Amino Acids, Proteins and DNA

Question 1

Structure of amino acids?

Answer 1

amine = -NH2

carboxyl = -COOH

organic side chain (R) (exception: glycine, which has H as an R group)

hydrogen

Question 2

Which amino acid does not rotate plane polarised light and why?

Answer 2

glycine, as it is not a chiral molecule

Question 3

Properties of amino acids

Answer 3

Amphoteric:
-can act as an acid or a base

Chiral:
-have a central carbon atom with 4 different groups around it

Question 4

What is a zwitterion?

What happens to it at a low/high pH?

Answer 4

-molecule with both positive and negative ions
-only exist at the amino acid’s isoelectric point
-isoelectric point = pH at which overall charge is 0 = dependent on the R group
-both the carboxyl and amino groups are ionised

When the pH is lower than the isoelectric point
-COO- is likely to accept a H+

When the pH is higher than the isoelectric point
-NH3+ is likely to lose a H+

Question 5

Method to separate and identify amino acids?

Why does it work?

Answer 5

Thin Layer Chromatography (TLC)
-works due to different solubilities

-stationary phase of silica or alumina is mounted on a glass/metal plate
-pencil base line is drawn
-drops of amino acid mixtures added
-place the plate in a solvent, with the base line above the solvent level (why?)
-leave until solvent has moved up to near the top of the plate
-remove, mark the solvent front and allow to dry
-chromatogram has been produced
-amino acids can be identified by their position on the chromatogram

Question 6

Why are amino acids not visible?

What can be used to see them?

Answer 6

colourless compounds

iodine/ninhydrin solution
-colourless spots block any glow from fluorescent dyes
-draw around spots to mark where they are

fluorescent dyes and UV light
-place chromatogram in a sealed jar with a few iodine crystals
-iodine vapour sticks to chemicals on the plate
-dyes them purple
-iodine vapour = locating agent

Question 7

Rf values are fixed for amino acids except when…

Answer 7

temperature, solvent or makeup of the TLC plate changes

Question 8

What are proteins?

Answer 8

-condensation polymer of amino acid monomers
-joined together by peptide link
-dipeptide = 2 amino acid monomers