Spectroscopy of Ferric Heme and
Protoporphyrin IX Ions In Vacuo
7
Jean Ann Wyer and Steen Brøndsted Nielsen
Abstract
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.
J.A. Wyer (*) • S.B. Nielsen
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: jeanwyer@phys.au.dk; sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_7,
# Springer-Verlag Berlin Heidelberg 2013
117
Protoporphyrin IX Ions In Vacuo
7
Jean Ann Wyer and Steen Brøndsted Nielsen
Abstract
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.
J.A. Wyer (*) • S.B. Nielsen
Department of Physics and Astronomy, Aarhus University, 8000 Aarhus C, Denmark
e-mail: jeanwyer@phys.au.dk; sbn@phys.au.dk
S. Brøndsted Nielsen and J.A. Wyer (eds.), Photophysics of Ionic Biochromophores,
Physical Chemistry in Action, DOI 10.1007/978-3-642-40190-9_7,
# Springer-Verlag Berlin Heidelberg 2013
117