AMINO ACIDS Lesson Note
LEARNING OBJECTIVES
This SS3 Chemistry lesson note covers AMINO ACIDS for First Term, Week 4.
Lesson Note
LEARNING OBJECTIVES
By the end of lesson, students should be able to:
1. Define amino acids
2. Draw general structure
3. Classify amino acids
4. Explain amphoteric nature / zwitterion
5. Describe peptide formation and properties
6. State uses and tests
DEFINITION
Amino acids are the building blocks of proteins. They are organic compounds containing both amino group (-NH2) which is basic and carboxyl group (-COOH) which is acidic in the same molecule.
1. General formula:
R-CH(NH2)-COOH
Where R = side chain that differs in each amino acid.
2. STRUCTURE OF AMINO ACID
H
|
H2N - C - COOH
|
R
If R = H, it is Glycine (simplest amino acid)
If R = CH3, it is Alanine
The central carbon is called alpha-carbon, so they are alpha-amino acids.
B. CLASSIFICATION OF AMINO ACIDS
a) Based on number of amino and carboxyl groups:
- Neutral: One -NH2 and one -COOH e.g. Glycine, Alanine
- Acidic: One -NH2 and two -COOH e.g. Aspartic acid, Glutamic acid
- Basic: Two -NH2 and one -COOH e.g. Lysine, Arginine
b) Based on essential / non-essential:
- Essential amino acids: Cannot be made by body, must be obtained from food. 10 of them e.g. Valine, Leucine, Lysine, Isoleucine etc.
- Non-essential amino acids: Can be synthesized by body e.g. Glycine, Alanine, Glutamic acid.
c) Based on R group:
- Aliphatic, aromatic, sulphur-containing e.g. Cysteine contains -SH
AMPHOTERIC NATURE AND ZWITTERION
1. Amino acids are amphoteric - they can act as both acid and base.
In aqueous solution, the H from -COOH moves to -NH2 to form:
+H3N - CH(R) - COO-
This is called a Zwitterion (dipolar ion). It has both positive and negative charge, but overall neutral.
2. In acidic medium, it acts as base and forms +H3N-CH(R)-COOH
3. In basic medium, it acts as acid and forms H2N-CH(R)-COO-
C. PEPTIDE FORMATION
When two amino acids combine, -OH from COOH of one and H from NH2 of another is removed as water (condensation). A bond -CO-NH- is formed called Peptide linkage.
- Two amino acids = Dipeptide
- Many amino acids = Polypeptide / Protein
Equation:
H2N-CH2-COOH + H2N-CH2-COOH -> H2N-CH2-CO-NH-CH2-COOH + H2O
This is why proteins can be hydrolysed back to amino acids.
D. PHYSICAL AND CHEMICAL PROPERTIES
Physical:
1. White crystalline solids
2. Soluble in water, insoluble in organic solvents
3. High melting points due to zwitterion
4. Exist as solids not gases
Chemical:
1. Amphoteric / Zwitterion formation
2. Condensation to form peptides
3. React with acids and bases
4. Decarboxylation
5. Ninhydrin test
E. TEST FOR AMINO ACIDS
1. Ninhydrin Test: Amino acid + Ninhydrin solution -> Purple/blue colour on heating. Used to detect protein / amino acid.
2. Biuret Test: For proteins (many peptide bonds) - Protein + NaOH + CuSO4 -> Violet colour.
F. USES / IMPORTANCE
1. Building blocks of proteins for growth and repair of tissues
2. Used in production of drugs
3. Used as food supplements / seasoning (e.g. MSG - monosodium glutamate from glutamic acid)
4. Used in animal feeds
Evaluation
OBJECTIVE QUESTIONS
1. Amino acids are building blocks of:
A. Carbohydrates
B. Proteins
C. Fats
D. Vitamins
2. The general formula of amino acid is:
A. R-CH2-COOH
B. R-CH(NH2)-COOH
C. R-CO-NH2
D. R-OH
3. The simplest amino acid where R = H is:
A. Alanine
B. Glycine
C. Lysine
D. Valine
4. An amino acid with one amino group and two carboxyl groups is:
A. Neutral
B. Basic
C. Acidic
D. Amphoteric
5. Amino acids that cannot be synthesized by the body are called:
A. Non-essential
B. Essential
C. Neutral
D. Basic
6. The dipolar ion formed by amino acids in solution is called:
A. Cation
B. Anion
C. Zwitterion
D. Peptide
7. The bond formed when two amino acids combine is called:
A. Glycosidic
B. Peptide linkage
C. Hydrogen bond
D. Ionic bond
8. The reaction that forms peptide with loss of water is:
A. Hydrolysis
B. Condensation
C. Sublimation
D. Neutralization
9. Which test gives purple/blue colour with amino acids?
A. Biuret
B. Ninhydrin
C. Iodine
D. Fehling's
10. Amino acids are amphoteric because they contain:
A. Only acid group
B. Only basic group
C. Both acid and basic groups
D. No functional group
THEORY QUESTIONS
1. a) Define amino acids and draw the general structure, indicating the amino group, carboxyl group and R group.
b) Explain what is meant by Zwitterion with equation.
c) State two physical properties of amino acids.
2. a) Classify amino acids based on:
i) Number of -NH2 and -COOH groups (with examples)
ii) Essential and non-essential
b) Describe how a dipeptide is formed with chemical equation.
3. a) List three importance / uses of amino acids.
b) Describe two tests for amino acids / proteins.
c) Why do amino acids have high melting points?