16.3
Rx of Amino Acids
Isoelectric Point (IEP)
 pH where AA is a Zwitterion and overall neutral:
positive and negative charges equal each other out.
 Carboxyl is COO-
-1
 Amino group is NH3+ +1
 Net charge
0
Use the textbook (pg 536) and fill the IEP into your AA
sheet….
Charge of AA at specific pH’s
below IEP
at IEP
[H+]
[H+]
[H+]
neutral
negative
positive
above IEP
IEP pH ranges
 Polar neutral and non-polar AA:
IEP 5 - 6
 Acidic AA: IEP ≈ 3 (higher [H+] keeps
sidechain protonated as COOH
 Basic AA: IEP ≈ 7.0-11.0 (lower [H+]
prevents sidechain from protonation,
stays NH2)
Charge of an AA at specific pH’s
 What is the charge of Isoleucine (Ile) at:
a. pH 4.0
b. pH 6.0
c. pH 10
IEP (Ile) = 6.0 (non-polar AA)
Means it’s neutral at exactly pH=6!
a.pH 4: Positive +1
b.pH 6.0: Neutral 0
c. pH10: Negative -1
Acidic AA at different pH’s
 What is the charge of Aspartic Acid (Asp) at
a. pH 2
b. pH 2.8
c. pH 6
IEP is 2.8 it’s a non-polar AA
Means it’s neutral at exactly pH=2.8!
a.pH 2: Positive +1
b.pH 2.8: neutral
b.pH 6: Negative -1 or negative -2
(We can’t tell: need to know the dissociation constant of both
acid groups)
Basic AA at specific pH’s
 What is the charge of Arginine (Arg)at
a. pH 6
b. pH 10.8
c. pH 11
IEP is 10.8
Means it’s neutral at exactly pH=10.8!
a.pH 6: Positive +1 or +2 (don’t know kPa for the two
aminogroups)
b.pH 10.8: Neutral
c. pH11: Negative -1
Peptide bonds
 Carboxyl and Amino
groups react in a
Dehydration Rx
 Amide is formed called a
Peptide bond → Dipeptide
 Order of AA is important
 Glycine + Alanine: Glycylalanine
Alanine + Glycine: Alanylglycine
Naming Peptides
 N-terminal AA and center AA: Replace –ine with –yl
 C-terminal AA name stays same
 Example: Glycine Valine Alanine becomes:
Glycylvalylalanine
1.
2.
3.
 Name reflects number and order of AA
 Defines structure and function
 Name all the other possible versions of how Gly, Val,
Ala can be linked together….
Glycylalanylvaline, Valylglycylalanine, Valylalanylglycine
Alanylglycylvaline, Alanylvalylglycine
o
1
Structure of Peptides
 Beginning and end: N- and C-Terminus
 Peptide backbone: central core of peptide bonds:
 NCCNCCN…..
Peptide Drawing Guide
Draw glycylalanylserine
1. # of AA? 3
2. Draw Peptide backbone: 3 x N-C-C
N-C-C-N-C-C-N-C-C
3. Attach R-side chains to respective central carbon
H
CH3
OH
CH2
N-C-C-N-C-C-N-C-C
 Finish by adding H/O to Peptide Backbone
OH
H
CH3
CH2
H2N-C-C-N-C- C-N-C-COOH
H OH HOHH
a. Finish N/C terminus
b. Add H to central carbon
c. Add Carbonyl to Carboxyl carbon
d.
Add H to Nitrogen