Question

Except for cysteine, only \(S\) amino acids occur in proteins. Several \(R\) amino acids are also found in nature, however. \((R)-\) Serine is found in earthworms, and \((R)\) -alanine is found in insect larvae. Draw Fischer projections of \((R)\) -serine and \((R)\) -alanine. Are these \(D\) or \(L\) amino acids?

Short answer

Both \\(R\\)-serine and \\(R\\)-alanine are \\(L\\) amino acids.

Step-by-step solution

Understand Fischer Projections

Fischer projections are used to represent 3D configurations of carbohydrates and amino acids. In these diagrams, the vertical lines project away from the viewer, while horizontal lines project towards the viewer.

Identify Stereocenters

Serine and alanine each have one chiral center. These chiral centers are the carbon atoms bonded to the amino group (-NH₂), carboxyl group (-COOH), side chain, and hydrogen atom.

Draw \\(R\\)-Serine Fischer Projection

For \(R\)-serine, place the amino group on the left and the carboxyl group on the top in the Fischer projection. The side chain (a hydroxymethyl group: -CH₂OH) should be on the right, and the hydrogen atom on the bottom.

Draw \\(R\\)-Alanine Fischer Projection

For \(R\)-alanine, place the amino group on the left and the carboxyl group on the top, in the same fashion as serine. The side chain is a methyl group (-CH₃) for alanine, placed on the right with hydrogen on the bottom.

Determine D/L Configuration

In Fischer projections, if the amino group is on the left, the amino acid is an \(L\) configuration; if on the right, it is \(D\). Therefore, since both \(R\)-serine and \(R\)-alanine have the amino group on the left, they are \(L\) amino acids.

Key concepts

Understanding Chiral Centers

A chiral center is an atom in a molecule that is attached to four different groups or atoms. This unique configuration results in molecules that can exist as non-superimposable mirror images. This concept is crucial in understanding the three-dimensional structure of molecules, especially in organic chemistry. These configurations are like your hands, where the left hand and right hand are mirror images but cannot be perfectly aligned on top of one another. In the case of amino acids like serine or alanine, the chiral center is typically the carbon atom bonded to an amino group, carboxyl group, side chain, and a hydrogen atom. This setup allows for different optical isomers, known as enantiomers which exhibit distinct properties in biological systems.

Exploring Stereochemistry

Stereochemistry refers to the study of the spatial arrangement of atoms in molecules and how this influences the properties and reactions of the molecules. It plays a vital role in the behavior of molecules. In organic molecules, different spatial arrangements can lead to significant differences. These differences can affect the way molecules interact with other molecules, undergo reactions, and exhibit specific functions. In the case of amino acids, stereochemistry is crucial because it determines how an amino acid fits and functions within proteins. The "R" and "S" notations express the absolute configuration of chiral centers, derived through specific rules set by the Cahn–Ingold–Prelog priority system. Within biological contexts, even slight changes in stereochemistry can translate to vastly different biological activities.

The Role of Amino Acids

Amino acids are the building blocks of proteins and play several essential roles in biological systems. They link together through peptide bonds to form the endless variety of proteins critical for life processes. Each amino acid has a distinct side chain that influences the protein's final structure and function. In terms of composition, they typically feature an amino group, a carboxyl group, a hydrogen atom, and a unique side chain attached to the central carbon atom. This diversity of side chains allows amino acids to contribute to the protein's overall shape and function, influencing everything from enzyme dynamics to signaling pathways. Moreover, the exact structure, including the stereochemistry of the amino acid, is crucial in determining the properties of the resultant proteins.

D/L Configuration Explained

D/L configuration in amino acids is a way to describe the orientation of the amino acid relative to the central chiral carbon atom. The D- and L- nomenclature originates from the Latin words "dexter" and "laevus," meaning "right" and "left," respectively. It is important to note that this system is distinct from the R/S configuration and is specifically used to describe the general orientation in biomolecules. In Fischer projections, which help visualize these three-dimensional configurations, the amino group placement indicates D or L configuration. If the amino group is aligned to the left of the chiral center in the standard Fischer projection, it is considered an L amino acid, while the right orientation corresponds to a D configuration. In the biological context, most naturally occurring amino acids in proteins are of the L configuration, crucial for consistent protein structures in living organisms.