So you might see a molecular formula written as C₁₀₀ H₁₅₀ N₂₀ O₃₀ S₂. The small numbers tell us how many of that atom there are. The formula is a brief description of the overall composition of the molecule, which is not the same as a chemical name.
Determining a molecular formula
The molecular formula for a peptide is determined by the amino acid sequence and any chemical modifications present in the molecule. The combination of amino acids with peptide bonds releases water in the reaction. Hence, the overall molecular formula of a peptide is not a simple sum of the formulas of the individual free amino acids.
Modifications can also change the formula. For example, acetylation, amidation, phosphorylation or oxidation can add, remove or alter particular atoms. The reported formula should therefore account for the complete structure being tested, rather than only the basic amino acid chain.
Why molecular formula matters
The molecular formula is useful for researchers to understand the elemental composition of a peptide, and it is the basis for calculating the theoretical molecular weight of the peptide.
It can also be useful to judge if a reported peptide identity is reasonable. Molecular weight alone is not always enough information. Two different molecules can have the same molecular weight but different structures. The molecular formula gives another useful piece of information about the compound.
A molecular formula is also useful when comparing related peptide compounds or modified versions of the same peptide. A small structural change can alter the number of certain atoms and therefore produce a different molecular formula. This can be particularly useful when reviewing analytical data or checking whether a modification has been included in the reported compound.
Molecular formula vs molecular structure
A molecular formula tells you what atoms are present and how many of each, but it doesn’t tell you how those atoms are connected. So, two compounds can have the same molecular formula but different arrangements of atoms. These are called isomers.
The amino acid sequence for peptides gives more structural information than the molecular formula alone. The formula should therefore be considered alongside the sequence and stated modifications when identifying a peptide.
How molecular formula is verified
The molecular formula is normally calculated from the known peptide structure or sequence. Mass spectrometry (MS) can then provide measured mass information that can be compared with the theoretical value calculated from the formula.
When verifying a peptide product or Certificate of Analysis (CoA), compare the reported molecular formula to the peptide’s sequence, modifications and molecular weight. These details should be consistent with one another.
Related terms: Molecular Weight · Molecular Mass · Peptide Sequence · Amino Acid · Peptide Bond · Chemical Modification · Mass Spectrometry · Molecular Structure · Isomer · Certificate of Analysis
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Research Use Only. This glossary explains terminology used in published research. It is not medical advice and does not describe any product for human or animal use.