Amino  acids  are  composed  of  a
   carboxyl group, an aminoamino group, and an R group bound to a central
   carbon

   The  R  (radical)  group  confers specific chemical properties to each
   amino acid.

   The R groups can be divided into four main classes:
<Li> nonpolar (hydrophobic)
<Li> polar (hydrophilic)
<Li> negatively charged
<Li> positively charged