Poul Petersen, Ph.D.
Proton Transfer Mechanisms Revealed by Ultrafast Continuum Infrared Spectroscopy
Chemistry and Chemical Biology
Photoinduced proton transfer is of fundamental importance for energy transfer in biological systems and for the photostability of DNA. The 7-azaindole homodimer is a prototypical model system for the excited-state proton transfer within DNA base pairs. This system has been studied extensively both theoretically and experimentally utilizing ultrafast visible spectroscopies but the double-proton transfer mechanism following UV-excitation remains controversial due to the difficulties of interpreting the transient visible signatures. Ultrafast IR spectroscopy offers is capable of directly monitoring the location of the protons through the vibrations of the NH bonds involved in the proton transfer reaction and thus is more direct probe of the proton transfer mechanism. However, conventional ultrafast IR spectroscopy does not have the bandwidth coverage to span the broad spectral features associated with proton transfer. We recently developed a new method for generating ultrafast IR pulses that span the entire vibrational spectrum. Using ultrafast continuum IR spectroscopy we will follow the broad spectral changes associated with proton transfer. In addition to the classical 7-azaindiole homodimer we also study the more biological relevant asymmetric heterodimer of 7-azaindole with acetic acid. Initial experiments have focused on describing the vibrational dynamics in the electronic ground state of the heterodimer of 7-azaindole with acetic acid show a surprisingly strong coupling between the OH and the NH modes despite being separated by 750 cm-1. We are currently extending these measurements to involve UV excitation to follow the excited-state proton transfer mechanism through the NH/OH bonds that are breaking and forming during the reaction.