Amino acids Flashcards
What is an amino acid?
Molecule that contains at least one amine group and one carboxylic acid group
These groups are bonded to 1 carbon in between = alpha amino acid
This carbon has an r group
Essential amino acids
Amino acids that cannot be synthesised by the body but must be obtained in diet
Neutral amino acids
1 basic NH2 group and 1 acidic COOH group
Acidic amino acids
One basic NH2 group and 2 acidic COOH groups
Basic amino acids
Two basic NH2 groups and 1 COOH group
Naming amino acids
Carbón Number the amine group is
- amino
Branching
Carboxylic acid
Pure amino acids at room temperature state
Solid
What do amino acids exist as?
Zwitterions
What are zwitterions?
When the COOH group transfers H+ to the basic NH2 group:
COOH becomes COO-
NH2 becomes NH3+
Overall charge of a zwitterion
Neutral: positive NH3+ balanced by negative COO-
But the separate groups in the molecule have a charge
Zwitterions attraction to each other
Positive charge on NH3+ is attracted to negative charge on COO- of the neighbouring zwitterion
Why are amino acids solid at room temperature?
Because the Zwitterions can form strong ionic bonds between each others charges which require lots of energy to overcome
So these are much weaker than hydrogen bonds which are weaker
Why are Zwitterions soluble in water?
Because ion-dipole forces of attraction are formed between hydrogen dipoles and the charges on the Zwitterions
Adding an alkali to amino acid solution
We have a zwitterion in solution
The OH- acts as a base and removes H+ from NH3+ to form water, and the amine group on amino acid becomes NH2 as a result
COO- stays as is
Adding an acid to amino acid solution
H+ is donated to the carboxylate ion to form COOH
Acid/alkali to amino acid always forms..
A Salt of amino acid
Isoelectronic point
The pH at which the zwitterion exists in aqueous solution
Where the NH2 becomes NH3+ and COOH becomes COO-
Do amino acids have a chiral carbon?
Most of them do:
COOH, NH2, H and the R group bonded to a chiral carbon
What happens if amino acids have a chiral carbon?
They can display optical isomerism:
Exist as non superimposable mirror images of each other that rotate the direction of plane polarised light in opposite but equal directions
Racemic mixture of amino acids
Contain equal concentrations of 2 enantiomers
so the rotational effect of plane polarised light in opposite but equal directions is therefore cancelled out: has no effect on the plane of plane polarised light
A peptide bond
Formed in a condensation reaction between the COOH group of 1 amino acid and the NH2 group of a different amino acid
Releases H2O (eliminated)
Peptide bond symbol
C-N
What is a peptide bond also known as and why?
A secondary amide functional group
Because the nitrogen is bonded to C (from its previous amino acid) to C (from new amino acid) to H (from remaining Hydrogen from the amine group that did not get eliminated)
So nitrogen bonded to 1 hydrogen, 2 Carbon (inc C=O) secondary amide
What type of reaction is the formation of a peptide bond?
Acid base condensation polymerisation reaction
H+ from amine group donated and accepted by OH- group from COOH group to form water
Water is eliminated to join 2 molecules = condensation polymerisation
Dipeptide
Condensation reaction between 2 amino acids to form this molecule
Contains a peptide bond
Tripeptide
Condensation reaction between 3 amino acids to form this molecule
Contains 2 peptide bonds
How can we break a peptide bond?
In a hydrolysis reaction which uses water to break the bond and reform the constituent amino acids
How can we separate the amino acids formed?
By chromatgraphy
How do you hydrolyse the polypeptide into amino acids
By refluxing for a long period of time wit concentrated HCl
Protonates the NH2 groups
Polypeptides
Molecule formed from a Condensation polymerisation reaction between 4 or more amino acids, joined by peptide bonds
Amphoteric
A substance that acts as both a base and an acid
What does the solubility in water depend on, outside of forming ion dipole forces to water?
The size and nature of the R group
Why do amino acids not dissolve in non polar solvents?
Because there is a lack in attraction between non polar molecules and zwitterions.
So cannot pull zwitterions apart in the ionic lattice
Optical activity of amino acids
Most have a chiral carbon so can form different enantiomers which rotate the plane of plane polarised light in opposite but equal directions
