Principles of Bioinorganic Chemistry - 2004

Magnetic Splitting of Line in 63/65Cu EPR Spectroscopy

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Electron Transfer (ET) in Living Systems PRINCIPLES: M-binding sites tailored to minimize structural changes upon ET One-electron transfer processes preferred Coupling of H+ with electron transfer controls redox potential ET can occur over long distances; ~ 11-13 Å is most common Parameters: distance, driving force, reorganizational energy, path TOPICS: Three major bioinorganic ET units: FenSn clusters; Cu; hemes Long-distance electron transfer: dependence on distance, driving force, reorganization energy Electron supply in the methane monooxygenase system

The Major Metal Units in ET Proteins (1) Iron-Sulfur Clusters

Properties of Iron-Sulfur Clusters (A) Rubredoxin Fe–S, 2.25 - 2.30 Å in oxidized (FeIII) and reduced (FeII) states Reduction potentials: - 50 to + 50 mV (B) 2Fe-2S Ferredoxins (Fd) Reminder: eo = -RT/nF lnQ + pH, where Q = [Mn]/[Mn-1] Thus, at pH 7, the biological H2/2H+ standard couple is - 420 mV. FeII FeII FeII FeIII FeIII FeIII reduced mixed-valent oxidized all physiological uses Reduction potentials: -490 to - 280 mV (C) 3Fe-4S Ferredoxins (cube missing a corner) FeIII 3S4 FeIII 2 FeII S4 Reduction potentials: -700 to - 100 mV

Properties of Iron-Sulfur Clusters, cont’d (D) 4Fe-4S Ferredoxins and High-potential Iron Proteins (HiPIPs) The three state hypothesis: FeII3 FeIII FeII2 FeIII2 FeII FeIII3 Ferredoxin HiPIP Reduction potentials: -650 to - 280 mV (Fd); + 350 mV (HiPIP) minimal reorganizational energy

The Physical Properties of Iron-Sulfur Clusters

Structure of an 8Fe-8S Ferredoxin Primary structure (sequence) does not dictate the tertiary structure of a metalloprotein, as revealed by this 8-iron ferredoxin crystal structure.

The Major Metal Units in ET Proteins (2) Blue Copper and CuA Depicted at the right are the three copper sites in the enzyme ascorbate oxidase. Type 1, or blue, copper is the ET center. Below is depicted CuA . Blue Copper CuA

The Physical Properties of Blue Copper Centers The deep sky blue color of these proteins facilitated their purification on columns; the optical band is Cu–S C.T.

Structure of Poplar Plastocyanin The copper(II) thiolate center is difficult to model. The oxidized, reduced and apo plastocyanin structures are nearly identical.

EPR Spectra Distinguish the Three Types of Copper Found in Metalloproteins

CuA Model Chemistry: Reversible 1-Electron Transfer These complexes demonstrate that constrained dicopper(I/II) units afford good 1-electron reversible transfer centers and display the possible environments that could be encountered in biology. LeCloux Chuan He

The Major Metal Units in ET Proteins (3) Cytochrome c from tuna showing coordination of the iron porphyrin group by the protein side chains from Met (left) and His (right) residues.

Electronic Properties of Low-Spin Metalloporphyrins Note again, minimal reorganization energy upon electron transfer