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# The amino acids

## General formula

$R$ is a group of atoms characterizing the amino acid

## Example

The asparagine (asn) is an amino acid which can be extracted from asparagus. Its $R = CH_2CONH_2$ group contains an amide function

## Equilibria in aqueous phase

- As polar substances, all amino acids are soluble in water. - As acids (carboxyl group) and bases (amine group), they undergo acid-base equilibria:

(I) is more favored - in a basic medium - if the $R$ group has a basic character (II) is more favored - in an acidic medium - if the $R$ group has an acidic character (III) is more favored - in a neutral environment - if the $R$ group has no acid or basic character The proportions of the forms (1), (2), (3) and (4) may vary significantly depending upon the $pH$ of the medium or the group $R$.

## Configuration

The "central" $C$ atom of amino acids is asymmetric (except glycine with $R = H$) : Surprisingly, all the natural amino acids have the L Fischer configuration : That does not mean they all have the same R or S configuration :

## Importance

All living creatures function based on proteins that are amino acid polymers.

- The protein insulin -

(Amino acids are designated by their common abbreviations.)