Pyrimidine ionization constant

The basic ionization constants of a large number of 2,4-diaminopyrimidines and condensed pyrimidine derivatives have recently been measured and earlier values verified (Roth and Strelitz, 1969). The first protonation occurs at the N-1 ring atom (pAT = 7 40) and the effect of 5-substituents is correlated quite well with -constants for the -t-M substituents (p = 4 85). The —M substituents (—NO2, —CN) show an enhanced base-weakening effect, which may be accounted for in terms of the direct conjugation shown in [65]. This provides additional evidence for protonation at N-1. 

As mentioned by Tipson,10 the question arises as to whether ultraviolet absorption spectra of the type described112-114 can serve as proof for the positional assignment of the carbohydrate moiety on the pyrimidine ring of nucleosides. The increasing usage of spectrophotometry in the isolation, and in structural and enzymic studies, of nucleosides and nucleotides warrants a detailed consideration of this technique. The studies described above,112 114 for example, may be open to question (despite the fact that the conclusions were correct), since the pH of the curves selected was not predicated upon the ionization constants of the compounds under investigation. A series of comprehensive spectral studies of nucleosides of nucleic acids, their parent pyrimidines, and related compounds have been reported,87-110 and much of the following discussion is derived from them. 

C-Hydroxy-8-azapurines do not exist as such but as equilibrium mixtures (e.g., 8 9) in which the cyclic amide tautomers greatly preponderate over the hydroxy tautomer. This behavior parallels what was found in the pyridine, pyrimidine, and purine series on evidence from ionization constants and the UV spectra of C- and A -methyl derivatives. A formal name for9 is 1,6-dihydro-8-azapurin-6-one but such specification of the hydrogen atom s position is, in the absence of data, risky for example, pyrimidin-4-one is an equilibrium mixture in which tautomers with mobile hydrogen on N-3 and N-1 preponderate in a 5 2 ratio, respectively. Hence the simpler names, such as 8-azapurin-6-one, will be used in this review. 

This table lists some of the important purine and pyrimidine bases that occur in nucleic acids. The pAlj values (negative logarithm of the acid dissociation constant) are given for each ionization stage. The last column gives the aqueous solubility S at the indicated temperature in units of grams per 100 grams of solution. 

Application to electron attachment reaction. The effective biphotonic ionization of quinones can be used to generate radical-anions by electron attachment reactions. This method was used to study the radical-anions of the pyrimidine-type bases uracil and thymine. The comparison of the highly resolved FT EPR spectra in H2O and D2O allows a new assignment of the hfs coupling constants of... 

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