Electron Injection into DNA: Synthesis and Spectroscopic Properties of Pyrenyl-Modified Oligonucleotides

The nucleoside 5‐(1‐pyrenyl)‐2′‐deoxyuridine (1) was prepared by a Suzuki–Miyaura cross‐coupling reaction and subsequently used as a DNA building block in order to prepare a range of modified oligonucleotides using phosphoramidite chemistry. The DNA duplexes contain a pyrenyl group covalently attached to the nucleobase uracil. Upon excitation at 340 nm an intramolecular electron transfer from the pyrenyl group to the uracil moiety takes place which represents an injection of an excess electron into the DNA base stack. Based on the results obtained by steady‐state fluorescence and time‐resolved pump‐probe laser spectroscopy it was possible to show that base‐to‐base electron transfer can occur from the Py‐dU group only to adjacent thymines. Thymine does it! A range of modified oligonucleotides was prepared bearing a covalently attached pyrenyl group located outside the DNA base stack. The injection of excess electrons into DNA upon photoexcitation was then investigated spectroscopically using the pyrenyl‐modified DNA duplexes. It was possible to show that electron transfer can occur from the Py‐dU group only to adjacent thymines (see scheme).