Uracil photolysis

The rate of uracil photolysis is affected by the fraction of oxygen content in the solution,7,291 64 and is also affected by dissolved N20.65 The data of Burr and Park are shown in Table III. The effect of N20 upon the rate suggests the intervention of solvated electron intermediates in the reaction mechanism if N20 is, as is commonly supposed, a scavenger specific for electrons. 

The relative rates of uracil photolysis in acetonitrile-water mixtures is nearly independent of the water content,64 but the fraction of hydrate formed is a linear function of the water content.64 Thus the rate of hydrate formation is a linear function of the water content, and the rate of dimer formation is also proportional to the water content of the solvent mixture, except for an initial curvature (Fig. 5). 

There is evidence32,45 that two photo-reactivated products are formed in uracil photolysis one of these is the known dimer. The relative amounts of these two products are a function of initial uracil concentration and the time between irradiation and examination, since the... 

All subsequent studies on uracil photolysis reported here were, therefore, carried out in oxygen-saturated solution for reasons of uniformity, photolysis of the substituted uracils was also done in oxygen-saturated solution. 

Figure 2. The rates of uracil photolysis (uracil loss) in 10 M unbuffered aqueous solution at different partial pressures of oxygen...

Both the dimer and the photohydrate from photolysis of uracil have been isolated not only as spots on a chromatogram7 but also as crystalline or amorphous solids—the dimer by Smietanowska and Shugar45 and Swenson and Setlow48 and the hydrate by Gattner and Fahr.45 Many physical properties have not been recorded for these materials. The melting points of the principal dimer is 380°.34 The infrared spectrum is not reported. The elementary composition of the hydrate has been reported.45 ... 

The dimer has been shown by Blackburn and Davies40 to have a geometrical structure similar to that of thymine dimer (10). The proof consisted in observation of the 13C—H satellite NMR spectrum of 5,5 -dideutero-,3,3 -dimethyl uracil dimer (prepared by methylation of the dimer from photolysis of uracil-5-rf). The magnitude of the H6H6 coupling constant showed that these hydrogens were vicinal, and was... 

The dimer has also been isolated from photolysis of dried uracil deposits on filter paper,33 and its elementary composition and molecular weight determined. It has an ultraviolet spectrum similar to that of thymine dimer (15). Uracil can be recovered by photolysis of the dimer in solution at 240 nm, but only to the extent of 75%. The photoreversal of dimer in solution gives, in part, the photohydrate.33 The thermal reversibility of the photolyzed aqueous solution ranges from 17 to 44% recovery of absorbance. 

Photolysis of a frozen mixture of 5 x 10 4M thymine-2-14C and 5 x 10" M uracil-2-14C gave a thymine dimer, a uracil dimer, and a mixed dimer of thymine and uracil,10,32 47 with the identities established as usual by the stability to heat and by photoreversion to monomers in solution. Wacker reports that the uracil dimer obtained in that investigation showed some instability to heat. These collected bits of evidence taken together with the reported441 thermal instability and chromatographic similarity of a synthetic tram dimer suggests that the dimer ordinarily obtained from uracil may be a mixture.46... 

Smith32 reported that the absorbance of frozen cytosine solutions (0.5 mg/ml) decreased only 3-5% when irradiated with light from a mercury resonance lamp, but that the rate of loss of cytosine doubled when the frozen solution contained both cytosine and uracil. In solutions containing cytosine and thymine, a mixed dimer was apparently formed. Dried films of cytosine were apparently stable to the resonance radiation under conditions where there was 9% conversion of uracil and 17% conversion of thymine. There is a report that uracil dimer was formed in low yield in the photolysis of frozen cytosine solutions.32,81... 

Only a confused picture is revealed by these essentially qualitative studies. Many questions are left unanswered. The probable formation of uracil hydrate in cytosine photolysis has not been reported and the question of dimer formation, either in solution or in ice, is still unresolved. There are thus large gaps in our knowledge about the photolysis of cytosine and cytidine. Much more detailed work has been carried out on cytidylic acid and on dinucleotides of cytosine (Sect. XI-C). 

The hydrate formed by photolysis of this substance is one of the few such products (the others are uracil hydrate, 5-fluorouracil hydrate, and uridine hydrate) that have actually been isolated and compared with authentic material of known structure.7 It is nearly the only product formed in the photolysis, is definitely stable at room temperature and neutral pH, and the thermal reversal to dimethyluracil is nearly quantitative. These properties, as Moore observed, make the reaction ideal for mechanistic investigation. Burr and Park have investigated the reaction mechanism by measuring the photolysis rate of dimethyluracil in mixtures of water with several nonaqueous, nonreactive solvents as a function of water concentration.64 The photolysis rate for 10" iM DMU was found to be the same in water-saturated cyclohexane (about 5 x 10-3M in water) as in dry cyclohexane. The photolysis rate in dry, highly purified dioxane was quite insensitive to water, and it was observed that hydrate formation (measured by thin-layer chromatography and by thermal absorbance reversal) became appreciable only at water concentrations above 40%. 

