Psoralen group

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). 

The synthesis of the psoralen containing a methyl rather than methoxy group on the aromatic ring starts with construction... 

Psoralen, or derivatives of 9-methoxy-7H-furo[3,2-g]chromen-7-one tricyclic ring structures, are used as photoreactive groups in crosslinkers, biotinylation compounds, and nucleic acid probes. Psoralens have been used for many years as photochemotherapy agents for treatment of psoriasis and vitiligo (Smith and Barker, 2006). Psoralens react when exposed to UV light... 

Psoralen-PEOj-Biotin has been used to label double-stranded DNA for detection using (strept)avidin reagents (Henriksen et al., 1991 Wygrecka et al., 2007). The psoralen photoreactive group provides better insertion yields than typical phenyl azide-based systems, such as the standard photobiotin probe discussed previously in this section. 

Oser, A., Roth, W.K., and Valet, G. (1988) Sensitive non-radioactive dot-blot hybridization using DNA probes labeled with chelate group substituted psoralen and quantitative detection by europium ion fluorescence. Nucleic Acids Res. 16, 1181-1196. 

A well-investigated group of potent phototoxic compounds are the/wra-nocoumarins (psoralens) (Fig. 11.16), compounds that are also used as fish poisons. Furocoumarins absorb ultraviolet at around 330 nm and are converted to triplet stages these, in turn, produce singlet oxygen that damages amino acids (Fig. 11.17). 

Phenolics. The phenolics of citrus represent a varied and widely studied class of compounds. They range from the lipid-soluble methoxylated coumarins and psoralens to the water-soluble glycosides of the flavanones and flavones. They include the intensely bitter naringin and the highly insoluble hesperidin. Different classes within the group have characteristic UV spectra which have been used to detect adulterations in juices and oils. 

The last major group of reagents are the psoralens which have earlier been exploited to form crosslinks in double-stranded DNA and RNA (Song and Tapley, 1979 Parsons, 1980 Hearst, 1981). Reaction occurs primarily by 2 + 2 addition to pyrimidines but also with purines (Fig. 2.9). 

The substitution pattern of TfOH-mediated electrophilic aminomethylation of psoralens (furo-coumarins) by V-(hydroxymethyl)phthalimide has been elucidated <85JHC73>. Multiple phthal-imidoylated adducts were obtained when a B-ring hydroxy or methoxy activating group was present, and these resisted simple cleavage with NH2NH2. However, this two-step procedure to aminomethyl group introduction worked well when the psoralens contained only methyl substituents. 

Compositions of volatiles in different orange oils are shown in Tables 4 and 5. It is evident that most of the constituents belong to the terpene family and may be arranged into two groups, terpene hydrocarbons (terpenes and sesquiterpenes) and oxygenated terpene products (21). Aside from the volatile components, there are small amounts (2-15%) of nonvolatile residues in citrus peel oils that possess antioxidative property these include coumarins, psoralens, and polymethoxylated flavones (30-34). 

Figure 4 Puromycin (A) is an antibiotic analog of tyrosyl tRNAthat differs by the groups in red. Once joined to a DNA linker at the 3 -end and a psoralen (B) at the 5 -end it is ready to covalently link the mRNA to nascent peptide. Photoactivation of the psoralen (green), cross-links the 5 -end of the linker and the mRNA. Once the ribosome stalls at the end of the mRNA, the puramycin enters the ribosome A-site and is transferred to the end of the newly formed protein. However, the ribosome is unable to hydrolyze the ribamine (red) amide bond thus forming a permanent link between mRNA and the encoded protein (C).

Fig. 7.20. Psoralen (I) reacts only with ds DNA (II), at a rate of 5-10 groups/lOO bp, and can be derivatized with different side-groups (III). These side-groups can then be detected. M13 or phagennid clones, hybridized with M13 or phagemids without inserts, are convenient. This leaves only the insert of the clone single-stranded, which will be the only region not labeled.

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