Amino acids are the building blocks of proteins. The general structure of amino acids includes an α-amino group, an α-carboxylate group and a variable side chain (R).

 Examples of the Biological significance of amino acids ; Although over 200 different amino acids occur in nature only about one tenth of these occur in proteins, indicating other biological functions performed by these small organic molecules; 1- They are the building blocks of peptides and proteins. 2- As Precursors. Many biologically important molecules are derivates of amino acids. For example Tyrosine is the precursor of the hormone thyroxine and the skin pigment melanin, Tyrosine is also the precursor of a compound abbreviated as DOPA (dihydroxy-phenylalanine). It acts as a neurotransmitter, i.e., trasnmission of impulses in the nervous system, Tryptophan is the precursor of a vitamin named nicotinic acid (B3). 3-As Source of Sulphur. Derived from the sulfur containing amino acids. 4- Amino acids are involved in many metabolic pathways such as in Gluconeogenesis where it is involved in glucose synthesis.

1- Optical activity; All amino acids show optical activity except for glycine, the rest of the amino acids contain at least one asymmetrical carbon atom What is an asymmetrical carbon atom ? It is a carbon atom that is attached to four different chemical groups(four different substituted groups). Why does glycine lack optical activity? Since its” R-group” is a Hydrogen atom thus its α-carbon atom is not asymmetrical.

What does possessing optical activity mean? It means that the amino acid in solution can be present in two isomers; The (dextrarotatory)(+) isomer which has the ability to rotate the plane of polarized light to the right. The (laevorotatory)(-) isomer which has the ability to rotate the plane of polarized light to the left. So both isomers can rotate the plane of polarized light by the same magnitude but in opposite directions.

2- Acid-Base properties of amino acids; All amino acids contain at least two ionizable groups the α-amino group and the α-carboxyllic group,some contain an additional acidic or basic group in their side chain,which are responsible for the amino acids, acid- base behaviour. As a result of their ionizability the following equilibrium reaction can be written; R-COOH R-COO - + H + R-NH 2 R-NH 3 +

Since these reactions are reversible this indicates that they can act as acids (as demonstrated by the forward reaction) or as bases (as demonstrated by the reverse reaction) which explains the acid-base behavior and thus the Amphoteric property of amino acids. What is the Zwitterions form ?  All neutral amino acids are present in the Zwitterions form at physiological pH (around 7.4) the carboxyl group will be unprotonated and the amino group will be protonated.

Amphoteric properties of amino acids; amino acids due to the presence of their ionizable α-amino and α-carboxylic group can act sometimes as acids and sometimes as bases depending on the pH of their media..

 In solutions more basic than the pI of the amino acid, the amino group —NH3 + in the amino acid donates a proton.  In solution more acidic than the pI of the amino acid, the carboxylic group COO- in the amino acid accepts a proton.  Thus behaving sometimes as an acid and other times as a base depending on the pH of the solution.

a)Polarity of amino acids; Since amino acids are present in the Zwitterions form at physiological pH. Where it carries a +ve charge on the α-amino group and a –ve charge on the α-carboxylic group, thus creating two opposite charges On both sides of the molecule thus showing polar properties. b)Isoelectric point pI ; It is that pH at which the amino acid net electrical charge is equal to zero,and thus cannot move in an electrical field.

3-Ultraviolet Absorption Spectrum of Aromatic amino acids; The aromatic amino acids tryptophan, tyrosine, histidine and phenyl alanine absorb ultraviolet light at 280nm,which explains the absorption of proteins at 280nm.

 Amino acids can be present in two Configurations the L-isomer and the D-isomer which are mirror images, which differ in the arrangement of the chemical groups around the α-carbon atom. All amino acids in proteins are in the L-configuration, L-amino acids.

Physical properties of amino acids; 1-Amino acids are mainly water soluble which is explained by its polarity and the presence of charged groups. They are soluble thus in polar solvents and not soluble in non-polar solvents. 2-They have a high melting point reflecting the high energy needed to break the ionic forces maintaining the crystal lattice. It is important to note that the general properties of amino acids is shared by all the amino acids and is in many cases contributed by its α-amino and α- carboxyl group. Amino acids can posses other specific properties dictated by they unique side chain.