Amino acids
Introduced| English | Meaning |
|---|---|
| amino acid/əˈmiːnəʊ ˈæsɪd/ | An organic molecule containing amino and carboxyl groups; these molecules join to form proteins. |
| peptide bond/ˈpeptaɪd bɒnd/ | A covalent bond joining amino acids, formed between an amino group and a carboxyl group with water removed. |
| zwitterion/zwɪˈtɪərɪən/ | A particle with both positive and negative charges on different parts, but no overall charge. |
| isoelectric point/ˌaɪsəʊˈlektrɪk pɔɪnt/ | The pH at which an amino acid carries no net charge and does not move in electrophoresis. |
| electrophoresis/ɪˌlektrəʊfɔːˈriːsɪs/ | Separating charged molecules such as amino acids by their movement in an electric field. |
The building blocks of proteins
- An amino acid 氨基酸 has both a basic –NH₂ group and an acidic –COOH group.
- Its charge depends on the pH.
- Amino acids join into proteins by peptide bonds 肽键.
Amino acid lab
Classify amino acid behaviour by the group that reacts.
An amino acid contains:
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Having both groups is what makes amino acids behave as they do (zwitterions, peptide bonds).
Amino acids contain both an amino group and a carboxylic acid group.
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They can act as both acid and base (zwitterions).
Zwitterions 两性离子 and the isoelectric point 等电点
- The –COOH can give its $\text{H}^+$ to the –NH₂ in the same molecule → a zwitterion ($\text{H}_3\text{N}^+\text{–CHR–COO}^-$): both ends charged, no overall charge.
- in acid (low pH): gains $\text{H}^+$ → positive.
- in alkali (high pH): loses $\text{H}^+$ → negative.
- at the isoelectric point: mostly the zwitterion, no net charge.
An amino acid is positive, zwitterionic or negative with pH
A zwitterion is:
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The –COOH gives H⁺ to the –NH₂, so both ends are charged but the molecule is overall neutral.
In acidic solution (low pH), an amino acid is:
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In acid it gains H⁺ (positive); in alkali it loses H⁺ (negative); at the isoelectric point it is neutral.
Match each term to its meaning.
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Each item links the term to its correct meaning.
Peptide bonds
- Two amino acids join by condensation (lose water) forming a peptide bond (an amide link) → a dipeptide.
- Repeating this builds a polypeptide, and then a protein.
Two amino acids condense to form a peptide bond
Two amino acids join by a:
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Condensation between –COOH and –NH₂ forms a peptide (amide) bond and releases water.
Worked example: electrophoresis 电泳
- Electrophoresis separates amino acids by charge at a chosen pH.
Glycine has an isoelectric point of about pH 6.
- At pH 2 (below pI): it is positive → moves to the negative electrode (cathode).
- At pH 11 (above pI): it is negative → moves to the positive electrode (anode).
- At pH 6 (its pI): no net charge → it stays put.
So the direction and distance moved identify the amino acid.
Electrophoresis separates amino acids by charge
You've got it
- an amino acid has –NH₂ (basic) and –COOH (acidic)
- zwitterion = no net charge; positive in acid, negative in alkali; neutral at the isoelectric point
- two amino acids condense to form a peptide bond (amide link), losing water
- electrophoresis: below pI → moves to cathode; above pI → anode; at pI → stays put