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[en] The photocycloaddition reaction of 4',5'-dihydropsoralen with thymine was carried out in solution and in the frozen state. A major photoadduct was isolated and characterized by elemental analysis and physical methods. The photoadduct was proven to be the 1:1 C4-cycloaddition product, an analogue of furocoumarine-DNA biadduct, with the stereochemistry of anti-head-to-head formed through 2+2 addition reaction between the pyrone double bond of 4',5'-dihydropsoralen and 5,6-double bond of thymine. (author)
[en] This work established the mechanism of the post-ionisation opening of cyclic molecules derived from thymine after irradiation of the aqueous solution. A study of fragmentation of hydroxy-5 methyl-5 barbituric acid and its linear isomer, formyl pyruvyl urea, shows that in the former the ring opens after ionisation without initial rearrangement whereas in urea the hydrogen atom has been displaced chemically. The two spectra are entirely different. The hydanthoinic ring opens with rearrangement after electron impact with the result that most of the ionisation is distributed on fragments derived from urea
[fr]Cette etude a permis de preciser le mecanisme d'ouverture, apres ionisation, des molecules cycliques derivees de la thymine apres irradiation de la solution aqueuse. L'etude de la fragmentation de l'acide hydroxy-5 methyl-5 barbiturique et de son isomere lineaire, la formyl pyruvyl uree, montre que dans le premier l'ouverture du cycle a lieu apres ionisation sans rearrangement initial, alors que dans l'uree l'atome d'hydrogene a ete deplace chimiquement. Les deux spectres sont totalement differents. Le cycle hydanthoinique s'ouvre avec rearrangement apres impact electronique si bien que la plus grande partie de l'ionisation est repartie sur des fragments derives de l'uree
[en] As pharmacophore hybridization has been used frequently in the discovery of new drugs, ursolic acid/glycyrrhetinic acid-uracil/thymine hybrids have been synthesized and evaluated their antiproliferative activity by the MTT assay. The results displayed that, the hybrids of glycyrrhetinic acid (7a–7b to 10a–10b) exhibited slightly better antiproliferative activity than that of ursolic acid hybrids (3a–3b to 6a–6b). Two single glycyrrhetinic acid-thymine hybrids (8a and 10a) possessed good antiproliferative activity against tested A549 cell line (IC50 = 10.7 and 18.3 µM, respectively). And three hybrids (3a, 5a, and 10b) exhibited about 80% inhibitory ratio against tested Hela cell line.
[en] Nucleobases are the carriers of the genetic information in ribonucleic acid and deoxyribonucleic acid (DNA) for all life on Earth. Their presence in meteorites clearly indicates that compounds of biological importance can form via non-biological processes in extraterrestrial environments. Recent experimental studies have shown that the pyrimidine-based nucleobases uracil and cytosine can be easily formed from the ultraviolet irradiation of pyrimidine in H2O-rich ice mixtures that simulate astrophysical processes. In contrast, thymine, which is found only in DNA, is more difficult to form under the same experimental conditions, as its formation usually requires a higher photon dose. Earlier quantum chemical studies confirmed that the reaction pathways were favorable provided that several H2O molecules surrounded the reactants. However, the present quantum chemical study shows that the formation of thymine is limited because of the inefficiency of the methylation of pyrimidine and its oxidized derivatives in an H2O ice, as supported by the laboratory studies. Our results constrain the formation of thymine in astrophysical environments and thus the inventory of organic molecules delivered to the early Earth and have implications for the role of thymine and DNA in the origin of life.
[en] Primary high-energy radiations interacting with biomolecules generate secondary species such as radicals, ions and low-energy electrons (LEE) along the radiation track through the local energy deposition to the targets. Most of LEEs in this energy region do not have energies enough to directly ionize DNA molecules. But these secondary LEEs in vivo can subsequently react with DNAs and their constituents, such as bases, sugar and phosphate, and lead to various DNA damages indirectly. Therefore, the fundamental understanding of the interactions of LEEs with DNA constituents is very important to understand indirect DNA damages, which are comparable to direct DNA damages by high energy radiations. In this report, X-ray photoelectron spectroscopy (XPS) was used for analyzing the bond cleavage in thymine with and without LEE irradiation. Thymine was irradiated with 5-eV electrons and X-ray photoelectron spectroscopic technique was used for analyzing the bond dissociation in thymine. We concluded that, with 5 eV electron irradiation, C-C and N-C=O bond are primary dissociations occurred in thymine molecule. Further detailed studies at different electron energies and for other nucleobases will be underway
[en] The structures of all diastereoisomers of 5,6-dihydroxy-5,6-dihydrothymine (thymine glycol) and 5,6-dihydrothymine, two important DNA lesions, have been optimized with ab initio quantum chemical methods at a 6-31G level of calculations. The methyl group on C5 of thymine glycol shows a strong preference for a pseudo axial orientation. In contrast, in 5,6-dihydrothymine a pseudo equatorial methyl is preferred. Consequently, the thymine glycol lesion is much more bulky than 5,6-dihydrothymine. This observation may explain the different biological consequences observed for the two lesions. (author)
[en] On exposure to ultraviolet light, especially in the range of 320-400 nm, psoralens form chemical crosslinks to the DNA bases in the three steps. The first step is intercalation of psoralen between to adjacent base-pairs. The second step is formation of monoadduct; that is one psoralen molecule photo-react with one strand of DNA. The third step is the cross-linking of the same psoralen molecule to the other strand of DNA, thus covalently linking the two strand of a double stranded DNA. Psoralens have two photochemical reaction sites, 3,4-pyrone double bond and 12,13-furan double bond, both of which are engaged in cross-linking of DNA bases. The formation of interstrand cross-linking through C4-cycloaddition of 3,4- and 12,13-double bonds to the 5,6-double bond of the pyrimidine bases, especially thymine, in DNA has been correlated with the biological effects of photoexcited psoralens