Digression of amino acidsCissy Gao / Irene Meng / Zero Yao

Today’s Object• Identify amino acids and understand their structure

•Identify the peptide bonds in dipeptides and polypeptides

•Explain why amino acids are soluble in water

What is amino acid?

• An amino acid is a carboxylic acid with an amine group at the 2-position.

– A carboxylic acid contains a –COOH group.– An amine group contains a NH₂ group.

• Amino acids are the building blocks of proteins.A general 2-amino acid

CH₃—CH—COOH

NH₂2-aminopropanoic acid [alanine]

Amino group Carboxyl group

Hydrogen atom

Radical group

Central carbon

S t r u c t u r e

• An amino acid has a central carbon atom bonded to a hydrogen atom and three groups:

– An amino group(-NH₂)– An carboxyl/acidic group(-COOH)– An R group [radical group] “R" can be quite a complicated group containing

other active groups like -OH, -SH, other amine or carboxylic acid groups, and so on.

• There are only 20 different amino acids.– Amino acids differ from one another by their R group

Structure Summary:

Reaction:

• Amino acids can react with both acids and bases.

CH₃—CH—COOH + H⁺

NH₂

CH₃—CH—COOH

+NH₃

Reaction with acids

Reaction with bases

CH₃—CH—COOH + OH⁻

NH₂

CH₃—CH—COO⁻

NH₂

+ H₂O

[alanine]

After reacting with either an acid or base the amino acid is ionic and remains soluble in water.

Two important properties:

1. Amino acids are highly soluble in water.

Amino acids have both acid and base groups arranged such that the acid and base groups can “neutralize” each other.

H atom is lost from COOH and gained by NH₂

IonicAcidic

Basic

zwitterion

2. Amino acids link with each other to form “dipeptides” and “polypeptides”.

Two important properties:

• An amino acid can be referred to as a peptide.• When two amino acids bond together, they produce a dipeptide.

Question #1: When three amino acids bond together, what is it called?

Answer: Tripeptide

Question #2: When more than three amino acids bond together, what is it called?

Answer: Polypeptide

2. Amino acids link with each other to form “dipeptides” and “polypeptides”.

Two important properties:

H—N—CH₂—C—OH + H—N—CH₂—C—OH

H H

O O

H—N—CH₂—C—N—CH₂—C—OH + H₂O

H

O O

H Peptide bond[Amide linkage]

Dipeptide

Dehydration synthesis-water is removed

2. Amino acids link with each other to form “dipeptides” and “polypeptides”.

Two important properties:

• A series of amino acid molecules can be joined by a series of amide linkages to form a polypeptide.

• A polypeptide is a single chain of amino acids.• The order or combination of these amino acids determines which protein is produced.

Practice:

1. Identify which of the following is an amino acid.

2. What is this reaction called ?

Dehydration synthesis

Bonus:

Hydrogen bondBesides peptide bond, what other bond is presented?

Video