Photocleavage

Initiation of radical reactions with uv radiation is widely used in industrial processes (85). In contrast to high energy radiation processes where the energy of the radiation alone is sufficient to initiate reactions, initiation by uv irradiation usually requires the presence of a photoinitiator, ie, a chemical compound or compounds that generate initiating radicals when subjected to uv radiation. There are two types of photoinitiator systems those that produce initiator radicals by intermolecular hydrogen abstraction and those that produce initiator radicals by photocleavage (86—91). 

Some commercial photoinitiatois (Table 11) undergo a Noirish Type I photocleavage to form two initiating radical fragments direcdy for a benzoin... 

Photocleavage of nucleic acids with participation of metal complexes with heterocyclic ligands 98CRV1171. 

Two more recent studies of the nature of the sulfur-carbon bond cleavage in sulfoxide photolyses have been conducted, using ESR5 and CIDNP6 detection methods, respectively. In the latter case there is some evidence for a triplet process being involved in the photocleavage of aryl methyl sulfoxides. 

The fragmentation of Ca -ATPase by proteolytic enzymes [42,85,235,236] and by vanadate-catalyzed photocleavage [104,105] occurs at well defined and conforma-tionally sensitive cleavage sites that delineate functional domains within the Ca -ATPase. The functional changes that follow the cleavage of the polypeptide chain provide useful hints about the role of various domains in the mechanism of Ca transport. 

Vanadate-catalyzed photocleavage of the Ca -ATPase was observed after illumination of sarcoplasmic reticulum vesicles or the purified Ca -ATPase with ultraviolet light in the presence of 1 mM monovanadate or decavanadate [104]. Two sites... 

The photocleavage is accompanied by loss of ATPase activity and inhibition of the crystallization of Ca -ATPase both processes are accelerated by Ca [104,105]. The faster loss of ATPase activity during illumination in the presence of calcium is consistent with simultaneous binding of vanadate and calcium to the Ca -ATPase [259-261]. 

The target of vanadate-catalyzed photolysis is presumably an amino acid near the catalytic site of the Ca " -ATPase. The vanadate-catalyzed photocleavage at the V cleavage site in the absence of Ca " is 500 amino acids away from the C cleavage site which is attacked in the presence of Ca. Both sites are probably adjacent in the native structure to the catalytic site of the Ca -ATPase. 

Photochemical reactions have the principal advantage of clean chemistry , as they use light of defined energy [72, 74], Synthesis of vitamin D and photocleavage of protection groups, for example, are accepted organic synthesis routes. Nevertheless, no widespread use of photochemistry has been made so far as this technique... 

In most cases, however, noble metals such as Pt, Ru and Rh or RUO2 and Rh203 have been deposited on different semiconductors such as CdS , Ti02 , SrTiOa and WO3 although these catalysts do not form an ohmic contact . In connection with the photocleavage of H2O mainly Gratzel introduced the idea of using semiconductor particles loaded even with two types of catalysts, one (e.g. Pt)... 

In connection with this problem it should be mentioned that 02-formation was found at CdS electrodes coated with polypyrrole and RUO2 under anodic polarization whereby the anodic decomposition could be considerably reduced. Under open circuit conditions only H2-evolution was observed, whereas O2 could obviously not be detected. This result is not in contradiction to the first experiment because the Fermi level can pass the electrochemical potential of H2O/O2 under bias. Very recently it was reported on photocleavage of H2O at catalyst loaded CdS-particels in the... 

Fig. 16. Volumn of produced by photocleavage of H2S at a CdS mono-grain membrane vs. irrdiation time ...

Dining the last couple of years CdS-containing Nafion membranes have been apphed for the photocleavage of H2S . They are not comparable with the monograin membranes because the CdS particles are at randomly distributed in a rather thick Nafion membrane. This technique is attractive for some applications because the semiconductor particles are immobilized . On the other hand, problems may arise because of diffusion problems in the nafion membrane. Mainly the photoassistol Hj-formation at CdS was investigated in the presence of a Pt-catalyst and with coprecipitated ZnS CdS without a catalyst . 

Fig. 2 Photocleavage of GG-containing oligomers complementary to the corresponding dCNBPu-containing strands. dCNBPU was located opposite to the A at position 0 (shown in red). Partial sequences of ODNs are shown and the sites of strand cleavage are underlined. Single Gs proximal to the dCNBPU-A base pair are shown in bold face. Efficiencies at the major cleavage sites were 18.5% (at position -4, ODN 5), 1.9% (at position +5, ODN 1), and 1.5% (at position -5, ODN 7)...

As an aid in the qualitative understanding of why photocleavage occurs, it is interesting to consider the results of semiempirical extended Hiickel theory (EHT)t86) for the following model compounds<87) ... 

Fonseca ASC, Gonfalves MST, Costa SPG (2007) Photocleavage studies of fluorescent amino acid conjugates bearing different types of linkages. Tetrahedron 63 1353-1359... 

Such enantiomeric excesses have not been observed in analyses of the corresponding a-H-a-aminoalkanoic acids. According to the authors, the excess of the L-forms could be due to a partial photocleavage of the racemic amino acid mixture as a result of the influence of circularly polarized UV light in a presolar cloud (Cronin and Pizzarello, 2000). 

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