Ligand substitution and stability constants 216-217

Question 1

What is haemoglobin in terms of structure?

Answer 1

Haemoglobin is a complex protein composed of four polypeptide chains.

Each protein chain contains for non-protein components called haem groups. Each haem group has an Fe²⁺ ion at its centre.

Question 2

Draw the haem group in haemoglobin.

Describe the bonds made around the Fe²⁺ ion.

Answer 2

• There are four coordinate bonds between the Fe²⁺ ion and the nitrogen atoms in the haem structure.
• A further coordinate bond is formed to the protein globin.
• A final coordinate bond can form to an oxygen molecule, which is the transported.

Question 3

Explain why carbon monoxide suffocates humans from oxygen.

Answer 3

• Carbon and oxygen can both bind to haemoglobin at the same binding site.
• Carbon monoxide binds more strongly to haemoglobin than oxygen does.
• Carbon monoxide cannot be removed from haemoglobin, which leaves fewer haemoglobin molecules to bind to oxygen molecules. Tissues are then starved of oxygen because carbon monoxide rather than oxygen is being carried round the body.

Question 4

Why is carbon monoxide referred to as the silent killer?

Answer 4

It is a colourless and odourless gas, so we have no warning if its present in the air around us.

Question 5

Explain the symptoms when exposed to carbon monoxide.

Answer 5

Low levels of carbon monoxide in the blood can cause headaches, nausea and potential suffaction.

Burning tobacco releases the gas and this is one of the reasons why long-term smokers become short of breath.

Question 6

Define the stability constant, K_stab.

Answer 6

The equilibrium constant for an equilibrium existing between a transition metal ion surrounded by water ligands and the complex ions formed when the same ion has undergone ligand substitution.

Question 7

In K_stab why is the water concentration left out in the equation?

Answer 7

This is common practice because all the species are dissolved in water, which is in large excess and its concentration is virtually constant.

Question 8

What does a large value of K_stab mean?

Answer 8

A large value of K_stab indicates that the position of an equililbrium lies to the right. Complex ions with high stability constants are more stable than those with lower values. A large value of a stability constant shows that the ion is easily formed.

Question 9

Why are multidentate complexes so stable?

Answer 9

Bidentate and multidentate ligands, such as EDTA, form particularly stable complex ions. The additional stability of these complex ions results from an increase in entropy.

Ligand substitution by EDTA is accompanied by a large increase in the total number of moles, which increases the entropy.