Amino terminus Carboxyl terminus Basic chemical structure of an amino acid alpha (  ) carbon R = side Chain

Carbons attached to the  -carbon are designated , , , etc. Serine

The alpha  carbon of amino acids is chiral (except glycine). There are two stereoisomers of amino acids (L and D). Proteins contain L-amino acids Figure 3.1 Nearly all amino acids are chiral

Figure 3.2 Ionization State as a function of pH. At physiological pH, what form of the amino acid dominates?

The 20 amino acids can be separated into 4 major categories: 1. Hydrophobic nonpolar groups 2. Neutral R groups with polar functional groups (e.g., -OH, amide group, -SH groups attached) 3. Positively charged R groups at physiological pH 4. Negatively charged R groups at Physiological pH

Fig 3.3 Amino acids with hydrophobic R groups Stereochemical form Fischer projection

These amino acids are important in the hydrophobic effect. Fig 3.3 Amino acids with hydrophobic R groups

Neutral R groups with polar functional groups Figure 3.4

Neutral R groups with polar functional groups Figure 3.4

Positively charged R groups at physiological pH Figure 3.5 Sometimes called basic amino acids

Positively charged R groups at physiological pH Figure 3.6 Imidazole ring pK a = 6.0

Negatively charged R groups at physiological pH Figure 3.7 Sometimes called acidic amino acids

Figure 3.2 Ionization State as a function of pH. What do we know about these two regions?

Histidine has a third ionizable group Imidazole ring pK a = 6.0 Figure 3.6

Ionization of histidine

Note: the terminal amino group on all Amino acids has a pK a ~ 8 to 9.5. Note: the terminal carboxyl group on all amino acids has a pK a ~ 2 to 3

On your own: Ionization of glutamate What is the complete ionization reaction for glutamate? What will the titration curve look like for this weak acid?

Assignment Read Chapter 3 Read Chapter 4