Amino terminus

Carboxyl terminus

Basic chemical structureof an amino acid

alpha (a) carbon

R = side Chain

COO-C+H3N

R

H

Carbons attached to thea-carbon are designatedb, g, d, 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

Stereochemicalform

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 groupsFigure 3.4

Neutral R groups with polar functional groupsFigure 3.4

Positively charged R groups at physiological pHFigure 3.5

Sometimes calledbasic amino acids

Positively charged R groups at physiological pHFigure 3.6

Imidazole ring pKa = 6.0

Negatively charged R groups at physiological pHFigure 3.7

Sometimes calledacidic 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 pKa = 6.0

Figure 3.6

Ionization of histidine

Note: the terminal amino group on allAmino acids has a pKa ~ 8 to 9.5.

Note: the terminal carboxyl group on all amino acids has a pKa ~ 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 3Read Chapter 4