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Summary Chemistry Alevel Unit 3.3.13 - Amino Acids Notes
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Chemistry Alevel Unit 3.3.13 - Amino Acids Notes
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Amino AcidsAmino acids have the functional groups - amine (NH2) and carboxylic acid (COOH).
Amino acids are amphoteric so can act as acids and bases.
COOH <=> COO- + H+ (acidic proton donor)
NH2 + H+ <=> NH3 (basic accepts electrons)
They are chiral molecules because the central carbon atom is attached to 4 different functional
groups. They rotate plane polarised light.
The carboxylic acid part of the amino acid can lose a proton to act as an acid.
The amine part can gain a proton to act as a base.
Zwitterions
The no charge form of an amino acid never exists. Amino acids exist as dipolar zwitterions.
This means that it has both a positive and negative charge in different parts of the molecule.
Zwitterions only exist near an amino acids isoelectric point where the pH of the amino acid is 0.
An amino acid becomes a zwitterion when its amino group is protonated to NH3+ and COOH
group deprotonated to COO-.
When conditions are more acidic than the isoelectric point the NH2 is likely to be
protonated and COOH unchanged. The amino acid would carry a positive charge.
When conditions are more basic than the isoelectric point the COOH group is likely to lose
a proton and NH2 group would remain unchanged. The amino acid would carry a negative
charge.
At isoelectric point both the carboxyl group and amino acid group are likely to be ionised
forming a zwitterion.
The ionic bonding between the zwitterions (-NH3+) (-COO-) explains the high melting points of
the amino acids - opposed to the weak hydrogen bonding present in the non charged version.
The charges in a zwitterion mean that strong intermolecular forces of attraction are present
between amino acids. Amino acids are present as soluble crystalline solids.
Isoelectric Point
A solution of amino acids in water will exist as zwitterions with both acidic and basic properties.
They act as buffer solutions which resist changes in pH when small amounts of acids or alkali
are added.
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