Question

The amino acid threonine, \((2 S, 3 R)-2\) -amino- 3 -hydroxybutanoic acid, has two chirality centers. (a) Draw threonine, using normal, wedged, and dashed lines to show dimensionality. (b) Draw a diastereomer of threonine, and label its chirality centers as \(R\) or \(S\).

Short answer

For part (a), draw the correct (2S, 3R) configuration of threonine. For part (b), change one chirality center, e.g., (2S, 3S) or (2R, 3R) for a diastereomer.

Step-by-step solution

Understand the molecular structure of threonine

Threonine is a 4-carbon amino acid with two chirality centers at C2 and C3. The components are: an amino group (-NH₂) attached to C2, a hydroxy group (-OH) at C3, and a methyl group (-CH₃) as the side chain.

Applying chirality and wedge-dash notation

For the given configuration - At C2: (2S) configuration implies that the priority order is such that the lowest priority group (likely hydrogen) is directed away, and other groups are viewed counterclockwise. - At C3: (3R) configuration implies that the groups should be viewed clockwise when arranged in priority, with the lowest priority group again facing backwards. Use the wedge to show the bond coming out of the plane (above), the dash for the bond going into the plane (below), and solid lines for bonds in the plane to show spatial arrangement.

Draw threonine

First draw the backbone carbon chain. Then place the amino group on C2 and hydroxy group on C3 according to the priority of the attached groups, keeping in mind the (2S, 3R) notation. Adjust the orientation using wedges and dashes to reflect stereochemistry.

Understand diastereomers and chirality

Diastereomers are stereoisomers that are not mirror images. By changing the configuration at just one of the chirality centers (either C2 or C3) while keeping the other the same, you create a diastereomer.

Draw a diastereomer of threonine

To create a diastereomer, change either C2 or C3 configuration. - Keeping C2-S, change C3 to S. At C2, maintain the original orientation and flip configuration at C3 such that it is viewed counterclockwise (3S) considering priority. - Alternatively, keep C3-R and change C2 to R following a similar change process. Adjust wedges and dashes accordingly to show the new spatial arrangement.

Key concepts

Amino Acids

Amino acids are the building blocks of proteins. They are organic compounds made up of an amine group

• Visualize: contains -NH₂ group
• Serves as a basis for peptide bonds in protein formation
• Each has a unique side chain that defines its properties

Amino acids like threonine have a standard structure with a central carbon atom, which is referred to as the alpha carbon. Attached to this alpha carbon are typically

• an amino group (-NH₂)
• a carboxylic acid group (-COOH)
• a hydrogen atom
• a distinct side chain or R group

The side chain can vary greatly between different amino acids, impacting their chemical behavior and role in proteins.

Stereochemistry

Stereochemistry revolves around the 3D arrangement of atoms in molecules. This field is crucial in determining how molecules interact with biological systems. It considers how the different spatial arrangements of atoms affect the properties and reactions of the molecules. When we talk about molecules like threonine,

• Stereochemistry dictates how the atoms are configured in space
• It influences the reactivity and behavior of molecules in biological systems
• Threonine, for instance, has two stereocenters

These are points in the molecule where the interchange of two groups produces a stereoisomer.

Diastereomers

Diastereomers are a type of stereoisomer. Unlike enantiomers, which are mirror images of each other, diastereomers are not mirror images. In amino acids like threonine, diastereomers can be formed by altering the configuration at one or more chirality centers. Important points:

• Diastereomers differ at one or more stereo centers
• They have different physical and chemical properties
• Almost always, they are easier to separate and identify compared to enantiomers

In practice, finding diastereomers involves changing the R/S configuration at specific chiral centers in the molecule.

Threonine

Threonine is one of the essential amino acids for humans, meaning it must be obtained through our diet. Structurally, it is a bit complex:

• Contains two chiral centers
• Has a side chain with a hydroxy group (-OH), which is a key feature for its function
• Plays important roles in protein construction and is involved in tissue synthesis and immune function

The unique structure of threonine allows it to form secondary structures in proteins such as alpha helices and beta sheets, contributing to the overall protein shape and function.

Chiral Center

A chiral center is an essential feature in stereochemistry. It is a carbon atom bonded to four different groups, resulting in non-superimposable mirror images, known as enantiomers. In threonine, there are two chiral centers located at the

• Second carbon (C2): labeled as (2S) in threonine
• Third carbon (C3): labeled as (3R) in threonine

These chiral centers are critical because their spatial arrangement affects the molecule's interaction with light and other molecules. Understanding the stereochemistry of these centers helps in predicting threonine’s behavior and interactions within biological systems.