Question

The amino acid threonine, \((2 S, 3 R)-2\) -amino-3-hydroxybutanoic acid, has two chirality centers. Draw the structure using plain, wedged, and dashed lines to indicate stereochemistry.

Short answer

Threonine's structure: - 2S: NH2 is wedged, H is dashed. - 3R: OH is wedged, H is dashed.

Step-by-step solution

Understand the Stereochemistry Notation

The notation (2S, 3R) refers to the configuration of the chiral centers at the second and third carbon atoms of threonine. 'S' stands for 'Sinister' which means the priorities are arranged counterclockwise, and 'R' stands for 'Rectus' which means the priorities are arranged clockwise.

Identify the Chiral Centers

Threonine has a chiral center at the second carbon (with the amino group) and another at the third carbon (with the hydroxy group). Consider each of these centers independently for assigning stereochemistry.

Assign Configuration to Each Chiral Center

For the second carbon, prioritize the groups: NH2 is highest, followed by CHOH, CH3, and H. Arrange them so NH2, CHOH, CH3 form a path counterclockwise (2S). For the third carbon, prioritize: OH, CH3, and CH as seen from the second step and arrange so OH, CH3, CH forms a path clockwise (3R).

Draw the Structure with Stereochemistry

Draw the four carbon backbone of threonine. Attach the NH2 group with a wedge to indicate it coming out of the plane, and the H with a dashed line to indicate it going behind the plane at the second chiral center (2S). For the third chiral center (3R), draw the OH group with a wedge and the H with a dashed line. CH3 remains in-plane in both steps.

Key concepts

Understanding Chiral Centers

Chiral centers are a crucial concept in stereochemistry, specifically in molecules like amino acids. A chiral center is typically a carbon atom that is bonded to four different groups. This molecular arrangement leads to chirality, meaning the molecule can exist in two non-superimposable mirror-image forms, also known as enantiomers. In the case of threonine, there are two such chiral centers. These centers are located at the second and third carbon atoms in its structure, each affecting the molecule's three-dimensional arrangement.

Stereochemistry Notation Explained

When discussing stereochemistry, notation such as R and S is used to denote the configuration of chiral centers. This notation follows the Cahn-Ingold-Prelog priority rules. The label 'R' (Rectus) indicates that when the substituents are prioritized and arranged, they lay in a clockwise order. Conversely, 'S' (Sinister) means the arrangement is counterclockwise once prioritized. For threonine, this notation is used to precisely indicate the spatial arrangement of groups around the chiral centers, providing insight into the molecule’s overall 3D shape.

Configuration Assignment Process

Assigning configuration to a chiral center involves a few steps. First, you prioritize the attached groups based on established rules: atoms with higher atomic numbers take precedence. Take the second chiral center of threonine: the groups are NH2, CHOH, CH3, and H. With NH2 having the highest precedence, these groups are arranged counterclockwise, fitting the '2S' configuration. For the third carbon, the groups OH, CH3, and the carbon backbone create a clockwise arrangement, leading to the '3R' designation. Learning to perfectly assign these configurations helps in understanding the molecule's stereochemistry.

Threonine in Amino Acids Chemistry

Amino acids are the building blocks of proteins, essential to biological processes. Threonine is one such amino acid, notable for its stereochemistry, which affects both its chemical properties and biological function. The presence of multiple chiral centers in threonine means there are several stereoisomers, each with unique properties. In biochemistry, the specific configuration of amino acids like threonine can influence protein structure and function. Understanding the stereochemistry allows chemists and biologists to predict and manipulate these influences for various applications in science and medicine.