Heme Photolysis Occurs by Ultrafast Excited State Metal-to-Ring Charge Transfer

Abstract

Ultrafast time-resolved resonance Raman spectra of carbonmonoxy hemoglobin (Hb), nitroxy Hb, and deoxy Hb are compared to determine excited state decay mechanisms for both ligated and unligated hemes. Transient absorption and Raman data provide evidence for a sequential photophysical relaxation pathway common to both ligated and unligated forms of Hb* (photolyzed heme), in which the excited state 1Q decays sequentially: Q 1 → H b I ∗ → H b II ∗ → Hb Hb ground state. Consistent with the observed kinetics, the lifetimes of these states are <50 fs, ≈300 fs, and ≈3 ps for 1Q, H b I ∗ and H b II ∗ , respectively. The transient absorption data support the hypothesis that the H b I ∗ state results from an ultrafast iron-to-porphyrin ring charge transfer process. The H b II ∗ state arises from porphyrin ring-to-iron back charge transfer to produce a porphyrin ground state configuration a nonequilibrium iron d-orbital population. Equatorial d– π* back-bonding of the heme iron to the porphyrin during the lifetime of the H b II ∗ state accounts for the time-resolved resonance Raman shifts on the ≈3 ps time scale. The proposed photophysical pathway suggests that iron-to-ring charge transfer is the key event in the mechanism of photolysis of diatomic ligands following a porphyrin ring π− π* transition.