**10. References**

Ananyev G., Nguyen T., Putnam-Evans C., & Dismukes, C. (2005) Mutagenesis of CP43 arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II. *Photochemical and Photobiological Sciences*, Vol. 4, pp. 991–998.

association between PSII and cytochrome c550 after purification of these particles, regardless of the media in which the cells were grown. Thus, even a conservative substitution (lysine) at this site, which results in a fairly functional mutant under normal growth conditions, prevents the strong association of cytochrome c550. It is possible that some cytochrome c550 associates with these mutants in vivo, but that the relatively harsh conditions used to isolate the particles disrupt this interaction. We are currently probing the kinetics of binding of

Site-directed mutagenesis has been employed extensively to probe the function of the CP43 protein of PSII. In particular, substitutions at Arg320 have revealed a potential role of this amino acid in the interaction of CP43 with the extrinsic PSII protein, cytochrome c550. In the present study, we have further defined the requirement at position 320 by constructing and characterizing additional mutant strains R320K and R320D. The results indicate that both the R320K and R320D strains are defective in their ability to utilize chloride to carry out efficient oxygen evolution and the effect is magnified under low chloride conditions. Spectrophotometrically detectable cytochrome c550 is lacking in these mutants. Additionally, we have preliminary evidence that these mutants also lack PsbU (Bricker and Putnam-Evans, unpublished). The phenotype of the R320S mutant previously described is intermediate between that of the K and D mutants. Thus, the order of effectiveness of amino acids at this site in supporting normal PSII function is R>K>S>D. A positively charged residue appears to be required at this site for proper function. The 1.9 Å crystal structure reveals a number of amino acid residues within both cytochrome c550 and PsbU that are potential hydrogen bonding partners to Arg320. Our operating hypothesis is that CP43, cytochrome c550 and PsbU interact via a hydrogen-bonding network that is involved in promoting a stable environment for chloride incorporation into the active site. While the position and ligands of chloride around the manganese cluster are now known, what other proteins bind chloride, how chloride is funneled to the active site, and where potential chloride channels occur remain unclear. Continuing studies to address these questions will no doubt expand our knowledge of the protein-protein and protein-ligand interactions in

cytochrome c550 to His-tagged PSII from these strains in order to examine this.

PSII that contribute to the proper functioning of the oxygen-evolving complex.

E, and a U.S. Department of Energy grant (DE-FG02-98ER20310) to TMB.

This work was supported by a U.S. National Science Foundation grant (MCB9982981) to CP-

Ananyev G., Nguyen T., Putnam-Evans C., & Dismukes, C. (2005) Mutagenesis of CP43-

arginine-357 to serine reveals new evidence for (bi)carbonate functioning in the water oxidizing complex of Photosystem II. *Photochemical and Photobiological* 

**8. Conclusion** 

**9. Acknowledgment** 

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**5** 

*Italy* 

**Semiconductors in Organic Photosynthesis** 

*Department of Chemistry, Materials and Chemical Engineering "Giulio Natta",* 

It is commonly accepted that "Organic Synthesis" was born in 1828, when the german chemist Friedrich Wöhler succeeded to make urea from simple materials (Friedrich, 1828). After only few years, Becquerel reported the first example of photo-induced electrochemical reaction (Becquerel 1839). In his work, he found that a voltage and an electric current were obtained by illuminating a silver chloride electrode immersed in an electrolyte solution and connected to a counter electrode. But the world had to wait until 1955, when Brattain and Garret, working on germanium semiconductor electrodes, well understood the origin of that strange photovoltaic phenomenon (Brattain & Garret, 1955). The modern "Photo-

However, the year was 1900 when it appeared the first article onto the use of light to promote an organic reaction (Albini & Fagnoni, 2008; Ciamician & Silber 1900). Ciamician and his colleague Silber carried out the first systematic studies on the behaviour of organic substances in the presence of sunlight, thus, nowadays they are regarded as the fathers of

Nowadays, if we open SciFinder Scholar client and we write the word "Synthesis" in the "Research topic" field, almost 10 millions of references will be shown. This means that in the last two centuries more than 50000 papers per year onto the argument have been published. Moreover, we are considering only an average value, whereas it is well known that the number of publications has known an exponential-type growth in the last decades. However, only 70000 references are reported for "Photocatalysis", indicating how relatively young is this research field, although the first paper was published more

The question is: why in the last two-three decades the use of light in chemistry has aroused a such worldwide attention? The answer could be summarized mainly in one word:

A great impetus to the development of photocatalysis is derived from the growing demand to reduce the environmental pollution (air and ground). This has led to the development of several photo-induced protocols for the oxidative degradation of organic pollutants, in which semiconductors play a key role in the reaction mechanism, focusing the attention

**1. Introduction** 

electrochemistry" era was born.

modern Organic Photochemistry.

onto the photodegradation of water and air pollutants.

than a century ago.

environment.

Cristian Gambarotti, Lucio Melone

and Carlo Punta

*Politecnico di Milano* 

Zouni, A., Witt, H., Kern, J., Fromme, P., Krauss, N., Saenger, W. & Orth., P. (2001) Crystal structure of photosystem II from *Synechococcus elongatus* at 3.8 Å resolution. *Nature*, Vol. 409, pp. 739-743.
