2.2. Amino Acids and their Asymmetric Synthesis

2.2.1. Introduction

• Amino acids are building blocks of proteins.
• Proteins are composed of 20 different amino acid (encoded by standard genetic code, construct proteins in all species).
• Their molecules containing both amino and carboxyl groups attached to the same a-carbon
• 19 are 1°-amines, 1 (proline) is a 2°-amine
• 19 amino acids are “chiral” and 1 (glycine) is achiral (R=H)
• The configuration of the “natural” amino acids is L (L-a-amino acids).

• Their chemical structure influences three dimensional structures of proteins.
• They are important intermediates in metabolism (porphyrins, purines, pyrimidines, creatin, urea etc).
• They can have hormonal and catalytic function.
• Several genetic disorders are cause in amino acid metabolism errors (aminoaciduria - presence of amino acids in urine)

2.2.2. The Basic Structure of Amino Acids

• It differs only in the structure of the side chain (R-group).
• L-isomer is normally found in proteins.

2.2.3. Nonionic and Zwitterionic Forms (Dipolar Structure) of Amino Acids

• Zwitterion = in German for hybrid ion
• The zwitterion predominates at neutral pH
• Isoelectric point (pI): pH at which the amino acid exists in a neutral, zwitterionic form (influenced by the nature of the side chain).
• At acidic pH, the carboxyl group is protonated and the amino acid is in the cationic form 
• At neutral pH, the carboxyl group is deprotonated but the amino group is protonated. The net charge is zero; such ions are called Zwitterions 
• At alkaline pH, the amino group is neutral –NH2 and the amino acids are in the anionic form.

• Amino Acids Carry a Net Charge of Zero at a Specific pH

• Amino acids have characteristic titration curves:

• Henderson/Hasselbach equation and pKa

• Amino Acids Can Act as Buffers