Alanine (Ala, A (
 
 The amino acid alanine is incorporated into the nascent polypeptide chain during protein biosynthesis in response to four codons—GCU, GCC, GCA, and GCG—and represents approximately 8.3% of the residues of the proteins that have been characterized. The alanyl residue incorporated has a mass of 71.09 Da, a van der Waals volume of 67 ų, and an accessible surface area of 113 Ų. Ala residues are usually relatively variable during divergent evolution, as they are frequently interchanged in homologous proteins with serine, threonine, valine, glutamic acid and proline residues.
 The Ala side chain is simply a methyl group: 

This nonpolar side chain makes Ala residues unreactive chemically, relatively hydrophobic, and not very hydrophilic; consequently, 38% of the Ala residues are fully buried in the folded conformations of proteins. There, the methyl side chain undergoes rapid rotations about the C_a–C_b single bond. Ala has the greatest tendency of all the normal amino acid residues to adopt the alpha-helical conformation in model peptides. It occurs frequently in that conformation in folded protein structures, but at about only half that frequency in beta-sheets and in reverse turns.
References 
T. E. Creighton (1993) Proteins: Structures and Molecular Properties, 2nd ed., W. H. Freeman, New York.