Spin cascade and doming in ferric hemes: Femtosecond X-ray absorption and X-ray emission studies

Abstract

The structure–function relationship is at the heart of biology, and major protein deformations are correlated to specific functions. For ferrous heme proteins, doming is associated with the respiratory function in hemoglobin and myoglobins. Cytochrome c (Cyt c) has evolved to become an important electron-transfer protein in humans. In its ferrous form, it undergoes ligand release and doming upon photoexcitation, but its ferric form does not release the distal ligand, while the return to the ground state has been attributed to thermal relaxation. Here, by combining femtosecond Fe Kα and Kβ X-ray emission spectroscopy (XES) with Fe K-edge X-ray absorption near-edge structure (XANES), we demonstrate that the photocycle of ferric Cyt c is entirely due to a cascade among excited spin states of the iron ion, causing the ferric heme to undergo doming, which we identify. We also argue that this pattern is common to a wide diversity of ferric heme proteins, raising the question of the biological relevance of doming in such proteins.

Short-lived metal-centered excited state initiates iron-methionine photodissociation in ferrous cytochrome c

“…For the first 300 fs, our model qualitatively reproduces the observed XSS difference signal of cyt c without the need to invoke heme doming 19 , 33 . This is consistent with the observed delayed appearance of the 5 MC state that has been suggested as the primary origin of the doming motion due to the antibonding nature of the singly occupied orbital with respect to the Fe(II)-N(Por) bonds 27 , 34 , 35 . We do observe a delayed rise in the positive peak at Q = 1.265 Å −1 strongly correlated with the rise time for quintet state formation (Fig.…”

Interplays of electron and nuclear motions along CO dissociation trajectory in myoglobin revealed by ultrafast X-rays and quantum dynamics calculations

“…In carbonmonoxymyoglobin (MbCO), the low-spin (LS) Fe(II) center has a pseudo-octahedral coordination geometry, ligated with four nitrogens (N p ) from the heme, the nitrogen of an axial histidine (N His , His93), and CO, a strong field ligand. Previous studies have pointed out that upon excitation of the heme Soret or Q band, photolysis occurs within ∼50 fs, although there is an ongoing debate about the mechanism of CO photodissociation and the subsequent relaxation of the heme, as well as the possible role of intermediate spin states, similar to those observed in photoexcited iron Tris(bipyridine) (20) and ferrous cytochrome c (14,15,21). With the loss of CO, the LS state of Fe(II) transforms to a high-spin (HS) state and adopts square-pyramidal pentacoordination with the axial histidine His93 moving ∼0.3 Å out of the porphyrin plane, perturbing the position of the alpha helix in which it sits (Fig.…”

“…Because the dissociation of diatomic ligands, such as CO and NO, can be synchronized through optical excitation of the porphyrin, diatomic ligand binding in hemoproteins is amenable to scrutiny by dynamic structural and electronic spectroscopies (1,(7)(8)(9)(10)(11)(12)(13)(14). Several X-ray diffraction, solution scattering, and X-ray spectroscopy (including X-ray absorption and emission) studies have been carried out using intense X-ray pulses from synchrotron and X-ray free electron laser sources (11,(14)(15)(16)(17)(18)(19). In this report, we focus on the correlations between the electronic structural change of the iron center and these nuclear motions.…”

Capturing photochemical and photophysical transformations in iron complexes with ultrafast X-ray spectroscopy and scattering

“…116,121 This is consistent with the delayed appearance of the 5 MC state that has been suggested as the primary origin of the doming motion due to the antibonding nature of the singly occupied d x 2 Ày 2 orbital with respect to the four Fe(II)-N(porphyrin) bonds. 118,121,122 Taken in total, the following mechanism for the photodissociation emerges. Fe-S bond dissociation requires the transition of the 1 p-p* excitation to the Fe center.…”