Labile photoproducts

The first synthesis of 9-azabicyclo[6.1.0]nona-2,4,6-triene (235) has been achieved by irradiation of l//-azonine (236) at 0°C.179 An oxonin structure (237) has tentatively been assigned to the labile photoproduct of the azonine 1,2-oxide 238.180... 

TABLE I Biologically Labile Photoproducts Formed from Freshwater and Marine DOM... 

The apparent differences in identified photoproducts between marine and freshwater systems may be due to fundamental differences in DOM composition (see Chapters 3 and 5) or to differences in analytical approaches (e.g., capillary electrophoresis has often been the method of choice to identify DOM photoproducts in freshwater systems, but this method is not appropriate for high-salinity marine samples Table I). The fact that most of the labile photoproducts found only in freshwater environments have been identified by more than one analytical approach, however, suggests that methods alone cannot explain the 14 nonoverlapping photoproducts. On the other hand, studies conducted by the same researcher(s) tend to report the same suite of compounds, even across marine/freshwater boundaries (e.g., Kieber et al., 1990), suggesting that optimization of the analytical approach and/or researcher focus may be influencing the data. More studies identifying DOM photoproducts have been conducted in freshwater environments than in marine environments (12 vs. 6), a factor that is also likely to influence the number of reported photoproducts. At this point, evidence is insufficient to determine whether DOM photoproducts that are currently unique to either marine or freshwater environments can be attributed to inherent DOM compositional differences or to analytical approach. 

TABLE IV Ecological Significance of Biologically Labile Photoproducts in Surface Waters (A) and on a Depth-Integrated Basis (B)... 

As DOM in surface waters is exposed to solar radiation for increasing periods of time, the formation of labile photoproducts either may continue at ecologically relevant rates or may slow (or stop completely) as the DOM becomes progressively photobleached. Which of these two scenarios better describes DOM photodegradation kinetics has important consequences for the validity of models addressing long-term yields of DOM photoproducts, because most of the models assume that photoproduct formation (which has been measured primarily over time frames from hours to days) occurs as a first-order reaction over time frames of months to years. The fact that... 

Figure 6. Seasonal changes estimated for potential biologically labile photoproduct production from terrestrially-derived CDOM at various latitudes in the Northern Hemisphere [67].

W.L. Miller, M.A. Moran, W.M. Sheldon, R.G. Zepp, S. Opsahl (2002). Determination of apparent quantum yield spectra for the formation of biologically labile photoproducts. Limnol Oceanogr., 47, 343-352. 

An alternative approach to the use of chiral sensitizers is to form a complex of a prochiral photosubstrate or labile photoproduct with a chiral host, and then... 

Fe(Im )(CO). The Fe(TPP)(Im )(CO) complex is also photo-labile, and the photoproduct also returns to the starting materials with a rate a[CO]. The kinetic difference spectrum upon photolyzing Fe(TPP)(Im )(CO) does not correspond to the static [Fe(TPP)(Im )2] - [Fe(TPP)(Im )(CO)] difference obtained by subtraction of the appropriate absorbance spectra, (Figure 5B). It also does not agree with what would be observed if the tetracoordinated Fe(TPP) were formed (41). Instead the red-shifted peak with maximum at 444 nm corresponds to the formation of the high-spin, pentacoordinated Fe(TPP)(Im ) (40). In addition, the isosbestic point observed at 436 nm is independent of... 

Population of the a orbital in M2(CO)io should labilize the M-CO bond, (8, 9) and such is likely the case. However, prompt CO loss apparently does not compete with scission of the M-M bond. Irradiation of M2(CO)jo in the presence of potential entering ligands such as PPh3 does lead to substitution ( 5, 6, 11)> but the principal primary photoproduct suggests a mechanism other than dissociative loss of CO to give M2(C0)9. For example,... 

Another major form of base damage generated in DNA by UV light is one in which the C6 position of a 5 pyrimidine is covalently linked to the C4 position of the 3 adjacent pyrimidine (Fig. 6). These lesions [which are readily detected by their lability in alkaline conditions at 80-100° C (43)], are called pyrimidine-pyrimidone [6-4] adducts, or simply [6-4] photoproducts ([6-4]PP). The pyrimidine planes in ([6-4]PP) are almost perpendicular. Hence, they result in prominent distortions of the double-helical structure of DNA. ([6-4]PP) can involve adjacent TC, CC, or (less often) TT sequences. Their formation at CT sequences is infrequent. Cytosine methylation at the C5 position inhibits the formation of [6-4]PP (44). The yield of [6-4]PP is proportional to the incident UV fluence in the range 100-500 J/m and continues to increase after exposure to several thousand J/m (1). In UV-C irradiated DNA, the ratio of CPD [6-4]PP is 3 1 (1). 

Wagner and Nahm have observed the photoaddition of a remote double bond to the benzene ring of acetophenone derivatives affording (179) and (180) from the irradiation of (181) and (182) respectively. In another publication they have reported further on the process and have demonstrated that the final product from the reaction is a secondary photoproduct. Thus the irradiation of (181) has been shown by n.m.r. spectroscopy to afford an initial photoproduct (183). This is thermally labile and.is converted into the triene (184) which undergoes a photochemical cyclization to yield the stable product (179). ... 

An alternative reagent (216) has been prepared for the synthesis of biotinylated oligonucleotides. In this case the protection used for the biotin moiety is base labile rather than the conventional dimethoxytrityl. The photo-crosslinking behaviour of oligonucleotide constructs, incorporating photoactive residues 217a-d at a defined position, has been examined in the presence of their DNA and RNA complementary targets. The X-ray crystal-structure of the photoproduct formed between 4-thiothymidine and adenosine upon near UV irradiation has been reported. 

Photoproducts and quantum yields for representative Co(III) complexes are summarized " in Table 2. As noted above, charge-transfer excitation of the Co(lll) amines is accompanied by photoreduction to Co(II) plus oxidations of ligands. For LF excitation, quantum yields of photosubstitution are wavelength dependent. Even the nature of the predominant substitution is dependent, as illustrated for [Co(NHj)jCl], for which NHj aquation is favored for excitation of the lowest-energy singlet LF absorption (488 nm), whereas Cl labilization is predominant for excitation of the triplet LF absorption (647 nm). In contrast, the Co(CN)jX photoaquations are independent of... 

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