Dertinger report the thymidine positive ion in powdered thymine crystals.27 The resolution is sufficient to resolve only the large methyl proton splitting of 19 G. This value is in good agreement with that found in our previous work8 considering the differences in molecular environment.
The protonation reactions at carbon sites of the anion radicals of certain unsaturated compounds as well as aromatic amino acids and their analogs have been investigated in alkaline and neutral aqueous glasses by electron spin resonance spectroscopy. For the compounds acrylic acid, 3,3-dimethylacrylic acid, crotonic acid, acrylamide, and acrylonitrile protonation or deuteration is found to occur ß to the electronegative group (R3) to form an «-carbon radical: RiR2C=CHR3~+ H20 -* R1R2CH-CHR3 + OH-. The anions were found to be stabilized when Rj (or R2) was a carboxyl (fumaric) or methyl (crotonic and 3,3-dimethylacrylic acid) group. The anions of acetylenedicarboxylic acid and hydrogen cyanide were also found to protonate in these aqueous glasses. The anions of four aromatic amino acids were found to protonate at carbon sites on the aromatic ring. Analogs of these amino acids which contain the aromatic ring were found to protonate at positions equivalent to those found in the amino acids. Molecular orbital calculations of the spin density and free valency show that the magnitude of these parameters can be correlated with the site of protonation and perhaps the tendency to protonate.
ANION radicals of pyrimidine DNA bases are likely primary intermediates in the radiol ysis of pyrimidines and pyrimidine-containing molecules such as DNA. Anions or radicals which could be considered as protonated anions have been reported in ESR studies of irradiated single crystals of pyrimidines [l-4]. It is therefore of interest to study these radicals separate from other possible radicals which are produced during radiolysis; for example cation radicals would be expected in radiolysis of crystalline compounds and radical species produced by .OH or Hattack would be expected in the radiolysis in aqueous media. In several previous studies pyrimidine anions have been produced by electron attachment in alkaline matricesr5-71. ESR spectra from such glassy matrices are not as readily interpreted as those in solution. However attempts to prepare anions of pyrimidines in solution have been successful in only a very few cases [8, 91. Holroyd and Glass report the ESR spectrum of the anions of the pyrimidine derivatives to be ca. 24 G wide singlets in an 8N NaOH glass at 80"K [5]. Only in the case of uracil anion at 200°K is resolution found. Here the ESR spectrum consists of a 16 G doublet. Srinivasan et al. also report 23 to 27 G singlets for these anions in 2.5M NaOH at 77"K[6]. It is known that the use of D,O in place of H,O in alkaline matrices can significantly reduce linewidths and consequently increase resolution of ESR spectra[5, 121. Employing an 8N NaOD glass, Sevilla has found a resolved spectrum for the thymine anion consisting of a ca. 16 G doublet [12]. In the work presented in this note such a deuterated alkaline glass is used in the study of a series of pyrimidine derivatives.The experimental technique has been described in detail previously [5,10-121. In this method K,Fe(CN), (5 X 10-3M) is photooxidized to produce trapped electrons in an 8N NaOD (92% D) glass at 77°K. The trapped electrons are photobleached and react with the pyrimidine derivative (5 X 10-4M).We find that the pyrimidines cytosine, uracil and thymine show 15-16 G doublets at -160°C [ Fig. l(a)]. ESR spectra of the nucleosides thymidine, cytidine and uridine also show doublet splittings; however the doublets are approximately 10 per cent less in magnitude than those found for the pyrimidines at this temperature [ Fig. l(b)]. Table 1 reports the hyperfine splittings for the various anions. Apparently substitution at the nitrogen reduces the hyperfine splitting. This is confirmed by the results found for the anion of I ,3-dimethyluracil which gives an even smaller splitting of 12-3 G.
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