Just a proton: distinguishing the two electronic states of five-coordinate high-spin iron(II) porphyrinates with imidazole/ate coordination.

We report detailed studies on two S = 2 electronic states of high-spin iron(II) porphyrinates. These two states are exemplified by the five-coordinate derivatives with either neutral imidazole or anionic imidazolate as the axial ligand. The application of several physical methods all demonstrate distinctive differences between the two states. These include characteristic molecular structure differences, Mossbauer spectra, magnetic circular dichroism spectroscopy, and integer-spin EPR spectral distinctions. These distinctions are supported by DFT calculations. The two states are characterized by very different spatial properties of the doubly occupied orbital of the high-spin that are consonant with the physical properties.

EPR detection of an O(2) surrogate bound to heme c(n) of the cytochrome b(6)f complex.

The ligand-binding properties of the unique heme c(n) of the cyt b(6)f complex, which is bridged to the heme b(n), are studied with EPR spectroscopy. Despite an open coordination site, high-spin heme c(n) in the oxidized state does not bind typical heme ligands such as cyanide, indicating their inaccessibility to the heme. In the reduced state, heme c(n) binds the O(2) surrogate NO to give a five-coordinate S = (1)/(2) [FeNO](7) complex, indicating that the site is accessible in the reduced state of the protein. The binding of NO implies that the heme c(n) can also bind O(2). Given the significant number of experimentally documented pathways for which a plastoquinol oxidase has been proposed, but the actual oxidase not identified, it is proposed that one of the functions of heme c(n), the only prosthetic group in the electron transport chain with oxidase-like properties, is the putative oxidase.