Redox potential light absorption

The naturally occurring Zn-bchl a and Mg-bchl a show large structural similarities and have very similar physicochemical characteristics. Likewise, Zn-chl a exhibits features similar to Mg-chl a with regard to redox potential and absorption maxima in organic solvents. The light-harvesting efficiency of Zn-chl a and Mg-chl a are very similar allhough... 

Most mechanisms which control biological functions, such as cell respiration and photosynthesis (already discussed in Chapter 5, Section 3.1), are based on redox processes. In particular, as shown again in Figure 1, it is evident that, based on their physiological redox potentials, in photosynthesis a chain of electron carriers (e.g. iron-sulfur proteins, cytochromes and blue copper proteins) provides a means of electron transport which is triggered by the absorption of light. 

Any endoergonic photochemical reaction converts light energy into chemical energy. If the photoproducts are kinetically stable, they have to be considered fuels as they can be stored, transported and then converted to other chemical species with evolution of energy11 ). As we have seen in Section 5, electronic excitation increases the oxidation and reduction potentials of a molecule. Light absorption can thus drive a redox reaction in the non-spontaneous direction (Fig. 16). 

The difference between the redox potentials of reductive and oxidative surface centers cannot be higher than the absorbed light energy divided by the elemental charge. In the case of irradiation near to the absorption onset (620 650 nm) this difference is ca. 2 V. 2 pt (ci/ci > can be estimated as described above at ca. —0.4 V + 2.0 V = 1.6 V (Scheme 2). 

In Chapter 5 (Section 5.5B), we introduced the various molecules involved with electron transfer in chloroplasts, together with a consideration of the sequence of electron flow between components (Table 5-3). Now that the concept of redox potential has been presented, we will resume our discussion of electron transfer in chloroplasts. We will compare the midpoint redox potentials of the various redox couples not only to help understand the direction of spontaneous electron flow but also to see the important role of light absorption in changing the redox properties of trap chi. Also, we will consider how ATP formation is coupled to electron flow. 

Figure 6-4. Energy aspects of photosynthetic electron flow. The lengths of the arrows emanating from the trap chi s of Photosystems I and II represent the increases in chemical potential of the electrons that occur upon absorption of red light near the Xmax s of the trap Chip s. The diagram shows the various midpoint redox potentials of the couples involved (data from Table 5-3) and the three types of election flow mediated by ferredoxin. Spontaneous electron flow occurs toward couples with higher (more positive) redox potentials, which is downward in the figure.

It was found that surface derivatives modify the redox properties of Ti02 particles if a surface modifier is an electron-donating species, and that the crucial parameter for effective removal of heavy metal ions is the trade-off between enhanced redox properties of Ti02 by surface modification and the enhanced redox potential of chelated metal ions. Surface modification can lead to the appearance of a charge transfer complex with small-particle Ti02 colloids that have an optical absorption threshold at 730 nm. The red shift of the optical absorption provides improved optical properties for use of visible light, i.e., for solar energy conversion [63]. 

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