Real-time observation of hydrogen bond-assisted electron transfer to a DNA base

Electron transfer (ET) in a pyrene-labeled DNA base (PydU) has been investigated with fs-broadband pump-probe spectroscopy. By comparing the dynamics of PydU in a nonprotic solvent (acetonitrile) and a protic solvent (methanol) with similar dielectric properties, one can observe the formation of a planar intramolecular charge transfer state and the build-up of the pyrene radical cation population in real time. The temporal evolution of the excited state absorption spectrum provides direct evidence for a greatly enhanced ET yield in methanol. The results emphasize the critical role of hydrogen bonding for ET involving DNA bases. No spectroscopic evidence for (de)protonation of the ET product could be manifested.