Purine nucleotides, irradiation

Nuclear spin polarization can be induced in a variety of biologically important molecules by laser irradiation of solutions in the presence of a dye. Initially we had found that using flavin dyes it is possible to polarize the amino acids tyrosine, histidine and tryptophan, both as free amino acids and as surface residues in proteins (, O. This occurs in cyclic reactions with a high degree of reversibility of the type discussed in the previous section. More recently, we found that the purine nucleotide bases can be polarized similarly (16). It appears that flavins are remarkable in that upon photo-excitation they react reversibly with several classes of compounds such as aromatic phenols, aromatic amines, and secondary and tertiary amines. In all these cases CIDNP can be observed. We shall now discuss the photo-CIDNP spectra of the amino acids in more detail. 

There has been continued interest in the radiation chemistry of the purines since early reports on oriented DNA by Graslund et al. [35] which suggest that the main trapping site of one-electron oxidation in DNA is the guanine base. It is remarkable that in aqueous solution, the electron adducts of the purine nucleosides and nucleotides undergo irreversible protonation at carbon with a rate constant 2 orders of magnitude higher than that for carbon protonation of the electron adduct in thymidine [36]. It is therefore important to know the properties of the various purine reduction products and to ask why they have not been observed in irradiated DNA. 

Yields of Destruction of Base Components of Various Purine and Pyrimidine Nucleotides on Irradiation of Their Aqueous Solutions (2 x 10 iM, pH = 7.0) with X-Rays (200 kV) in the Presence of Oxygen... 

The purine bases, caffeine, theophylline, and theobromine, yield the corresponding 6-monothio- and 2,6-dithio derivatives upon reaction with LR without a solvent under microwave irradiation. Thionation of the carbonyl groups in the nucleobases of nucleotides leads to interesting special types of thiolactams, which are expected to exhibit modified biological activities. P3u imidine nucleosides are selectively thionated in the 4-position of the nu-cleobase. The mild reaction conditions applied allow the conversion of nucleosides with labile glycosidic bonds, such as 5,6-dihydropyrimidine-(113) and 2, 3 -dideoxynucleosides. Thiononucleosides with unnatural bases (114) or sugar moieties (115), (116) can also be prepared. 

It is well known that basic functions and a-carbon atoms can react readily with nucleotide bases. This is well demonstrated in the photo-cross-linking of 1-propyIamine to 1,3-dimethyluracil both the amino group and the a-carbon form adducts with the pyrimidine ring. Irradiation of solutions of cysteine and uracil give 5-/S-cysteine-6-hydrouracil 5-heteroadducts have also been identified after irradiation of thymine and cysteine. With purines, the C-8 position of adenosine as well as of guanosine can photo-cross-link to 2-propanol. ... 

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