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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
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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