In your chemistry journey, you might have seen the letter "R" in a molecule. Looking at the periodic table, you'll notice that there is no element's symbol is just the letter "R". Is this a mistake? Did the person drawing the molecule forget a letter and is actually referring to radon (Rn) or maybe radium (Ra)?
The truth is, R isn't an element at all, it's actually an abbreviation for a molecule. Keep reading to find out more!
- This article is about R-groups
- First, we will define what an R-group is and why we would use this notation
- Next, we will look at some examples of an R-group
- Then, we will learn about the importance of R-groups for amino acids and learn about the different types
R-Group Definition
Let's start by looking at the definition of an R-group.
An R-group is an abbreviation for any group that contains a carbon and hydrogen atom that is attached to a molecule
You can think of "R" as standing for "Rest of molecule". It is essentially an abbreviation.
R-group chemistry
We use R-groups for two reasons:
To refer to a general functional group without referring to a specific molecule
To focus on another part of the molecule during a reaction
For example, below is what the general formula for a ketone looks like:
Figure 1. General form of a ketone.
Functional groups are classes of molecules with the same reactivity and basic structure. When I mention ketones, I might not want to refer to a specific ketone, which is why you would write down an R group instead. The superscript is to denote that these are different groups. R-groups can also be written like R' or R" to mark that they are different groups.
As another example, take a look at this reaction:
Figure 2. The Hurtley reaction.
In this reaction, the identity of the R-group doesn't matter, which is why it is abbreviated. We instead want to focus on the parts of the molecule that affect its reactivity.
Basically, R-groups are used when the general chemistry/reactivity does not matter, so we want to focus on the "stars" of the molecule.
Example of an R-group
Now that we have a general idea of what an R-group is, let's look at an example of what one looks like. Remember that ketone from earlier? Well, here are some examples of what those R-groups could be:
Figure 3. Examples of R-groups for a Ketone.
When we write molecules like this, each "point" represents a carbon with however many hydrogens it needs to be balanced (i.e., have a total of four bonds). So in our first example, both R1 and R2 are CH3. Even those these are the same, the molecules still count as a ketone. Having R1 and R2 means that these groups can be different.
R-groups and Amino Acids
While R-groups aren't the "star" in many organic molecules, they are incredibly important for identifying amino acids.
Amino acids are molecules containing both a carboxyl (-COOH) and an amino (-NH2) group. These molecules combine to form proteins
Each of the 20 standard amino acids has a specific side chain called an R-group that is also attached to the carbon.
With other organic molecules, R-groups are either stand-ins or simplifications. For amino acids, R-groups are side chains that are specific to each amino acid
There are many shapes, sizes, charges, and reactivities in the R groups. This lets amino acids be put into groups based on how their side chains behave chemically. Some amino acids, like serine, threonine, and asparagine, have side chains that are polar and dissolve in water. Other amino acids, like isoleucine, phenylalanine, and valine, don't like water and are called hydrophobic.
The side chain of the amino acid cysteine is chemically active and can bond with another cysteine. Some amino acids, like lysine, are basic, while others, like glutamic acid, are acidic. Each protein can fold into a certain three-dimensional shape and do a biological job because of the order and interactions between the side chains of the different amino acids.
Structure of Amino Acids and R groups
Below is the general structure of an amino acid:
Figure 4. General Structure of an Amino Acid.
The general structure shows an alpha-carbon bonded to an amino group, a carboxyl group, and an R-group.
An "alpha-carbon" is the first carbon that attaches to a functional group. This separates it from other carbons, such as the one in the carboxyl group.
Types of R-groups
Amino acids can be classified in a few different ways. For example, we can divide them based on polarity, types of functional groups, and acidity.
In this case, we are going to divide these R-groups like this:
Positively charged side chain
Negatively charged side chain
Polar, uncharged side chain
Hydrophobic side chain
Special cases
Hydrophobic species will either repel or not mix with water
Polar species are species with a difference in electronegativity between the different sides of the group
Electronegativity is the tendency for a species to attract/gain electrons
Below is a chart showing the amino acids in these different groups:
Figure 5.The amino acids' side chains classified into different groups
One thing you might notice is the three letter "code" written below the names of each amino acid. These codes are shorthand for the specific side chain/R-group that each amino acid possesses. Instead of writing just "R", we can instead write the three-letter code to denote the chain.
Below is a table with these codes:
One-letter code | Three-letter-code | Name of Amino Acid |
A | Ala | Alanine |
C | Cys | Cysteine |
D | Asp | Aspartic Acid |
E | Glu | Glutamic Acid |
F | Phe | Phenylalanine |
G | Gly | Glycine |
H | His | Histidine |
I | Ile | Isoleucine |
K | Lys | Lysine |
L | Leu | Leucine |
M | Met | Methionine |
N | Asn | Asparagine |
P | Pro | Proline |
Q | Gln | Glutamine |
R | Arg | Arginine |
S | Ser | Serine |
T | Thr | Threonine |
V | Val | Valine |
W | Trp | Tryptophan |
Y | Tyr | Tyrosine |
R-Groups - Key takeaways
- An R-group is an abbreviation for any group that contains a carbon and hydrogen atom that is attached to a molecule
- We use R-groups for two reasons:
- To refer to a general functional group without referring to a specific molecule
- To focus on another part of the molecule during a reaction
- Different R-groups can be marked by number (R1, R2, etc.) or by apostrophes (R', R", etc.)
- Amino acids are molecules containing both a carboxyl (-COOH) and an amino (-NH2) group. These molecules combine to form proteins
- Each amino acid has a unique R-group
References
- Fig.2-The Hurtley Reaction (https://upload.wikimedia.org/wikipedia/commons/thumb/d/da/HurtleyReaction.svg/640px-HurtleyReaction.svg.png) by Dissolution (https://commons.wikimedia.org/wiki/User:Dissolution) licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/)
- Fig.4-General Structure of an amino acid (https://upload.wikimedia.org/wikipedia/commons/thumb/4/45/Amino_acid_generic_structure.png/640px-Amino_acid_generic_structure.png) by PJsg1011 on Wikimedia Commons licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
- Fig.5-The amino acids side chains classified into different groups (https://upload.wikimedia.org/wikipedia/commons/thumb/4/4f/ProteinogenicAminoAcids.svg/640px-ProteinogenicAminoAcids.svg.png) by TungstenEinsteinium on Wikimedia Commons licensed by CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/)
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