Amino Acids

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  • Created by: Hindleyc
  • Created on: 24-03-19 20:53
General structure of an a Amino acid
NH2-CH(R)-COOH
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What can R group be
Variety of different things depending on what AA it is
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What does a (alpha ) AA mean?
Both NH2, COOH groups are joined to the same C
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What is the simplest AA
Glycine, where the R is an H- NH2-CH2-COOH
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What all AA's except glycine
Chiral because there are 4 different groups around the C
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What do they do
Rotate plane polarised light
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How to draw
Enantiomer Line then reflect
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How to name
Use IUPAC eg aminoethanoic acid, 2-amino-3-hydroxypropanoic acid, 2-aminobutanedioic acid
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What do some AA have
An extra Carboxylic acid group or an amine group on the R group. These are classed as acidic or basic (respectively) AA's eg Lycine (basic)
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Solid
both change not R group??
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What form of an AA never occurs?
No charge
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What does the AA exist as
Dipolar zwitterion
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What are they often
Solids
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What does the ionic interaction between zwitterions explains
The relatively high MP of AA as opposed to the weaker hydrogen bonding that would occur in the no charge form
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What is the amine group
Basic
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What is the carboxylic acid group
Acidic
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In neutral solution?
Both
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In High PH (add alkali)
COO- (Na)
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In low PH (add acid)
N+H3 (Cl)
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What do AA act as
Weak buffers and will only gradually change PH if a small amount of acid or alkali are added to the AA
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What will also react and change form in alkaline and acidic conditions
Extra carboxylic acid or amine groups on the R group
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What is a dipeptide
Simple combination molecules of 2 amino acids with one amide (peptide) link
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What are there for any 2 different amino acids
2 possible combinations of the amino acids in the dipeptides
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How do they join
between COOH and NH2, C(O)-NH
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What can the carboxylic acid group and amine group in AA's undergo
the usual rxns of these FGs met in earlier topics
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eg
esterification- amino acid + alcohol with strong acid catalyst = ester + water
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eg
Amino acid + acyl chloride
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What if the R group contains an amine or carboxylic acid
then these will do the same reactions as the alpha amino and carboxylic acid groups
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What if proteins are heated with dilute acid or alkali they can be
Hydrolysed and split back into their constituent AA's
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How may the composition of the protein molecule may then be deduced?
By using TLC chromatography
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Protein + H+
Reform groups and H3N+
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Protein + NaOH
Reform and COO-
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How can a mixture of AAs can be separated and identified
by chromatography, from the amount they have moved
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Method
Thin layer chromatography
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Steps
1. wearing gloves, draw a pencil line 1cm above the bottom of a TLC plate and mark spots for each sample equally spaced along line
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step 2
Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry
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step 3
Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth
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4
place TLC plate into the chamber making sure that the level of the solvents below the pencil line, replace the lid to get a tight seal
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5
When level of solvent reaches about 1cm from the top of the plate remove the plate and mark the solvent level with a pencil, allow the plate to dry in the fume cupboard
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6
Spray paper with ninhydrin and put in oven, draw around them lightly in pencil
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7
Calculate Rf value
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Why wear plastic gloves
To prevent contamination from the hands to the plate
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Pencil line
Will not dissolve in the solvent
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Tiny drop
Too big a drop will cause different spots to merge
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Depth of solvent
If solvent too deep it will dissolve the sample spots from the plate
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Lid
Prevent evaporation of toxic solvent
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How will they get more accurate results
If the solvent is allowed to rise near the top of the plate but the Rf value can be calc if the solvent front doesn't reach the top of the plate
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Why dry in a fume cupboard
As solvent is toxic
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What happens if ninhydrin is sprayed or an AA and then heated for 10 mins
Red to blue spots appear - bc AA are transparent and cannot be seen
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Rf=
Distance moved by AA/ distance move by the solvent
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what does this measure
How far each spot travels relative to the solvent front and calc Rf value
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What won't some substances do
separate bc similar compounds have similar Rf values so some spots may contain more than 1 compound
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Then?
Each AA has its own Rf value then compare Rf values to those for known substances
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Other cards in this set

Card 2

Front

What can R group be

Back

Variety of different things depending on what AA it is

Card 3

Front

What does a (alpha ) AA mean?

Back

Preview of the front of card 3

Card 4

Front

What is the simplest AA

Back

Preview of the front of card 4

Card 5

Front

What all AA's except glycine

Back

Preview of the front of card 5
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