Amino Acids

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Amino Acids Intro

Has 2 functional groups. -An amino group (NH2) and a carboxylic acid group (COOH). Structure of all amino acids is the same except the alkyl side chain (R). General structure...

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They are amphoteric molecules--> have both acidic and basic properties.                                          Act as an acid due to -COOH acidic - donates proton:  -COOH   ==    -COO- + H+                               Act as a base because of -NH2 basic - accepts proton:   -NH2 + H+  ==    -NH3^+

They are chiral molecules because the carbon usually has 4 different groups attached. So a solution of a single amino acid enantiomer will rotate plane polarised light.

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Naming Amino Acids

Name an amino acid systematically:

1. Find longest carbon chain, includes carboxylic acid group and write down its name

2. Number the carbon in the chain starting with the carbon in the carboxylic acid group as number 1.

3. Write down positions of any NH2 groups and show that they are NH2 groups with "amino"

4. Write down names of any other funtional groups and identify the carbon they are on.

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Zwitterions

amino acids can exist as zwitterions. Zwitterions are dipolar (both - and + charge in different parts of the molecule). They only exsist near an amino acids isoelctric point - pH where average overall charge on amino acid is zero.

An amino acid becomes a zwitterion when its amino group gets protonated to NH3^+ and it OH group gets deprotinated to OH-.

  • Conditions more acidic than isoelectric point, -NH2 group is likely to be protonated but -COOH unchanged. Amino acid carry + charge but not -
  • Conditions more basic than isoelectric point, -COOH group likely to lose its proton but -NH2 group unchanged. Amino acid carry - charge
  • Onlt at or near isoelectric point are both carboxyl group and amino group likely to be ionised - forming switterion

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Seperating mixtures of amino acids

Use paper chromatography to seperate out and identify different amino acids that make up your solution.

-Draw pencil line at bottom of piece of chromatography paper. Put conc. spot of mixture of amino acids on

-Dip bottom of paper in solvent

-as solvent moves up paper, different amino acids move with it at different rate, so they seperate out.

-Identify each amino acid by looking how far it moves in relation to the solvent and comparing it to a standard.

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Forming Proteins

Proteins are condensation polymers of amino acids - they are made up of lots of amino acids joined by peptide links. Chain put together by condensation reactions and broken by hydrolysis reactions. How amino acids join to make dipeptide:

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To break a protein (Hydrolyse) need harsh conditions. Aqeous 6 M hydrochloric acid is added, and mixture is heated under reflux.

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Hydrogen bonding in proteins

Proteins are not just straigh chains of amino acids. Chains fold or twist due to intermolecular and intramolecuar forces. Hydrogen bonding is one force that holds proteins in shape, Hydrogen bonding exsist between polar groups (e.g. -OH and -NH2)

Groups contain electronegative atoms which induce a partial positive charge on the hydrogen atom, The H is then attracted to lone pairson adjacent polar groups and a hydrogen bond is formed

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