# 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.

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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 above IEP [H+] [H+] [H+] positive neutral negative

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

1 o 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 OH H CH3 CH2 N-C-C-N-C-C-N-C-C

Finish by adding H/O to Peptide Backbone OH H CH3 CH2 H 2 N-C-C-N-C- C-N-C-COOH H O H H O H H a. Finish N/C terminus b. Add H to central carbon c. Add Carbonyl to Carboxyl carbon d. Add H to Nitrogen

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