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Coordination Complexes in Biological Systems

Porphyrin

Porphyrins are heterocyclic macrocycles composed

of four modified interconnected pyrrole subunits.

They form coordination complexes with metal ionsand are found in biological systems.

Chlorophyll c2 Heme B Cobalamin

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Hemoglobin and Myoglobin

Notice that the hemoglobin is essentially a tetramer of myoglobin. (There are four

myoglobin like units in hemoglobin.)

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Oxygenated and Deoxygenated Forms

N

H

N

HN

N

Proximal Histidine (F8)

Distal Histidine (E7)

Fe

F helix

E helix

NH

N

Proximal Histidine (F8)

Fe

O

O

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The Role of the Protein

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Hemoglobin and Myoglobin

Fe-O-O = 150

Raman Spectra gives o-o at 1105 cm-1, indicating that the complex should be in asuperoxide state

o-o (cm

-1

) 1560 1100 850-740

O2

O2 O

22

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Coordination Environment of Fe

HN

N

NH

N

Fe2+

dxz dyz

dxy

dz2

dx2-y2

Total unpaired electrons = 4, S = 2

Deoxygenated form is high-spin andparamagnetic

Total unpaired electrons = 1, S = 1/2

Oxygenated form is low-spin

The magnetic moment of Fe3+ and the

superoxide radical involves in anti-

ferromagnetic coupling and the oxygenated

complex is not paramagneticNH

N

Fe3+

O

O

dxz dyz

dxy

dz2

dx2-y2

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Ferromagnetism

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Antiferromagnetism

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Hemoglobin and Myoglobin

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Bohr Effect

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Hemoglobin and Myoglobin