Photobiological Studies

It should be evident from this discussion that the dioxetane 25 represents an example par excellence for photobiological studies in the dark (photobiology without light ). It generates efficiently electronically excited ketones, which may be directly viewed by transient spectroscopy and which are convenient sources of active radical species through their facile a cleavage. 

Thus, considerable care has to be taken not to misinterpret photobiological studies using this OH-source. A further caveat has been expressed by Douki et al. (1999) who observed that product ratios are considerably different when ionizing radiation and N-hydroxypyridine-2-thione were used as -OH sources, and this has been explained by the H-donating property of the N-hydroxypyridine-2-thione (note the major dimers mentioned above). 

Fossa, P. Mosti, L. Menozzi, G. Marzano, C. Baccichetti, F. Bordin, F. Novel angular furo and thienoquinolinones synthesis and preliminary photobiological studies. Bioorg. Med. Chem. 2002, 10, 743-751. 

Oliveira HS, Goncalo M, Figueiredo AC. Photosensitivity to lomefloxacin. A clinical and photobiological study. Photodermatol Photoimmunol Photomed 2000 16(3) 116-20. 

Encinas, S., Bosca, E, and Miranda, M. A., Phototoxicity associated with diclofenac a photophysical, photochemical and photobiological study on the drug and its photoproducts, Chem. Res. Toxicol, 11, 946,1998. 

In order to computationally study these processes it is crucial to be able to model the radiationless nonadiabatic transitions. It is growth in this area that has catalyzed computational photochemistry and photobiology. The basics of the theory describing radiationless transitions are outlined below. 

Sakura, S. and Fujimoto, D. (1984) Absorbtion and fluorescence studies of tyrosine derived crosslinking amino acids from collagen. Photochemistry and Photobiology 40, 731-734. 

At a very optimistic conversion efficiency of 10 % and a price for H2 of 15 per GJ only 10 worth of H2/m2/year would be obtained thus severely constraining construction and operational expenditures. Obviously, the situation worsens if conversion efficiencies are lower. In spite of the critical importance of this consideration, relatively few studies on photobiological hydrogen production report conversion efficiencies, and when these are reported they typically fall well below 1 % (i.e. < 1 HVmVyear). 

Zachariasse K. A., Kozankiewicz B. and Kuhnle W. (1983) Micelles and Biological Membranes Studied by Probe Molecules, in Zewail A. H. (Ed.), Photochemistry and Photobiology, Vol. II, Harwood, London, pp. 941-960. 

Kusic, H, Koprivanac, N Srsan, L. Azo dye degradation using Fenton type processes assisted by UV irradiation A kinetic study. Journal of Photochemistry and Photobiology A Chemistry, 2006 181 (2-3), 195-202. 

L. and Miranda, M.A. (2000) Tiaprofenic acid-photosensitized damage to nucleic acids a mechanistic study using complementary in vitro approaches. Photochemistry and Photobiology, 71, 499-505. 

Rollag MD, Provencio I, Sugden D, Green CB 2000 Cultured amphibian melanophores a model system to study melanopsin photobiology. Methods Enzymol 316 291—309 Sancar A 2000 Cryptochrome the second photoactive pigment in the eye and its role in circadian photoreception. Annu Rev Biochem 69 31—67... 

Finally, a promising trend in the study of the photoelectrochemical behavior of objects, whose nature is close to that of semiconductors, is related to photobiology, in particular to processes of light conversion in natural photosynthesizing objects. Certain elementary stages of photosynthesis, particularly photoelectrochemical ones, can, apparently, be simulated in some cases within the framework of the concepts of photoelectrochemistry of semiconductors. 

Most interesting applications of intramolecular energy transfer between nonconjugated chromophores are found in the conformational studies of biomolecules like nucleic acids and proteins. The experiments on rotational depolarization of emission from intrinsic fluorescent groups on externally attached fluorescent probes, have resulted in a vast store of knowledge which has helped to enrich the subject of photobiology. 

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