Amino Acids

There are 20 essential amino acids in our body which can combine and form many different peptides and proteins. 

The structures of amino acids are determined by the pH of the solution. The pK1 (or pKa) is the point where caboxylic acid group (-COO-H) lose one proton (Hydrogen). At pK2 (pKb), the amine group (NH3+) will be deprotonated and form NH2. There are vary numbers of pKR, the point where the R-group will be deprotonated. 

Example: 

Aspartic Acid Titration Curve
Source: http://faculty.une.edu/com/courses/bionut/distbio/obj-512/Chap6-titration-aspartate.html