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    I recently sat an OCR chemistry past paper in which one of the questions was to suggest which R-groups on an amino acid would cause an isoelectric point to be above a pH of 10 and another for it to be below 3. I was wondering if anyone would be able to explain how it is that the R-Group of an amino acid effected its Isoelectronic point as it hasn't been covered in my class.

    Ps the answers were a carboxyl group for below 3 and an Amino group for above 10.
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    (Original post by TheBarnHouse)
    I recently sat an OCR chemistry past paper in which one of the questions was to suggest which R-groups on an amino acid would cause an isoelectric point to be above a pH of 10 and another for it to be below 3. I was wondering if anyone would be able to explain how it is that the R-Group of an amino acid effected its Isoelectronic point as it hasn't been covered in my class.

    Ps the answers were a carboxyl group for below 3 and an Amino group for above 10.
    The 'R' group is the 'other' bit of an amino acid, which also must have a carboxylic acid group and an amine group.

    At low pH values the acid exists as COOH and the amine group as NH3+ ions
    At high pH values the acid exists as COO- ions and the amine as NH2.

    At the isoelectric point the concentration of negative COO- ions equals the concentration of NH3+ ions.

    If the 'R' group contains a further acid group then this makes the amino acid much more acidic.

    A lower pH is then needed to ensure that the concentration of the NH3+ groups is equal to the concentration of the COO- groups (this reduces the concentration of COO- ions and increases the concentration of NH3+ ions)
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    (Original post by charco)
    The 'R' group is the 'other' bit of an amino acid, which also must have a carboxylic acid group and an amine group.

    At low pH values the acid exists as COOH and the amine group as NH3+ ions
    At high pH values the acid exists as COO- ions and the amine as NH2.

    At the isoelectric point the concentration of negative COO- ions equals the concentration of NH3+ ions.

    If the 'R' group contains a further acid group then this makes the amino acid much more acidic.

    A lower pH is then needed to ensure that the concentration of the NH3+ groups is equal to the concentration of the COO- groups (this reduces the concentration of COO- ions and increases the concentration of NH3+ ions)
    Ok so to summarise it just means the pH has to change to allow the concentration of OH and H ions to be at a point where the ratio of COO- and NH3+ are equal. If that is the case it makes sense so thankyou and I think I might have just been trying to overcomplicate things a little bit. Trying to find extra reasons that I didnt need, as I was unable to use what I already knew to explain it.
 
 
 
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