Thymine reaction with psoralen

Oxygen-free reactions of psoralens, when in close proximity to the target, proceed via the first excited states in which the 3,4-and the 4, 5 7r-bonds of the pyrone and furan moieties, respectively, can undergo C4-cyclization reactions with, e.g., unsaturated bonds of lipids, or the C5=C6 double bonds of thymine in DNA. In reactions with DNA the psoralen is believed to intercalate with DNA in the dark. Subsequent irradiation at 400 nm usually leads to furan-side 4, 5 -monoadduct formation, whereas irradiation at 350 nm increases the formation of crosslinks in which the furan and pyrone rings form C4 cycloadducts to thymines on opposite strands [95], Subsequent irradiation of the 4, 5 -monoadducts at 350 nm leads to formation of crosslinks and conversion into pyrone-side 3,4-monoadducts. Shorter wave-... 

Psoralen contains two double bonds that undergo [2+2] photocycloaddition reactions with the 5,6-double bond of thymines (Fig 2). When intercalated at a 5TA3 step in DNA, the two strands of DNA become covalently cross-linked, upon UV irradiation, with psoralen forming cyclobutane rings with two thymines, one on each strand. The structure of this bis-adduct has been determined by NMR studies and shows that the Cg atom of psoralen is located on the major groove side of DNA for the intercalation complex that leads to the cross-... 

The photochemical reactions of psoralens with DNA have been described in detail [1, 4, 5], and is proposed to proceed via a stepwise mechanism. First, the molecule intercalates between the base pairs in double stranded DNA interacting with the 7i-stack of the nucleobases. After irradiation with UVA light, the photo-excited furocoumarins react with DNA and form covalent adducts through a [2 + 2] pericyclic reaction. The photoaddition takes place mostly between the psoralen 4, 5 double bond (furan ring) and the C5, C6 double bond of a pyrimidine (usually thymine). The furan-side adduct can absorb an additional UVA photon, forming an interstrand cross-linked derivative through photoaddition at the 3, 4 double bond in the pyrone ring [6-8]. 

Modified oligonucleotides can be used to cross-link DNA sequences via a reactive group tethered to an oligonucleotide. When irradiated with uv light, psoralens (31) reacts with thymine bases, and the reaction yields a cross-link if the thymine residues are adjacent to each other on opposite strands. Psoralen linked to oligonucleotides have been shown to induce site-specific cross-links in vitro (51). 

Psoralens can react by two different routes upon photoactivation (Parsons, 1980 Pathak, 1984). The first route is through the well-known photoreaction mechanism that principally involves intercalation within double-stranded DNA or RNA with the formation of adducts with adjacent thymine bases. The furan-side and pyrone-side rings in psoralen both can form cycloaddition products with the 5,6-double bond of thymine to create a crosslink between two DNA strands (Reaction 57) or to a lesser extent, within double-strand regions of RNA. 

The interaction of MNPs with DNA is a well-known phenomenon [86-89], and single- or double-strand DNA is decorated by MNPs, which can easily be imaged by transmission electron microscopy (TEM) or AFM [90-92]. Various surface-functionalized AuNPs have been prepared to attach AuNPs securely to DNA molecules. In numerous studies, such AuNPs were prepared by surface modifications with cationic thiols [54, 55, 92] or intercalators [93, 94]. The ID chains of AuNPs coated with cationic trimethyl(mercaptoundecyl) ammonium monolayers were electrostatically assembled along DNA molecules in solution by the relative molar quantities of AuNPs and DNA base pairs. Since psoralen acts as a specific intercalator for A-T base pairs, the functionalized AuNPs were assembled onto the pA-pT ds-DNA [93]. Furthermore, UV irradiations induced the reaction between the psoralen units and the thymine of DNA, and then covalently fixed AuNPs to the DNA. 

Figure 5.19 The possible mechanism for the reaction of linear furanocoutnarins (psoralens) with nu-cleobases in DIMA (e.g. thymine residues) under the influence of UVA radiation resulting in the formation of interstrand cross-linb in DNA that may lead to photodermatitis and/or other health disorders such as cancer.

A psoralen photoadduct to adenine was also isolated and characterized upon irradiation of a mixture of adenine and psoralen derivative, however, the latter photoadduct does not appear to be generated in cellular DNA. The psoralen adducts within nuclear DNA are formed primarily by a [2 -i- 2] photocycloaddition reaction between the 4, 5 -furan double bond of the psoralen molecule and the 5,6-double bond of thymine. Monoadducts involving the 3,4-pyrone double bond of the psoralen and the pyrimidine 5,6-double bond are also formed, but to a lesser extent (Figure 142.4). Photoisomerization can lead to 4, 5 -monoadducts being converted into 3,4-monoadducts, at least in isolated DNA. Interstrand crosslinks can be generated as the result of photoexcitation of the 4, 5 -monoadduct and its subsequent cycloaddition with the pyrone 3,4 double bond. The structures of the monoadducts and cross-links are shown in Figure 142.4. The cross-linking process depends on the structure of the psoralen derivative. Linear psoralens form cross-links efficiently with a yield, which may reach up to 50% of the overall adducts formed. 

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