Redox control of proton transfers in membrane b-type cytochromes: an absorption and resonance Raman study on bis(imidazole) and bis(imidazolate) model complexes of iron-protoporphyrin

Abstract

Optical absorption spectra and resonance Raman (RR) spectra, obtained with Soret excitation, are reported for bis(imidazole) and bis(imidazolate) complexes of iron(II)- and iron(III)-protoporphyrin IX, prepared in aqueous conditions. Perdeuteration experiments on the axial ligands permitted the assignment of the symmetric Fe-(ligand)2 stretching mode of Fe[x]PP(L)2 to RR bands at 203 (x = II; L = ImH), 212 (x = II; L = Im−), 201 (x = III; L = ImH) and 226 cm−1 (x = III; L = Im−). These frequency differences indicate a strengthening of the axial bonds when the imidazole deprotonations occur. The larger difference observed for the ferric derivatives reflects the stronger σ-donor capability of the Im− anion for iron(III) over iron(II). For the ferrous derivatives, the frequencies of several skeletal porphyrin modes (ν4, ν10, ν11 and ν38) are downshifted by 2–10 cm−1 upon deprotonation of the ligands. This effect corresponds to an increased back-bonding from the metal atom to the porphyrin ring when the axial ligand decreases its π-acid strength. Bringing further support to this interpretation, an inverse linear relationship is established between the frequencies of ν(Fe(Il)-L2) and ν11. This correlation is expected to monitor the overall H-bonding state of histidine ligands of reduced cytochromes b. On the other hand, absorption measurements have characterized large pKa differences for the sequential imidazole ionizations of Fe[x]PP(ImH)2 in aqueous cetyltrimethylammonium bromide (9.0 and 10.8 for x = 111; 13.0 and 14.1 for x = II). These titrations show that Fe(II)PP(Im−)2 and Fe(III)PP(ImH)2 are good proton-acceptor and proton-donor, respectively, and suggest a model by which heme, located in a favorable environment inside a cytochrome, could couple a cycle of electron transfer with a proton transfer. Based on sequence data and structural models, it is further proposed that, in several membrane cytochromes b (b, b 6, b 559), a positively charged amino acid residue and an imidazolate ligand of the ferriheme could form an ion pair involved in a redox control of proton transfer.