The rate of dimethyluracil photolysis in water is independent of pH,64 and neutral salt concentration HCN does not have any effect upon the photolysis rate. These elements of photochemical behavior are all strikingly different from the behavior of uracil. 

Two dimeric products are formed in the photolysis of frozen aqueous solutions of dimethyl uracil,346 with melting points of 252-253 and 263-264°C the structures and relationship of these products are not known. 

The existence of photoreversible, but not of heat-reversible, absorbance change in irradiated poly dI dC was taken to prove that the photoproducts are entirely dimers (in contrast to those in poly C irradiations where the product is almost entirely the hydrate82a). It was possible to detect dimers of uracil as well as those of cytosine, by means of the much slower photoreversal of uracil dimers. In the acid hydrolysates of irradiated dl-dC, both uracil dimers and uracil could be identified. Enzymatic hydrolysis (snake venom phosphodiesterase) does not split pyrimidine dimers, and the products of such hydrolysis of irradiated tritium-labeled poly dl dC contained trinucleotides shown by radioactivity to contain cytosine dimers. Thymine dimers were formed in the photolysis of the poly dA dT, and were detected and assayed by the same methods. The yield of thymine dimers in irradiated poly... 

The only information about the variation of the hydrate formation rate with light intensity is that the photolysis rate of 1,3-dimethyl uracil is independent of intensity over a 50-fold range.7 Since dimer formation must be bimolecular,106 and since it is a photoreversible phenomenon, the rates of dimer formation are complicated functions of the... 

However, although photolysis of dimethyluracil64 and of other pyrimidines may be genuinely oxygen independent, early observations of an oxygen effect on the rate of uracil photolyses have been confirmed (Sect. V) the kinetic evidence has been supplemented by the observation84 that dimer formation is almost entirely suppressed in oxygenated uracil solutions, and uracil hydrate is the whole product. This clear-cut... 

The quenching of thymine fluorescence in frozen aqueous solution has led Eisinger and Shulman168 to conclude that the precursor of thymine ice dimer is the singlet excited state of thymine Weinblum165 has shown that the ice thymine dimer and the mixed thymine-uracil dimer formed in frozen solutions are the same stereoisomers as those formed by photolysis of DNA, using infrared spectral evidence. 

Diphenylhydrazine (Ph2NNH2) is converted into indazole (215) by Cl2C = NMe2. Oxidative cyclizations are also known thus, fully methylated 6-(benzylidenehydrazino)uracils (216) on photolysis or in hot AcOH give the pyrazolo[3,4-d]pyrimidines (217) (75BCJ1484,92KGS219). 

The photoaddition of water to a variety of naturally occurring pyrimidine derivatives has been reported. Photolysis in aqueous solution of uracil (224 R = H), uridine (224 R=ribosyl), and uridylic acid results in the formation of the corresponding 6-hydroxy-5,6-dihydropyrimidine (225)208-210 these structures have been established by independent synthesis.209 Analogous photoadditions have been observed in 1,3-dimethyluracil211 and 5-fluorouracil.212 These additions are reversible. 

Photolysis is also widely applied to generate the Ura-5-yl radical from 5-halo-uracils. In aqueous solution, the reaction is a homolytic splitting of the C-X (X = Br, I) bond (Chap. 10) in DNA, the reaction seems to be much more complex (Chap. 12). 

Campbell JM, Schulte-Frohlinde D, von Sonntag C (1974) Quantum yields in the UV photolysis of 5-bromo-uracil in the presence of hydrogen donors. Photochem Photobiol 20 465-467 Chapman JD (1984) The cellular basis of radiotherapeutic response. Radiat Phys Chem 24 283-291 Chatterjee M, Mah SC, Tullius TD, Townsend CA (1995) Role of the aryl iodide in the sequence-selective cleavage of DNA by calicheamicin. Importance of thermodynamic binding vs kinetic activation process. J Am Chem Soc 117 8074-8082 Chaudry MA, Weinfeld M (1995) The action of Escherichia coli endonuclease III on multiply damaged sites in DNA. J Mol Biol 249 914-922... 

There are, nevertheless, experimental indications that a dimer radical cation Pyr +oPyr, even without a linking bridge, must have a certain lifetime. The splitting of t,s-l initiated by the oxidizing nitrate radicals (NCV), generated by photolysis of cerium(IV) ammonium nitrate in acetonitrile, leads besides the repaired uracil 9, also to a cyclobutane dimer species 8, presumably with a hy-... 

In exceptional cases, no electron transfer but energy transfer occurs from the electronically excited donor to the alkyl halide. For example, the population of the T2 state of 2-acetylnaphthalene by two-laser flash photolysis in CC14 yields Cf as transient (observed as Cl benzene n complex with added benzene)93. Possibly Br, which is supposedly responsible for the rather efficient formation of 5-bromouracils upon the photolysis of 1-substituted and 1,3-disubstituted uracils in CHBr3-CH2Cl292, is also formed via energy transfer. 

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