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# The amino acid classifications

## Acids, neutral or basic

If the $pH$ of the medium is low ( $[H_3O^+]$ high), the carboxylate ion becomes a neutral carboxyl group, the ammonium ion remains in the positive form:

On the other hand, if the $pH$ of the medium is high ($[OH^-]$ high), the ammonium ion turns into a neutral amino group, the carboxylate ion remains in negative form:

The same applies to ammonium or carboxylate groups in the side chains, for example:

We see that the overall charge of the amino acid depends on the $pH$ of the medium as well as on the functions present in the side chain: - At low pH and side chain already "acid" (presence of carboxylic acid function), the amino acid is globally positively charged. - At low pH and side chain already "basic" (presence of amine function or the like), the amino acid is generally negatively charged. - At $pH \approx 7$ and neutral side chain, the amino acid is globally neutral.

The isoelectric point ($pI$) is the $pH$ where the amino acid is globally neutral. - For basic amino acids ("basic" side chain), $pI \approx 9.5$ - For acidic amino acids ("acid" side chain), $pI \approx 3$ - For neutral amino acids ("neutral" side chain), $pI \approx 6$

## Polar and non-polar

If the side chain of the amino acids only has atoms $C$ or $H$, the difference of →     electronegativity is low, the interactions of the →     Van der Waals type between these side chains are very weak :

If the side chain of the amino acids has strong polar groups, ($O-H$, $-NH_2$ ...), the →   dipole-dipole or →     H-bond interaction between these side chains can be very strong:

Nonpolar amino acids have low non-polar side chains. Polar amino acids have strong polar side chains.

## Examples (Click !)

Lysine is .......... Valine is .......... Serine is .......... Aspartic acid is ..........