1 Department of Physics and Astronomy, Science and Technology, Aarhus University2 Department of Physics and Astronomy, Science and Technology, Aarhus University
This chapter deals with gas-phase spectroscopy of protoporphyrin IX and heme ions, two important biochromophores in nature. These ions strongly absorb blue and green light, which accounts for e.g. the red colour of blood. We present absorption spectra of four-coordinate ferric heme cations at room temperature and in cold helium droplets, obtained in both cases from light-driven dissociation processes. These spectra serve as references for protein biospectroscopy and provide a natural testing ground for advanced quantum chemical modelling. The role of axial ligands bound to the iron centre, i.e., amino acids and nitric oxide, on the electronic structure of the porphyrin ring is discussed, and gas-phase spectra are compared to relevant protein ones. Spectroscopy on intact multiply charged protein anions with the heme prosthetic group is possible by monitoring the detachment of electrons triggered by light absorption. Similar experiments on protein cations rely on the absorption of too many photons for dissociation to be of practical use. This is illustrated from results on cytochrome c in vacuo. Time constants for dissociation and the corresponding dissociation channels obtained from storage-ring experiments are presented and discussed in the context of vibrational cooling within a heme protein cavity. Finally, we show the time spectrum for dissociation of photoexcited protoporphyrin IX anions, from which it is concluded that intersystem crossing to triplet states is in competition with internal conversion to the ground state. This is somewhat supported by spectroscopic characterisation of the long-lived states based on pump-probe experiments. Hence from one time spectrum (a one-laser experiment), triplet quantum yields can easily be estimated.