Part I – The uncomfortable truth about peptide purification
Most peptide purification problems are not purification problems. They are impurity problems.
When people think about peptide manufacturing, the conversation often starts with purification technology:
– RP-HPLC.
– Prep LC.
– Columns.
– Gradients.
But the real question should come earlier. What exactly are we trying to remove?
In chemically synthesized peptides, the crude mixture is rarely simple.
Solid-phase peptide synthesis routinely generates a whole spectrum of peptide-related impurities such as:
• deletion sequences
• insertion sequences
• epimers
• aspartimides
• oxidized or deamidated variants
The difficult part is that many of these impurities differ from the target peptide by only a single amino acid or a small chemical modification.
From a separation perspective, that is a nightmare. Because now purification is not about removing completely different molecules. It is about separating molecules that behave almost identically.
This is the reason why peptide purification often looks the way it does today.
Not because chemists love RP-HPLC. But because the impurity profile leaves them little choice.
If these peptide-related impurities didn’t exist, many peptide processes could probably run on FPLC instead of RP-HPLC.
And that would change quite a lot.
Part II: If peptides had cleaner impurity profiles
Here is a thought experiment.
Imagine synthetic peptides without peptide-related impurities.
– No deletion sequences.
– No epimers.
– No near-identical variants.
Just the target peptide plus some process-related impurities.
In that world, purification would look very different.Because for most process-related impurities, you don’t need extreme chromatographic resolution.
Charge differences, size differences, or hydrophobicity gaps are usually sufficient to run a purification.
This means that many peptide purifications could run on FPLC-type workflows, such as:
• ion exchange
• mixed-mode chromatography
• size exclusion
• simple polishing steps
These methods are attractive for obvious reasons like the following:
• scalable
• water-based
• lower solvent consumption
• easier process robustness
But in real peptide manufacturing, they often hit a wall.
And the reason is simple: product-related impurities behave almost exactly like the target peptide. They differ by a single amino acid, stereochemistry, or a small modification, which makes them chromatographically extremely difficult to separate.
At that point, process developers need maximum resolution.
And that usually leads straight back to RP-HPLC.
So the real question is not – Why do we use RP-HPLC?
The better question is – Why do synthetic peptides generate so many product-like impurities in the first place?
That’s where the real story begins.
Part III- The real root cause
Why does peptide purification so often end with RP-HPLC?
The answer is surprisingly simple.
Chemical peptide synthesis generates peptide-related impurities and those impurities are extremely hard to separate.
During solid-phase peptide synthesis, several side reactions occur naturally such as:
• incomplete couplings → deletion sequences
• racemization → epimers
• aspartimide formation
• oxidation or deamidation
The result is a crude mixture containing molecules that are almost identical to the target peptide.
From a purification perspective, this creates the worst possible situation.
One is trying to separate molecules that have the following properties:
• have nearly identical size
• nearly identical charge
• nearly identical hydrophobicity
That’s why peptide purification often requires high-resolution RP-HPLC.
Not because it is convenient. But because the impurity profile demands it.
Interestingly, this raises a much bigger question. What would peptide purification look like if the production process generated fewer peptide-related impurities in the first place?
Because if those impurities disappear, a lot of downstream complexity disappears with them. And that’s exactly where the field is starting to move.
Recombinant systems, biochemical production strategies, and hybrid approaches are beginning to change the impurity landscape.
But that’s a bigger topic.
So in the next posts, we will look at:
How alternative production systems could fundamentally simplify peptein purification.