Sugar base release from

For the release of an unaltered base, the sugar moiety must be damaged. In principle, the base could already be released from a radical site at the sugar moiety, i.e. on the time-scale of the lifetime of the DNA radicals. The observation of 2-dRL incorporated into DNA as a product formed upon OH attack shows that a damage at C(l ) contributes to the release of an unaltered base. In the carbohydrate series, hydrolytic scission at the glycosidic linkage when this site contains a free-radical is a well-documented phenomenon, and it has been estimated that the rate of reaction must be faster than 35 s 1 (von Sonntag and Schuchmann 2001). As it stands, it cannot be excluded, that under certain conditions the base release from the C(l ) radical [reaction (38)] occurs in competition to its oxidation [reaction (2)]. In a cellular environment, there is also the reduction of DNA... 

It would be interesting to test with other Rh(III) complexes, whether the direct oxidation of the base (by photo-electron transfer) could also be a primary step responsible for photocleavages. Indeed, as outlined before in Sect. 5, radiation studies have shown that the radical cation of the base can produce the sugar radical, itself leading to strand scission [122]. Moreover base release, as observed with the Rh(III) complexes, can also take place from the radical cation of the base [137]. Direct base oxidation and hydrogen abstraction from the sugar could be two competitive pathways leading to strand scission and/or base release. 

It is hence obvious that a radical transfer must occur from the base to the sugar moiety [reactions (4)-(7)]. In agreement with this, strand breakage and the decay of the absorption of the base radicals follow the same kinetics (Jones and O Neill 1991). This radical transfer is also evident from the high yields of unaltered Ura (G(Ura) = 3.0 x 10 7 mol J-1 Deeble and von Sonntag 1984 Deeble et al. 1986 Hildenbrand et al. 1993). There must be more than one precursor. This is evident from the kinetics of base release only 20% are released during (or immediately after) irradiation, while 80% are liberated at a much later stage, 50% in a fast and 30% in a slow process. The fast and the slow processes are only observable at elevated temperatures (Table 11.3). 

Alkali-labile sites (ALS) may contain an AP such as 2-dRL or certain damaged bases that are released from the sugar moiety upon treatment with alkali (OH" or an organic base such as piperidine). Subsequent to this, a strand break is induced, and this procedure is often used to detect damaged bases within DNA. As the mechanism of the decomposition of 2-dRL by alkali is concerned, it is been suggested that the carbonyl function at C(l ) acidifies H2 and deprotonation [reaction (6)] leads to a (3-elimination of the phosphate group [reaction (7)]. 

The ADMET polymerization of sugar-based monomers is much less explored than the ROMP approach, and only a few examples have been reported to date. Bui and Hudlicky prepared a,oo-dienes derived from a biocatalytically synthesized diene diol, from which chiral polymers (up to 20 kDa) with D-c/uro-inositol units were prepared via ADMET in the presence of 1 mol% of C4 [169]. Furthermore, several ot,co-dienes containing D-mannitol, D-ribose, D-isomannide, and D-isosorbide have been synthesized by Enholm and Mondal [170]. Also in this study, C4 was used to catalyze the ADMET polymerizations at 1 mol% catalyst loading. As pointed out by the authors, the viscosity increased as the reactions progressed and vacuum had to be applied to efficiently remove the released ethylene. Unfortunately, the polymers obtained were not further analyzed. As already mentioned above, Fokou and Meier have also reported the ADMET polymerization of a fatty acid-/D-isosorbide-based a,co-diene [126]. Furthermore, Krausz et al. have synthesized plastic films with good mechanical properties by cross-linking fatty esters of cellulose in the presence of C3 [171-173]. 

One of the simplest methods of estimation of PolyPs in extracts is based on the assay of Pi, which is released from the PolyPs by hydrolysis with 1 M HC1 at 90 °C for 10 min. The Pi released under these conditions is defined as labile phosphorus . If the compounds containing organic labile phosphorus (i.e. nucleotide phosphates, sugar phosphates, etc.) were removed from the extracts by adsorption on Norit charcoal, the increase in Pj content after hydrolysis can be attributed to PolyP and pyrophosphate (PPi). Estimation of the PPj content (Mansurova, 1989) before hydrolysis may be needed in some cases for more precise calculations of the PolyP content. Pi may be determined by one of the well-known chemical methods (Fiske and Subarrow, 1925 Weil-Malerbe and Green, 1951). 

Complex carbohydrates released from glycoproteins were readily profiled by capillary gel electrophoresis with LIF-based detection of 1-aminopyrene-3,6,8-trisulfonic acid (APTS)-labeled sugar molecules (Figure 10) [125]. High-mannose-type oligosaccharides of ribonuclease B were derivatized by APTS and separated by capillary electrophoresis using polyethylene oxide separation medium [126]. 

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