making it . Afterward, in (b), the donor loses an electron, thus becoming , and that electron moves to . Later on, the shuttle in (c) receives the electron from .

Image of FIG. 4.
FIG. 4.

Energy levels involved in an artificial photosynthetic reaction center. This figure is similar to Fig. 3, but now the energy profile corresponds to the stage after the shuttle diffuses to the -reservoir. Here (d)–(f) correspond to the stages (d), (e), and (f) in Fig. 2. The left and right panels represent proton and electron energy levels, respectively. The thick brown arrows denote the path followed by the (e) electron and (f) proton. In (d), an electron on the shuttle moves to the donor site , neutralizing it in (e). This electron transition in the right panels increases the proton energy of the shuttle, as shown in the left panels [(d) and (e)]. The proton finally leaves the shuttle in the left panel of (f).

Image of FIG. 5.
FIG. 5.

(a) Stochastic motion of the shuttle with time. The horizontal black dashed lines denote the borders of the membrane , . Via this diffusion the shuttle transports protons and electrons through the membrane. (b) Variation in the electron and proton population on the shuttle. Note that the proton density (red curve) and the electron density (black curve) mostly coincide in (b). (c) Number of protons pumped vs time. The main parameters used here are the light intensity , temperature , and the chemical potentials and . The light intensity corresponds to the photosensitive -group with a dipole moment , where is the electron charge.

Image of FIG. 6.
FIG. 6.

Contour plots presenting the variations in the quantum efficiency with the reorganization energy and with the energy gap , where . The parameters used here are light intensity , temperature , and chemical potentials and . The detunings take the following values: (a) , (b) , and (c) .

Image of FIG. 7.
FIG. 7.

Proton pumping quantum efficiency vs resonant tunneling rate at different reorganization energies shown in (a) and (b) and for different detunings shown in (c) and (d). Note that here represents since we set . We use the following parameters: , , , , and the energy gap . Panels (a) and (b) are plotted at fixed , whereas in (c) and (d) the reorganization energy is fixed with .

Image of FIG. 8.
FIG. 8.

(a) Coulomb energies and vs the dielectric constant of the medium. (b) Proton pumping efficiency vs dielectric constant for different values of and for . (c) The pumping efficiency as a function of the dielectric constant for different reorganization energies and at the fixed detuning . The other parameters are the same as in Fig. 7: , , , , and .

Image of FIG. 9.
FIG. 9.

(a) Proton current vs light intensity for different temperatures at and . Notice that the proton current is roughly linear for small intensities of light but it saturates with higher light intensity. In this saturation region, the proton current is larger with higher temperatures. (c) The standard deviation of the number of pumped protons as a function of the light intensity for different temperatures. (b) The pumping quantum efficiency decreases with light intensity for all temperatures shown.

Image of FIG. 10.
FIG. 10.

(a) Proton current vs temperature for different values of the light intensity . (b) Pumping efficiency vs temperature. Here, the electrochemical gradient ( and ).

Image of FIG. 11.
FIG. 11.

Proton pumping current vs electrochemical potential of the positive side (-reservoir) of the membrane for different values of the potential of the negative side (-reservoir) for the light intensity and temperature .


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Modeling light-driven proton pumps in artificial photosynthetic reaction centers