Pyrimidine , bases, resonance

The pyrimidine bases, their derivatives, nucleotides, and nucleosides have been extensively studied by NMR techniques including proton magnetic resonance (PMR), carbon-13, nitrogen-15, and fluorine-19 magnetic resonances as well as nitrogen-14 nuclear qxiadrupole resonance (14N NQR). 

In a few papers an attempt has been made to interpret quantum-mechanically the zero-field splitting parameters, which have been determined from the analysis of the electron spin resonance spectra on the triplet state of the pyrimidine bases. 

The 15N magnetic resonance studies of the pyrimidine bases or their derivatives are scarce. Roberts et al.BB have measured the 1H and 15N magnetic resonance spectra of a number of pyrimidines including uracil and 1-methylcytosine. The most important result of this study was the elucidation of the dominant tautomeric structures of uracil and proto-nated 1-methylcytosine as the diketo, 32, and keto-amine form, 7, respectively (cf. Sections II and IV and the spectrum of 1-methylcytosine hydrochloride labeled only in the amino group62). In the case of uracil,85 the two 15N-bonded protons gave two doublets centered at 10.78 and 10.96 ppm (measured downfield from internal tetramethyl-... 

Hildenbrand K (1995) Spin-trapping studies of the reaction of the sulfate radical ion with N1-substituted pyrimidine bases. Comparison with continuous-flow electron paramagnetic resonance experiments.) Chem Soc Perkin Trans 2 2153-2162 Hildenbrand K, Schulte-Frohlinde D (1997) Time-resolved EPR studies on the reaction rates of per-oxyl radicals of polyfacrylic acid) and of calf thymus DNA with glutathione. Re-examination of a rate constant for DNA. Int J Radiat Biol 71 377-385 Hildenbrand K, Behrens G, Schulte-Frohlinde D, Herak JN (1989) Comparison of the reaction OH and S04- radicals with pyrimidine nucleosides. An electron spin resonance study in aqueous solution. J Chem Soc Perkin Trans 2 283-289... 

The temperature dependence of the chemical shifts of the base resonances in poly(dA-dT) and poly(dA-5brdU) are plotted in Figure 5. These data demonstrate that the adenosine H-8 and H-2 protons exhibit very similar behavior over the entire temperature range and are not perturbed by the substitution on the pyrimidine 5 position. 

Applying the same analysis to pyrimidines (3 and 6) leads to pyrimidones, examples of which are the pyrimidine bases in DNA and RNA. Thus deoxycytidine 46 and deoxythymidine 47 are two of the four 2 -deoxyribonucleosides that are the building blocks of DNA and uridine 48 is one of the four nucleoside building blocks of RNA. As for pyridones, the contribution of dipolar resonance hybrids to pyrimidones and other systems with exocyclic conjugation often helps to understand their properties, including their aromatic character. 

Nonadiabatic Through Bond Electron Transfer Involving Resonances of Pyrimidine Bases... 

Simons and co-workers proposed the first theoretical model of LEE-induced strand break formation by modeling C5 -0 sugar-phosphate bond dissociation in 5 -dTMP and 5 -dCMP. > According to Simons et /., the initial attachment of the electron occurs in a n valence molecular orbital (shape resonance) in the pyrimidine base. Electron transfer occurs from the base to a a molecular orbital of the C5 -0 bond as C-O bond elongation takes place, thereby leading to... 

Due to the recent development of the utilization of nuclear magnetic resonance for the study of molecular associations (in particular, stacking) involving the purine and pyrimidine bases (e.g., Ts o et the knowledge of the ring currents in these compounds... 

The interaction of platinum with DNA has been clearly established both in vitro (8-11) and in vivo (12-14), and most modelling studies indicate that, although platinum can interact with the phosphate group (15), the prime interaction is with the purine and pyrimidine bases. Ultra-violet-visible spectroscopy (16,17) nuclear magnetic resonance (18,19) and x-ray photoelectron spectroscopy ( ) studies have been interpreted to suggest that sites of attack could be... 

Electron spin resonance studies of irradiated DNA at 77°K permitted the detection of at least two distinct paramagnetic centers. Pulse radio-lysis technics have been used to study the reaction of DMA and related pyrimidine bases with water radlolysls products. The free radicals produced in animal tissues have kinetic properties that are related to the radiosensitivity of those tissues.In a continuing study of the repair of damaged macromolecules by destruction of free radical sites it was found that transfer of the unpaired electron to a thiol occurred best in acid solution, in which the thiol is undissociated.Transfer to disulfide... 

Alkylation of pyrimidin-2(or 4)-amine on a ring-nitrogen gives an imine, e.g. (8), of quite high basic strength (pjSTa 10.7) because its cation, e.g. (13 R = Me), has typical and effective resonance stabilization indeed, methylation of pyrimidine-2,4-diamine gives a still stronger base (pjSTa> 13) due to an even more resonance-stabilized cation (14). 

With this in mind, the coordination chemistry of 52 with different diazine structural isomers was investigated. There were no detectable changes in the H NMR spectrum of 52 in a THF-Jg solution when either pyrazine or pyrimidine were added in 1 1 or 1 2 molar ratios, which suggested that only weak interactions might occur between 52 and these bases. In contrast, incremental addition of pyridazine or phthalazine to a THF-Jg solution of 52 at 25 °C resulted in an upheld shift of the aromatic NMR resonances of the diindacycle 52 thus reflecting the formation of complexes between 52 and the 1,2-diazines. Analysis of the tritration data clearly indicated the formation of 1 1 Lewis acid-diazine complexes 52-pyridazine-(THF)2 and 52-phthalazine-(THF)2 whose stability constants are equal to 80 ( 10) and 1000 ( 150) M respectively (Scheme 29). These data, as a whole, indicate that 52 is a selective receptor for 1,2-diazines. 

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