19 Biomolecules: Amino Acids, Peptides, and Proteins

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19.2 Amino Acids and The Henderson-Hasselbalch Equation: Isoelectric Points In acid solution an amino acid is protonated, and exist primarily as a cation.

In basic solution an amino acid is deprotonated, and exist primarily as an anion.

In between these two the amino acid is perfectly balances between anionic and cationic forms --> Exist as neutral twitter ion

This pH (between anionic and cationic) is called the amino acid's Isolectrical point, pI.

The isoelectrical point of an amino acid depends on its structure:

- Neutral amino acids, have pI near neutrality

- Acidic amino acids have pI at lower pH so that the extra acidic group doesn't get deprotonated

- Basic amino acids have pI at higher pH so that the extra base group doesn't get protonated

Proteins also have isolectrical points

- with the technique "electrophoresis", a mixture of proteins can be separated into their pure constituents.

19.1 Structure of Amino Acids Zwitterion: A molecule that has an protonated amino group and a deprotonated carboxyl group

Amino Acids are amphiprotic; they can react either as acids or as bases

- In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yields a cation

- In aqueous base solution, an amino acid zwitterion is an acid that loses a proton to yields an anion

All 20 amino acid commonly found in proteins are alpha-amino acids

Alpha-amino acid: The amino acid is a substituent on the alpha-carbon

And 19 of them are primary amines, which means only the side chain on the alpha-carbon is the only difference between the 19 amino acids

The 20th one is "Proline" and is a secondary amine, its nitrogen and carbon atoms are part of a ring.

The 20 common amino acids can be classified as; neutral, acidic or basic - depending on the structure og their side chains.

- An extra amino group would make the amino acid basic.

- An extra carboxyl group would make the amino acid, acidic.