L-Threonine (2S,3R) vs L-allo-Threonine (2S,3S) - Isomer Relationship

Introduction

Threonine is an essential amino acid with two chiral centers. Its diastereomer, allo-threonine, has the same connectivity but a different configuration at the beta carbon.

Formula Comparison

Both have the formula C₄H₉NO₃ — an amino acid with a hydroxyl-bearing side chain. The molecular formula is identical.

Connectivity Analysis

Both molecules have the same bonding: NH₂–CH(COOH)–CH(OH)–CH₃. The amino group is on C2 (alpha carbon) and the hydroxyl is on C3 (beta carbon). Connectivity is identical.

Spatial Relationship

C2 has the S configuration in both molecules. The difference is at C3: threonine is (2S,3R) while allo-threonine is (2S,3S). The OH and H on C3 have swapped positions.

Chirality Check

Both C2 and C3 are chiral centers. C2 bears NH₂, H, COOH, and CH(OH)(CH₃). C3 bears OH, H, CH₃, and CH(NH₂)(COOH). Only C3 is inverted between the two molecules.

Classification

These are diastereomers. They have the same configuration at one stereocenter (C2 = S) but opposite at the other (C3: R vs S). Since not all centers are inverted, they are not enantiomers.

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