Psoralen may be incorporated at the 5’ terminus only, using either a C2 or a C6 spacer arm. Psoralen intercalates between the two strands of a DNA duplex. When exposed to long-wavelength UV light, intercalated psoralen reacts with thymidine residues to form a covalent photoadduct. When attached to an oligonucleotide, psoralen thus forms interstrand crosslinks. The photoadducts can be reversed by short-wavelength UV light. The wavelength of the light used for the crosslinking experiment is critical to obtain a maximum diadduct.
Due to its relative hydrophobicity, psoralen oligos have the ability to cross membranes with relative ease. The C2 psoralen derivatization serves as an intercalating and cross-linking reagent in double-stranded oligonucleotides. The 6-atom spacer arm of psoralen C6 allows cross-linking with a triplex oligonucleotide strand.