Photohydration, uracil

There is as yet insufficient evidence for proper evaluation of electron transfer reaction as the possible key process in the photohydration reactions (and perhaps in some of the dimerization processes). It has also been suggested that there is a distinct charge separation in the (overall) neutral excited pyrimidine molecule, and that the charge is sufficiently localized that reaction of the excited molecule with OH or H+ (or both successively) can become a competitive reaction pathway.116 Such a dipolar reactant species has also been specifically proposed by Wacker et al.60 (Chart 8). This is an electrophilic attack on water similar to that proposed above for uracil photohydration. 

Photochemical Genetics. I. The Ionic Nature of Uracil Photohydration... 

Figure 4. Rates of formation of uracil photohydrate as functions of pH...

Excitation of a ground state UH+ or U" can be easily eliminated as the responsible process for uracil photohydration. In the first place, the midpoint of the uracil curve in Figure 7, at about 4.5 does not agree with either of the known pK values of uracil, Reactions 8a and 8b. 

Table I. Uracil Photohydration Rate Constant, Kf, at Constant (UH+)...

The water species involved in this reaction must be neutral (and not OH") because of the fact that the rate of uracil photohydrate formation is independent of NaCl concentration up to 1M, and is the same in unbuffered water as in 0.1M phosphate buffer. The rate constant for photohydrate formation in CU was also observed, in a series of runs all made in the same day with the same initial CU concentration, to be 0.0418 0.010 at NaCl concentrations of 0, 0.001M, 0.01M, 0.1 M, and 1M. The lack of salt effect is consonant, according to Debye-Huckel theory (3) with the reaction of a charged species (UH+) with an uncharged species, as written in Reaction f, and eliminates reaction between two charged species in the product-forming process. 

It is evident from the non-zero rate for uracil photohydrate formation at high values of pH that the fast reaction of 1(UH+) with water is accompanied by a slower reaction which is presumably the reaction of 1U with water. This type of reaction is apparently quite fast for EU and CU, although still slower than the reaction of the protonated excited state. It is possible that both protonated and neutral excited uridine species react at the same rate. 

The rate of photohydration of uracil with light of 265 n.m. in mixtures of acetonitrile and water is a linear function of the water content of the mixture the deuterium isotope effect, R//2o/R/)2o, is 1.18. These observations are shown to be consistent with the mechanism for uracil photohydrate formation proposed to explain the pH dependence of the rates. The rate of photohydrate formation for 1,3-dimethy-uracil is approximately proportional to the square of the water concentration in acetonitrile-H 20 mixtures, aceto-nitrile-D20, and in dioxane-water mixtures, RH20/Rn2o = 2.84. These data are also interpretable in terms of a common photohydration mechanism. 

The rate of uracil photohydrate formation is a linear function of the H20 or the D20 concentration in acetonitrile, Figure 3. This variation in rate is not caused by a variation in hydrogen ion—i.e., solvated hydrogen ion—because the estimated variation in pH over this solvent range (1... 

The rate trends indicated in Figures 3 and 4 may be used with good assurance, therefore, to determine the molecularity of the neutral water species concerned in the photohydration reactions. The linear variation of uracil photohydration rate with water concentration indicates that this reaction is first order in water concentration. The product forming step in this reaction can now be written with assurance, employing the notation from our first paper (3), as Equation 1. 

This conclusion is reinforced by the direction of the deuterium isotope effects for photohydrate formation in both uracil and DMU hAd is 1.18 for uracil photohydration and 2.8 for DMU photohydration (Figures 3 and 4). The fact that kH/kI is greater than 1.0 for uracil photohydration is significant because in that photolysis the principal product forming reaction mechanism was shown (2) to include a equilibrium between excited uracil and hydrogen ion. This is now shown... 

This correlation of second-order water dependence with large primary isotope effect and first order water dependence with the small isotope effect is of course quite tentative, but it is interesting that it can be fitted so neatly into a reaction mechanism which was proposed to explain the variation of uracil photohydration rate with hydrogen ion concentration. This scheme, provisional as it is, seems to provide a framework which includes all of the known kinetic factors in these photolyses. 

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