Question

The systematic (IUPAC) name for the amino acid threonine is \((2 S, 3 R)-2\) -amino- 3 -hydroxybutanoic acid. The systematic name indicates that threonine has two stereocenters, at positions 2 and \(3,\) with \(S\) and \(R\) configurations, respectively. Draw the Fischer projection of threonine. How many other possible stereoisomers of threonine are there?

Short answer

Threonine as drawn by its IUPAC name has three other possible stereoisomers.

Step-by-step solution

Drawing the Fischer Projection

Fischer projections provide a simple way to represent molecules with chirality. Chirality is a concept in molecular structure where a molecule cannot be superimposed on its mirror image. We know that threonine is an amino acid with two chiral centers and its IUPAC name is (2 S, 3 R)-2-Amino-3-hydroxybutanoic acid. In a Fischer projection, the horizontal lines represent bonds coming out of the plane (towards you) and vertical lines represent bonds going into the plane (away from you). The vertical line is numbered from top to bottom where the highest oxidized carbon(in this case carboxylic acid) is at Carbon 1. Therefore, the Fisher projection of threonine is drawn as follows: the amino group (NH3+) is on the left (S) at the second position and the hydroxy group (OH) is on the right (R) at the third position.

Calculating the Number of Stereoisomers

Stereoisomers are molecules with the same molecular formula and sequence of bonded atoms, but they differ in the three-dimensional orientations of their atoms in space. A molecule with n chiral centers will have \(2^n\) stereoisomers. Threonine has two chiral centers, so it will have \(2^2 = 4\) stereoisomers, including itself.

Key concepts

Chirality

Chirality is a fundamental concept in stereochemistry, referring to the geometric property of a molecule that makes it non-superimposable on its mirror image. Imagine your hands: they are mirror images of each other, but no matter how you position them, they cannot be perfectly aligned on top of each other. This is analogous to chiral molecules. In chemically relevant terms, molecules with at least one carbon atom bonded to four different substituents exhibit chirality and are known as chiral centers or stereocenters.

Chirality has significant implications in chemistry and biochemistry because chiral molecules often exhibit different biological activities or properties, such as taste or smell. In our exercise scenario, the amino acid threonine contains two chiral centers, which underpins its ability to exist in multiple stereoisomeric forms.

Stereoisomers

Stereoisomers are like twins; they share the same genetic makeup (molecular formula) and have the same physical traits (sequence of bonded atoms) but have distinct personalities (spatial arrangement of atoms). To further illustrate, consider a pair of gloves—they share the same design but are not identical due to their opposite orientation.

In chemistry, stereoisomers are categorized primarily into two types: enantiomers and diastereomers. Enantiomers are mirror images of each other, just like your hands, while diastereomers are non-mirror-image stereoisomers with more than one chiral center. In the case of threonine, each chiral center can exist in two configurations ('S' for sinister, left, and 'R' for rectus, right), leading to four possible stereoisomers, as noted in the exercise.

IUPAC Nomenclature

Just as everyone has a name, every molecule has an 'official' name given by the International Union of Pure and Applied Chemistry (IUPAC). IUPAC nomenclature provides a systematic way to name chemical compounds, ensuring that each compound has a unique and universally accepted name. By considering factors like functional groups, chain length, and the presence of substituents, IUPAC names convey much information about the compound’s structure.

For example, the name '(2S, 3R)-2-amino-3-hydroxybutanoic acid' tells us that we are dealing with a four-carbon acid (butanoic acid) with an amino group (amino) on the second carbon and a hydroxyl group (hydroxy) on the third carbon. The '2S, 3R' designation indicates the stereochemistry around the chiral centers, helping us to draw the correct Fischer projection and distinguish this molecule from its stereoisomers.

Amino Acids

Amino acids are the building blocks of proteins and play a central role in biochemistry. They are organic compounds composed of an amino group, a carboxyl group, and a unique side chain attached to a central carbon, known as the alpha carbon. The side chain determines the characteristics and properties of the amino acid.

Amino acids can be classified into various categories based on their side chains, such as nonpolar, polar, acidic, or basic. Threonine, the amino acid in our exercise, falls into the polar category due to its hydroxyl-containing side chain. Proteins are formed through peptide bonds linking the amino group of one amino acid with the carboxyl group of another, creating a diverse range of structures and functions within living organisms.