Chapter 5 Bioinorganic Chemistry

Complexes of iron(II) Complexes: • in solution: [Fe(H2O)6]2+ (octahedral, pale green) • easy oxidation to iron(III) (in basic solution) • redoxpotential of Fe(III)/Fe(II) is changed by formation of complexes Fe3+/Fe2+: CN– +0,36 V H2O +0,77 V Phen +1,12 V

Complexes: • intermediate (hard/soft) acid: binding to O-, N- and S-donoratoms • most important ligands: aromatic nitrogen donors in chelatable position • bipiridine, fenantroline, porphyrins • usually octahedral complexes (some tetrahedral complexes)

Complexes of iron(III) • in solution: [Fe(H2O)6]3+ (pale violet) • stable complexes in acid and basic pH range • characteristic reaction: hydrolysis pH > 1: [Fe(H2O)6]3+ + H2O [Fe(H2O)5(OH)]2+ + H3O+ Ks = 1,8⋅10–3 • Dimerization → structure with oxo bridges (yellow) [(H2O)5Fe–O–Fe(H2O)5]4+ Complexes

Complexes: pH > 2: polynuclear structures, mixed hydroxo complexes → Fe(OH)3 precipitation • usually octahedral complexes • hard acid: • binding to F– and O-donors containing ligands • [Fe(SCN)4]– + 6 F– [FeF6]3– + 4 SCN– intensive red colorless

Biological role of iron • Human body: cca. 4 g iron (~3 g in hemoglobin) • uptaking of iron: 1 mg/day Iron proteins Hem proteins Non-hem proteins ~ 70 % ~ 30 % iron-sulphur proteins and others

Biological role of iron Iron proteins Hem proteins Non-hem proteins hemerythrin oxygen transport, storage Hemoglobin myoglobin electron transfer Cytochromes iron-sulphur proteins oxidases, oxygenases cytochrome c oxidase iron transport transferrin iron storage ferritin

Myoglobin (oxygen storage) Globin: contains153 amino acids Hem: Fe-porphyrin Hem is bound to globin protein via iron ion (without covalent bound) 5. coordination side: imidazole N

Hemoglobin (oxygen transport) It contains 4 globin units Fe(II) + O2: iron ion passes to porphyrin ring

Hemoglobin (oxygen transport) uptaking of oxygen: high partial pressure of oxygen giving down of oxygen: low partial pressure of oxygen

Iron-sulphur proteins Tetrahedral geometry of iron • Iron binding to inorganic and organic sulphur donors (Cys) S • one e–-pass • reduced ferredoxin – e– oxidized ferredoxin + e– • unusually low redox potenctial: –0,05 - –0,49 V → they behave as a reduction agents Iron-sulphur proteins • rubredoxin: [1Fe]3+(RS–)4 (–0,06 V) • ferredoxins: [2Fe-2S]2+(RS–)4 (–0,3 - –0,4 V) [4Fe-4S]2+(RS–)4 (–0,4 V) HIPIP: [4Fe-4S]2+(RS–)4 (0,35 V)