14.21

There are two common types of secondary structure of proteins:

(i) α–helix structure

(ii) β–pleated sheet structure

α–Helix structure

If the size of R-groups is quite large then the intramolecular bonds are formed between the C=O of one amino acid and the N-H group of the forth amino acid residue in the chain. This causes the polypeptide chain to coil up into a spiral structure called righ handed α-helix structure.

β -pleated sheet structure

In this conformation, the polypeptide chains lie side by side in a zig0zag manner with alternate R groups on the same side situated at fixed distances apart. The two such neighbouring polypeptide chains are held together by interbounded to form a sheet. These sheets are then stacked one above the other like the pages of the book to form a 3-D structure. This structure resembles pleated folds of drapery and hence is called β–pleated sheet structure. The polypeptide chains can link together in parallel and anti- parallel sequence. Such sheet like structure can easily slip on each other. Proteins of this structure are soft.

14.20 

When two molecule of –amino acids similar or different combine in this way that the amino group of one molecule with carboxy group of the other molecule, this result in the elimination of water molecule and the formation of new bond-CO NH- .

This type of bond is called peptide bond or peptide linkage. The product of reaction is called dipeptide. For example, COOH group of valine combine with NH₂ group of alanine, we get dipeptide Valylalanine (Val-Ala) as final product, as shown below:-

2- Proteins may have one or more polypeptide chains. Each polypeptide in a protein has amino acids linked with each other in a specific sequence and due to this sequence of amino acids the structure of proteins is said to be primary structure.

Any change in this sequence of amino acids creates different proteins. In other words, the sequence in which various amino acids are arranged in a protein is called primary structure. The amino acid sequence of a protein determines its function and it's critical of its biological activity.

3- Each protein in the biological system has a unique 3-D structure and has specific biological This is called native form of a protein. When a protein in its native form is subjected to physical changes such as change in temperature, pH, etc.

hydrogen bonds are broken. Due to cleavage of hydrogen bonds, unfolding of protein molecule occurs and the protein loses its biological activity. This loss of biological activity is called denaturation. During denaturation, 2- degree and 3-degree structures of proteins are destroyed but 1-degree structure remains intact.

As a result of denaturation, globular proteins are converted into fibrous proteins. In other words, denaturation leads to coagulation. That is why coagulated proteins are also called denaturated proteins.

Given -

(i) All the ions are in aqueous state.

Reaction in solution:

AgNO_{3 (s)} + aq → Ag ⁺ + NO^3-

H₂O → H ⁺ + OH ^-

At cathode:

Ag + (aq) + e - →Ag (s)

Ag ⁺ ions have lower discharge potential than H ⁺ ions. Hence, Ag ⁺ ions get deposited as Ag in preference to H ⁺ ions.

At anode:

Ag (s)→ Ag + (aq) + e -

As Ag anode is attacked by NO^3- ions, Ag of the anode will dissolve to form Ag ⁺ ions in the aqueous solution.

(ii) Reaction in solution:

AgNO_{3 (s)} + aq → Ag ⁺ + NO^3-

H₂O óH ⁺ + OH ^-

At cathode:

2Ag ⁺ _(aq) + 2e ^- →2Ag _(s)

Ag ⁺ ions have lower discharge potential than H ⁺ ions. Hence, Ag ⁺ ions get deposited as Ag in preference to H ⁺ ions.

At anode:

2OH^- (aq) → O₂ (g) + 2H⁺ (aq) + 4e -

As anode is not attackable, out of OH ^- and NO^3- ions, OH ^- having lower discharge potential, will be discharged in preference to NO^3- ions. These then decompose to give out O₂.

(iii) Reaction in solution

H₂SO₄ (aq) → 2H⁺ (aq) + SO^2-₄  (aq)

At cathode:

2H + (aq) + 2e - →H₂ (g)

At anode:

2OH^-  (aq) → O₂ (g) + 2H⁺ (aq) + 4e -

∴ H₂ gas is evolved at cathode and O_{2 (g)} is evolved at anode.

(iv) Reaction in solution: CuCl₂ (s) + aq → Cu²⁺ (aq) + Cl^- (aq)

H₂O óH ⁺ + OH ^-

At cathode:

Cu²⁺ (aq) + 2e - →Cu (g)

At anode:

2Cl^- (aq) - 2e^- → Cl₂ (g)

∴ Cu will be deposited at cathode and Cl₂ gas will be liberated at anode.

The electrode reaction is written as,

2Fe³⁺ + 2I ^- → 2Fe²⁺ + I₂

= 0.54V - 0.77V

∴ E⁰_cell = - 0.23 V

It is not feasible, as E⁰_cell is negative, ∴ ?G⁰ is positive.

• The electrode reaction is written as,
• 2Ag⁺ (aq) + Cu(s)→ Cu²⁺ (aq) + Ag(s)

= + 0.80V - 0.34V

∴ E⁰_cell = 0.46V

It is feasible, as E_cell ⁰ is positive, ∴ ?G⁰ is negative.

• (iii) The electrode reaction is written as,
• 2Fe³⁺ (aq) + 2Br^- (aq)→ 2Fe²⁺ (aq) + Br₂

k

= 0.77V - 1.09V

∴ E⁰_cell = - 0.32 V

It is not feasible, as E⁰_cell is negative, ∴ ?G⁰ is positive.

• (iv) The electrode reaction is written as,
• Ag(s) + Fe³⁺ (aq) → Fe²⁺ (aq) + Ag⁺ (aq)

= 0.77V - 0.80V

∴ E⁰_cell = - 0.03

It is not feasible, as E⁰_cell is negative, ∴ ?G⁰ is positive.

• (v) The electrode reaction is written as,
• Br₂ + 2Fe²⁺ (aq) → 2Br^- (aq) + 2Fe³⁺ (aq)

= 1.09V - 0.77V

∴ E⁰_cell = 0.32 V

It is feasible, as E_cell⁰ is positive, ∴ ?G⁰ is negative.