Amino Acid
Amino acids are the building blocks of proteins.

Following are some details about Amino Acids, concentrating on the 2 biologically significant Amino Acids. Let us glance into few details at their simple physical properties such as solubility and melting points.

What are amino acids?
Aminoethanoic acid glycine & alanineStructures and names
Amino acids are compounds containing amino group, -NH2, and a carboxylic acid group, -COOH.
The biologically significant Amino Acids have the amino group attached to the carbon atom next door to the -COOH group. They are identified as 2-amino acids, also known (slightly confusingly) as alpha-amino acids.

2-Aminoethanoic Acid and 2-Aminopropanoic Acid are two simplest amino acids

Due to the biological importance of molecules like these, they are, in general known by their traditional biochemical names.

2-aminoethanoic acid, for example, is usually called glycine, and 2-aminopropanoic acid is usually known as alanine.
The general formula for a 2-amino acid is:

. . . Where "R" can be quite a complicated group containing other active groups like -OH, -SH, other amine or carboxylic acid groups, and so on. It is definitely NOT necessarily a simple hydrocarbon group!
Physical properties of Amino Acid

Melting points
   • The Amino Acids are crystalline solids with astonishingly high melting points.
   • It is complicated to badge the melting points down exactly since the amino acids tend to decompose       before they melt.
   • Decomposition and melting tend to be in the 200 - 300°C range.

This is very high for the size of the molecules. Something unusual must be happening.

Have a look again at the general structure of an amino acid; you will observe that it has both a basic amine group and an acidic carboxylic acid group.

There is an internal transfer of a hydrogen ion from the -COOH group to the -NH2 group to leave an ion with both a negative charge and a positive charge.
This is called Zwitterions.

Zwitterion is a compound with no overall electrical charge, but contains separate parts that are positive and negatively charged.

Amino acids subsist in this form even in the solid state. In spite of the weaker hydrogen bonds and additional intermolecular forces that you might have expected, you actually have much stronger ionic attractions between one ion and its neighbours.

These ionic attractions take more energy to break and so the amino acids have high melting points for the size of the molecules.
Solubility of Amino Acids
   • Amino acids generally soluble in water & insoluble in non-polar organic solvents as hydrocarbons.
   • This again replicates the presence of the zwitterions. In water, the ionic attractions between the ions
     in the solid amino acid are replaced by strong attractions between polar water molecules and the
     zwitterions. This is much the same as any other ionic substance dissolving in water.
   • The extent of the solubility in water varies depending on the size and nature of the "R" group.
   • The lack of solubility in non-polar organic solvents such as hydrocarbons is because of the lack of
     attraction between the solvent molecules and the zwitterions. Without strong attractions between
     solvent and amino acid, there won't be enough energy released to pull the ionic lattice apart.

Optical activity
The general formula for an amino acid (apart from glycine, 2-aminoethanoic acid) and carbon at the centre of the structure has four different groups attached. In glycine, the "R" group is another hydrogen atom.

Because of these four different groups attached to the same carbon atom, amino acids (apart from glycine) are chiral.

The lack of a plane of symmetry means that there will be two stereoisomer’s of an amino acid (apart from glycine) - one the non-super imposable mirror image of the other.

For a general 2-amino acid, the isomers are:

All the naturally occurring amino acids have the right-hand structure in this diagram. This is known as the "L-" configuration. The other one is known as the "D-" configuration.

All the naturally occurring amino acids have the same L- configuration, but they include examples which rotate the plane clockwise (+) and those which do the opposite (-).

For example:
   • (+)alanine
   • (-)cysteine
   • (-)tyrosine
   • (+)valine

It is quite common for natural systems to only work with one of the optical isomers (enantiomers) of an optically active substance like the amino acids. Because the molecules have different spatial arrangements of their various groups, only one of them is likely to fit properly into the active sites on the enzymes they work with.
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