Problem 58 - A Complete Resource Book in Chemistry for JEE Main

Question

The coordination number of \(\mathrm{Fe}(\mathrm{II})\) in oxyhaemoglobin is (a) 10 (b) 8 (c) 6 (d) 4

Step-by-Step Solution

Answer

The coordination number of Fe(II) in oxyhaemoglobin is 6.

Step 1: Understanding Coordination Number

The coordination number refers to the number of atoms or ions directly bonded to a central atom in a molecule or an ion. In the context of metal complexes like haemoglobin, it specifically refers to how many ligands are attached to the metal ion.

Step 2: Identifying the Ligand Environment in Oxyhaemoglobin

In oxyhaemoglobin, an ext{Fe}( ext{II}) ion is located within the heme group. This Fe ion is coordinated with 4 nitrogen atoms from the porphyrin ring. Additionally, one oxygen molecule ( ext{O}_2) and a protein side chain, typically histidine, coordinate with the iron, making the number of attachments to the iron.

Step 3: Counting the Coordination Sites

The iron in oxyhaemoglobin has 6 coordination sites: 4 from the nitrogen atoms of the porphyrin ring, 1 from the oxygen molecule, and 1 from histidine. Thus, the coordination number of Fe(II) in oxyhaemoglobin is 6.

Key Concepts

Oxyhaemoglobin

Oxyhaemoglobin is a compound formed when oxygen binds to the hemoglobin in red blood cells. It plays a crucial role in transporting oxygen from the lungs to various tissues throughout the body.

• When hemoglobin picks up oxygen, it becomes oxyhaemoglobin, brightening the blood's color to a vibrant red.
• This transformation is reversible; oxyhaemoglobin releases oxygen to the tissues and returns to its deoxygenated form.

This process is critical in maintaining our body's metabolic processes, ensuring that cells receive the oxygen they need for function. Understanding oxyhaemoglobin is key to comprehending how oxygen is distributed throughout the human body.

Fe(II) in Hemoglobin

Iron in the form of Fe(II), also known as ferrous iron, is centrally located in the heme group of hemoglobin. This iron atom is responsible for binding oxygen, which is vital for oxygen transport in the bloodstream.

• Fe(II) can coordinate up to six ligands, allowing it to interact with various atoms within the hemoglobin structure.
• The oxidation state of iron remains +2 even when it binds to oxygen, which is a key aspect of its ability to reversibly bind oxygen.

In essence, Fe(II) is the centerpiece of the hemoglobin's oxygen-carrying capacity, directly participating in the process that allows breathing to support life.

Ligand Coordination

In the context of hemoglobin, ligand coordination refers to how atoms or molecules, known as ligands, attach to the central iron atom in the heme group. These ligands are critical in stabilizing and enabling the function of the hemoglobin molecule.

• In hemoglobin, ligands include four nitrogen atoms from the porphyrin ring, one oxygen molecule, and one histidine residue from the protein structure.
• Each of these ligands forms a coordinate bond with Fe(II), involving the donation of an electron pair from the ligand to the metal ion.

The arrangement and specificity of these ligands around Fe(II) define hemoglobin's ability to pick up and release oxygen efficiently, a process crucial for respiratory physiology.

Porphyrin Ring

The porphyrin ring is a large, cyclic structure that plays a vital role in the functionality of hemoglobin. It provides a stable environment for the iron atom, allowing it to coordinate with other essential molecules.

• This ring is composed of four linked pyrrole subunits, each contributing a nitrogen atom to bind iron.
• The specific arrangement of these nitrogen atoms forms a planar structure that precisely fits an Fe(II) ion.

Thanks to its structural design, the porphyrin ring anchors the iron atom, maintaining its position and facilitating its role in reversible oxygen binding, a key aspect of oxygen transport in the bloodstream.

Histidine Coordination

Histidine, an amino acid residue within the hemoglobin protein, plays a crucial role in the coordination of the Fe(II) ion. This coordination through histidine is essential for the structural integrity and function of hemoglobin.

• The side chain of histidine contains an imidazole group, which donates an electron pair to Fe(II), forming a coordinate bond.
• This interaction is critical for maintaining the heme's proximity to the protein structure throughout the oxygenation-deoxygenation cycle.

Histidine's position ensures that the iron remains properly aligned within the porphyrin ring, facilitating efficient oxygen binding and release. This coordination is a vital aspect of how hemoglobin operates within the body.