1.1 The 20 a-amino acids share common structural features Each has a carboxyl group and an amino group (but one has an imino group in proline) bonded to the same carbon atom designated as the a carbon ∈ 6 5 2 CH2—CH2—CH2—cH2—cH—COO +NH 13 +NH3 Lysine In protein chemistry, we use Greek letter nomenclature. To find all greek letters, go to http://www.astro.uiuc.edu/kaler/sow/greek.html1.1 The 20 -amino acids share common structural features. •Each has a carboxyl group and an amino group (but one has an imino group in proline) bonded to the same carbon atom, designated as the - carbon. In protein chemistry, we use Greek letter nomenclature. To find all Greek letters，go to： http://www.astro.uiuc.edu/~kaler/sow/greek.html