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Why Longer Peptides Cost More: The Yield Math of Stepwise Synthesis (With a Table)
A 10-amino-acid peptide and a 44-amino-acid peptide are not just a difference in size. They are a difference in how hard it is to get a pure result. The reason is arithmetic, and it is simple enough to do on a napkin.
Key takeaways
- Every coupling step has a yield slightly below 100 percent.
- The fraction of chains that are perfect equals the step yield raised to the number of steps.
- At 99 percent per step, a 30-mer is about 74 percent perfect; a 50-mer is about 60 percent; a 100-mer is about 37 percent.
- At 98 percent per step the numbers fall much faster.
- Native chemical ligation joins shorter pieces to reach longer targets.
The formula
If each coupling works 99 percent of the time, the chance that one chain survives all of them is 0.99 multiplied by itself once for each step. For a peptide with n amino acids, there are about n steps. So the fraction of perfect chains is 0.99 to the power n. This ignores losses in deprotection and purification, which only make things worse.
The table
- 10 amino acids: 90% at 99 per step, 82% at 98 per step.
- 20 amino acids: 82% at 99, 67% at 98.
- 30 amino acids: 74% at 99, 55% at 98, 86% at 99.5.
- 50 amino acids: 60% at 99, 36% at 98, 78% at 99.5.
- 70 amino acids: 49% at 99, 24% at 98.
- 100 amino acids: 37% at 99, 13% at 98, 61% at 99.5.
What the missing chains look like
The chains that are not perfect are the deletion and truncation products described in our impurities post. They are separated by chromatography, but because they closely resemble the target, the purification is demanding and costs yield. That is why longer peptides cost disproportionately more and why testing matters more as length increases.
Making it better
- Better coupling reagents and double coupling push step yields toward 99.5 percent or higher.
- Difficult sequences, such as hydrophobic stretches, can be tackled with special solvents and pseudoproline building blocks.
- Heating and microwave-assisted synthesis can shorten cycles.
- Capping unreacted chains makes the impurities easier to separate.
Joining pieces
In 1994, Stephen Kent and colleagues introduced native chemical ligation. One peptide with a C-terminal thioester reacts with another that has an N-terminal cysteine, and the two join with a normal peptide bond. Each piece can be made and purified separately, which avoids the exponential loss for a very long chain. The method opened chemical synthesis of small proteins. Manufacturers of long peptide drugs use related fragment strategies, as in the enfuvirtide story.
What this means when comparing peptides
A short peptide and a long peptide at the same stated purity are not the same achievement. Look for mass spectrometry that confirms the full length and for chromatograms with good resolution. Our guides to HPLC versus mass spectrometry and why purity alone is not enough explain what to look for.
The bottom line
The math of stepwise synthesis explains a lot about peptide quality and cost. Longer chains are not impossible, just harder. For the origin of the method, read Merrifield’s story. See documentation for our compounds on the lab reports page.
Related reading
- Merrifield and solid-phase synthesis
- Deletion and truncation impurities
- The enfuvirtide story
- Browse the full Research Library
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For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.