3.3.13 Amino acids , Proteins and DNA

Question 1

what’s the structure of an amino acid

Answer 1

have an amino group ( -NH2)
Carboxyl group (-COOH)
Have a hydrogen
Have an R group - but the exception of glycine where this would be a H

Question 2

what does it mean if amino acids are amphoteric

Answer 2

They have acidic and basic properties

Question 3

why are amino acids chiral molecules

Answer 3

They have 4 different groups around a central carbon atom
They rotate plane polarised light

Question 4

how do we name an amino acid

Answer 4

1) find the longest carbon chain
2) number the carbons
3) note the number where the NH2 group sits
4) name any other groups

Question 5

what is a zwitterion

Answer 5

A molecule with both positive and negative ions. Only exist at the amino acids isoelectric point

Question 6

what is an isoelectric point

Answer 6

The pH at which the overall charge is zero. This is dependent on the R group

Question 7

when is a zwitterion likely to be formed

Answer 7

when at pH at the isoelectric point
Both the carboxyl and amino groups are ionised

Question 8

What will happen if the pH is lower than the isoelectric point ( in acidic conditions )

Answer 8

The COO- is likely to accept an H+
The NH3 group become positive

Question 9

what will happen if the pH is higher than the isoelectric point

Answer 9

The NH3+ is likely to lose a H+
The COOH becomes COO-

Question 10

what does Thin layer chromatography (TLC) allow us to do

Answer 10

Allows us to separate and identify amino acids as they have different solubility’s

Question 11

what is the stationary phase of TLC

Answer 11

Uses a stationary phase of silica or alumina mounted on a glass metal plate .
A pencil base line is drawn and drops of amino acids mixture added

Question 12

why must the base line be above solvent line

Answer 12

because if not the amino acid drops would just dissolve in the solvent

Question 13

What happens after the stationary phase in a TLC

Answer 13

Place plate in a solvent
Leave until solvent has moved up to near the top of the plate .
Remove , mark the solvent front and allow to dry

Question 14

What does it mean if the spots of an amino acid are higher up

Answer 14

the amino acid is more soluble

Question 15

what does it mean if the amino acid spot are lower down the chromatogram

Answer 15

The amino acids are less soluble

Question 16

How can we identify amino acids using a chromatogram

Answer 16

We can identify amino acids using the positions on the chromatogram

Question 17

How can amino acids be seen even though they’re colourless

Answer 17

Using iodine / nihydrin solution
Or fluorescent dyes and UV light

Question 18

how can fluorescent dyes and UV light be used to identify amino acids on a chromatogram

Answer 18

Adding a fluorescent dye to the silica / alumina can be seen using a UV light
The colourless spots on the chromatogram will block any glow from the fluorescent dye.
You can then draw around these spots to mark where they are

Question 19

Why is a glass lid used in a TLC

Answer 19

prevents solvent evaporating

Question 20

how can we use iodine / ninhyrdin to find amino acids in a chromatogram

Answer 20

Place the chromatogram in a sealed jar with a few iodine crystals
The iodine vapour sticks to the chemicals on the plate dying them purple
The iodine vapor is known as a locating agent

Question 21

what mathematical value can amino acids be identified from

Answer 21

amino acids can be identified by calculating the Rf value from a chromatogram

Question 22

what do the number of amino acids spots on the plate in a chromatogram tell you

Answer 22

tells you how many amino acids make up the mixture

Question 23

what is the calculation to work out the Rf value of amino acid

Answer 23

Rf = distance travelled by spot / distance travelled by solvent

Question 24

what do we do when we calculated the Rf value to find an amino acid

Answer 24

compare it to the data books to see which amino acid it is

Question 25

What changes the Rf values for an amino acid

Answer 25

If the temperature , solvent or make up of TLC changes

Question 26

EQ : state why each amino acid has a different Rf value (1)

Answer 26

each amino acid has different attraction / solubility in stationary and mobile phases

Question 27

EQ : Suggest how the positions of the amino acids on the TLC plate were located (1)

Answer 27

Using ninhydrin or UV lamp

Question 28

EQ Suggest why it was necessary to use two different solvents (1)

Answer 28

some of the amino acids did not dissolve with the first solvent