Activity 2: The color of hemoglobin complexes




Activity summary

What you must remember :

  Skills linked to the curriculum :

Compétences Capacités à maîtriser
 RCO Décrire l'établissement de la liaison entre l'ion ou l'atome central et le ou les ligands selon le modèle accepteur-donneur de doublet électronique.
Reconnaître dans un complexe : l'ion ou l'atome central, le ou les ligands, le caractère monodenté ou polydenté du ligand.

APP

Extraire des informations pour illustrer des applications des complexes inorganiques et bioinorganiques.
 ANA Extraire des informations pour illustrer des applications des complexes inorganiques et bioinorganiques.
Reconnaître dans un complexe : l'ion ou l'atome central, le ou les ligands, le caractère monodenté ou polydenté du ligand.

COM

Utiliser un vocabulaire scientifique adapté et rigoureux.
Formuler et argumenter des réponses structurées.
Formuler et présenter une conclusion.


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Document 1: The heme-Fe-histidine complex

In hemoglobin, each subunit contains a heme group. Each heme group contains an iron atom that is able to bind to one oxygen (O2) molecule. Therefore, each hemoglobin protein can bind to four oxygen molecules.
One of the most important classes of chelating agents in nature are the porphyrins. A porphyrin molecule can coordinate to a metal using the four nitrogen atoms as electron-pair donors, and hence is a polydentate ligand. Heme is a porphyrin that is coordinated with Fe(II) and is shown below:

Document 1
Fig. 1


On the left is a three-dimensional molecular model of heme coordinated to the histidine residue (a monodentate ligand) of the hemoglobin protein. On the right is a two-dimensional drawing of heme coordinated to the histidine residue, which is part of the hemoglobin protein. In this figure, the protein is deoxygenated; i.e., there is no oxygen molecule bound to the heme group.

Note: The coordinate-covalent bonds between the central iron atom and the nitrogens from the porphyrin are colored in brown; the coordinate-covalent bond between the central iron atom and the histidine residue is green. In the three-dimensional model, the carbon atoms are gray, the iron atom is dark red, the nitrogen atoms are dark blue, and the oxygen atoms are light red. The rest of the hemoglobin protein is purple.


Source: Hemoglobin and the Heme Group: Metal Complexes in the Blood for Oxygen Transport Authors: Rachel Casiday and Regina Frey

Document 2a: Geometry of the heme-histidine-Fe-O2 complex

The heme group is nonplanar when it is not bound to oxygen; the iron atom is pulled out of the plane of the porphyrin, toward the histidine residue to which it is attached. This nonplanar configuration is characteristic of the deoxygenated heme group, and is commonly referred to as a "domed" shape. The valence electrons in the atoms surrounding iron in the heme group and the valence electrons in the histidine residue form "clouds" of electron density. (Electron density refers to the probability of finding an electron in a region of space.) Because electrons repel one another, the regions occupied by the valence electrons in the heme group and the histidine residue are pushed apart. Hence, the porphyrin adopts the domed (nonplanar) configuration and the Fe is out of the plane of the porphyrin ring (Fig 2, left). However, when the Fe in the heme group binds to an oxygen molecule, the porphyrin ring adopts a planar configuration and hence the Fe lies in the plane of the porphyrin ring (Fig 2, right).

Document 2a
Fig. 2


On the left is a schematic diagram showing representations of electron-density clouds of the deoxygenated heme group (pink) and the attached histidine residue (light blue). These regions of electron density push one another apart, and the iron atom in the center is drawn out of the plane. (The nonplanar shape of the heme group is represented by the bent line. On the right is a schematic diagram showing representations of electron-density clouds of the oxygenated heme group (pink), the attached histidine residue (light blue), and the attached oxygen molecule (gray). The oxygenated heme assumes a planar configuration, and the central iron atom occupies a space in the plane of the heme group (depicted by a straight red line).


Source: Hemoglobin and the Heme Group: Metal Complexes in the Blood for Oxygen Transport Authors: Rachel Casiday and Regina Frey

Document 2b: Binding of oxygen to a heme-Fe group

Document 2b


 Source : https://commons.wikimedia.org/

Document 3: Spectroscopy and the color of blood

The changes that occur in blood upon oxygenation and deoxygenation are visible not only at the microscopic level, as detailed before, but also at the macroscopic level. Clinicians have long noted that blood in the systemic arteries (traveling from the heart to the oxygen-using cells of the body) is red-colored, while blood in the systemic veins (traveling from the oxygen-using cells back to the heart) is blue-colored. Hence, a simple macroscopic observation, i.e., noting the color of the blood, can tell us whether the blood is oxygenated or deoxygenated.

Document 3
Fig. 3


What causes this color change in the blood? We know that the shape of the heme group and the hemoglobin protein change, depending on whether hemoglobin is oxygenated or deoxygenated. The two conformations must have different light-absorbing properties. The oxygenated conformation of hemoglobin must absorb light in the blue-green range, and reflect red light, to account for the red appearance of oxygenated blood. The deoxygenated conformation of hemoglobin must absorb light in the orange range, and reflect blue light, to account for the bluish appearance of deoxygenated blood.


Source: Hemoglobin and the Heme Group: Metal Complexes in the Blood for Oxygen Transport Authors: Rachel Casiday and Regina Frey

Acquiring vocabulary.

English French

to bind to
to be bound to

electron-pair donor
oxygenated/deoxygenated
shape
light-absorbing properties
the blue-green range  
to account for  
the bluish appearance  


Understanding.
How many coordinate bonds can a heme group establish? Explain why.

How many coordinate bonds does Fe(II) establish in the heme-histidine-Fe complex (in deoxygenated blood)?


Give the right color:

Oxygenated blood   •
•   Blue
 Deoxygenated blood   • •   Red



Can you explain the color difference in oxygenated and deoxygenated blood?


Modifié le: lundi 29 janvier 2018, 11:56