Plants, algae and some kinds of photosynthetic bacteria, widespread on both land and water, play a crucial role in the biosphere, as they use sunlight to generate biomass. An estimated energy value of about 4.0×1021 J is converted in one year by these organisms into valuable biomolecules and molecular oxygen. All life forms on Earth, as well as modern energy sources, strictly depend on photosynthesis and today we have a quite deep knowledge of this natural process at a molecular level. Photosynthesis inspires many research efforts aiming to develop artificial machineries for efficient energy conversion1. In bacterial photosynthesis, the pivotal enzyme is the photosynthetic reaction center (RC), possibly the transmembrane protein most studied at structural and functional level and used as model for the construction of biohybrid devices. In this contribution, I will focus on the thermodynamic aspects of the photoinduced electron transfer cascade, how it works in vivo and how it can be exploited in photoelectrochemical cells. As sketched in the figure, the energy of a photon is used by the cofactors embedded within the RC to promote a redox reaction between a specialized pair of bacteriochlorophylls (the primary donor D, with Em = 450 mV) and the quinone acceptor QB (with Em = +100 mV). This non-spontaneous process is made possible by the energy hc/? gained by D at 860 nm which generates a new redox couple D*/D+ with a very reducing Em = -980 mV. It will be shown that under normal daylight illumination D* is several order of magnitudes less than D, so that the actual yield of conversion of photonic energy in excitation energy is 59%. Moreover, the largest part of the excitation energy gained by D* is wasted during the charge transfer chain along the intermediate acceptors. However, the final charge separated state D+QB- is higher in energy that the initial ground state, and, remarkably, it is obtained with a unitary quantum efficiency and has a lifetime in the order of seconds. Such extremely efficient photoconverter can be isolated from its host organism and it is stable enough to be used for building photoelectrochemical cells that are presently at the proof-of-concept stage. In fact, in the presence of exogenous electron donors (the physiological cytochrome c2 or artificial ferrocenes) and a quinone pool, the light triggers a photocycle having as final products the reduced quinol and the oxidized exogenous donor. In a classical three-electrode cell, if the potential of the working electrode is set to the appropriate value, it will be able to give electrons to the photo-oxidized exogenous donor and the relevant signal is detected as photocurrent. The efficiency of this process depends on the nature and concentration of the involved species, the material of the working electrode and the interface between the RC and the electrode. All these aspects will be presented and discussed 1.Operamolla, A., et al., (2015). "Garnishing" the photosynthetic bacterial reaction center for bioelectronics. J. Mater. Chem. C. http://doi.org/10.1039/C5TC00775E
The photoelectrochemical domain of bacterial photosynthesis
F Milano
2016
Abstract
Plants, algae and some kinds of photosynthetic bacteria, widespread on both land and water, play a crucial role in the biosphere, as they use sunlight to generate biomass. An estimated energy value of about 4.0×1021 J is converted in one year by these organisms into valuable biomolecules and molecular oxygen. All life forms on Earth, as well as modern energy sources, strictly depend on photosynthesis and today we have a quite deep knowledge of this natural process at a molecular level. Photosynthesis inspires many research efforts aiming to develop artificial machineries for efficient energy conversion1. In bacterial photosynthesis, the pivotal enzyme is the photosynthetic reaction center (RC), possibly the transmembrane protein most studied at structural and functional level and used as model for the construction of biohybrid devices. In this contribution, I will focus on the thermodynamic aspects of the photoinduced electron transfer cascade, how it works in vivo and how it can be exploited in photoelectrochemical cells. As sketched in the figure, the energy of a photon is used by the cofactors embedded within the RC to promote a redox reaction between a specialized pair of bacteriochlorophylls (the primary donor D, with Em = 450 mV) and the quinone acceptor QB (with Em = +100 mV). This non-spontaneous process is made possible by the energy hc/? gained by D at 860 nm which generates a new redox couple D*/D+ with a very reducing Em = -980 mV. It will be shown that under normal daylight illumination D* is several order of magnitudes less than D, so that the actual yield of conversion of photonic energy in excitation energy is 59%. Moreover, the largest part of the excitation energy gained by D* is wasted during the charge transfer chain along the intermediate acceptors. However, the final charge separated state D+QB- is higher in energy that the initial ground state, and, remarkably, it is obtained with a unitary quantum efficiency and has a lifetime in the order of seconds. Such extremely efficient photoconverter can be isolated from its host organism and it is stable enough to be used for building photoelectrochemical cells that are presently at the proof-of-concept stage. In fact, in the presence of exogenous electron donors (the physiological cytochrome c2 or artificial ferrocenes) and a quinone pool, the light triggers a photocycle having as final products the reduced quinol and the oxidized exogenous donor. In a classical three-electrode cell, if the potential of the working electrode is set to the appropriate value, it will be able to give electrons to the photo-oxidized exogenous donor and the relevant signal is detected as photocurrent. The efficiency of this process depends on the nature and concentration of the involved species, the material of the working electrode and the interface between the RC and the electrode. All these aspects will be presented and discussed 1.Operamolla, A., et al., (2015). "Garnishing" the photosynthetic bacterial reaction center for bioelectronics. J. Mater. Chem. C. http://doi.org/10.1039/C5TC00775EI documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
