Understanding Condensation Polymers and Protein Structures

Condensation Polymers

Esterification Reaction: alcohol (OH) + carboxylic acid (COOH) = Ester (COO) + water (H₂O)
Amidation Reaction: amide (NH₂) + carboxylic acid (COOH) = amine + water

Any union of acid and amine produces water.

Natural Polymers:
Protein:
Composed of amino acids (aa). A-AAs are the only proteins involved in the process.
The amino acids are classified in four ways: acidic, basic, polar, and nonpolar.

Acid-Base Character of the Amino Acids:
Amino acids in pure form are solids that crystallize as hydrochlorides (the amino group is protonated). When dissolved in water, they behave as weak acids and bases, reaching a chemical equilibrium characterized by the acidity constant (pKa).

Cationic Form (aa¹⁺): The compound has a positive charge. Example: H₂N¹⁺-C-COOH
Form Zwitterion (aa⁰): The compound is neutral. Example: H₃N¹⁺-C-COO^1-
Anionic Form (aa^1-): The compound is negatively charged. Example: H₂NC-COO^1-

All balances are controlled by pKa (pKa = -log(k₀)). The predominant form depends on the comparison between pKa and pH (pH allows equilibria to shift in either direction).

[H¹⁺] = K_a1 [Haa¹⁺]
[Haa¹⁺] = [H¹⁺] ÷ K_a1

* To find the predominant form, we compare the pH with the pKa (this function predominates when the pH is not greater than the pKa).

Isoelectric Point (pI) = (pKa₁ + pKa₂) / 2
* (pKa₁ and pKa₂ are constants; k is the predominant form).

Peptide Bond: The bond between two amino acids, releasing a molecule of water. This allows the formation of proteins.

H₂NCC-OH + HNC-COOH -> H₂NCC-NC-COOH

Proteins have four levels of organization:
Primary Structure: A sequence of a chain of amino acids.
Secondary Structure: The amino acids interact through hydrogen bonds. There are two types: pleated sheet and alpha helix.
Tertiary Structure: Certain attractions occur between alpha helices and pleated sheets.
Quaternary Structure: A protein consists of more than one polypeptide chain.