Leucine or Isoleucine? That is the Question...

Leucine and isoleucine are two very similar amino acids with the only structural difference in the positioning of a methyl group on the carbon chain. However, there are substantial differences. The properties of these two amino acids are summed up in the description below. After reviewing this post, if you are confronted with these two amino acids, you should be able and equipped to answer the opening question, "Leucine or Isoleucine?"

Leucine^1,2

Three Letter Representation: Leu

Two Letter Representation: L

IUPAC Name: (2S)-2-amino-4-methylpentanoic acid

Formula: C₆H₁₃NO₂

Molecular Weight: 131.2

Acidic, Neutral, or Basic: Neutral

pKas: For α-COOH = 2.33, for β-NH₃⁺ = 9.74

pI: 6.04

Isoleucine^1,2

Three Letter Representation: Ile

Two Letter Representation: I

IUPAC Name: (2S,3S)-2-amino-3-methylpentanoic acid

Formula: C₆H₁₃NO₂

Molecular Weight: 131.2

Acidic, Neutral, or Basic: Neutral

pKas: For α-COOH = 2.32, for β-NH₃⁺ = 9.76

pI: 6.04

Structures of these amino acids are given below. On ball and stick representations, only hydrogens on stereogenic carbons are shown.

References:

1) Smith, J. Organic Chemistry, 3rd ed.; McGraw-Hill: New York, 2011; 1076

2) Smith, J. Organic Chemistry, 3rd ed.; McGraw-Hill: New York, 2011; 1078

3)  ACD ChemSketch, version 2009; Advanced Chemistry Development, Inc.; (Accessed March 2, 2013).