Plasmid DNA and mRNA purification
Nucleic acids are large, highly charged and buffer-hungry. What that means for the chromatography step — and, for mRNA, an honest account of which parts of the process we cover and which we do not.
Plasmid DNA and mRNA are grouped together because they share a problem rather than a process. Both are large, both carry a high negative charge that makes anion exchange the natural handle, and both are produced in processes where buffer consumption is a real cost rather than a rounding error. Beyond that the routes diverge considerably.
Plasmid DNA
A therapeutic plasmid is typically 5 to 16 kbp — a molecule of a few hundred nanometres in its relaxed form, far larger than any protein. Size is what shapes the purification: the supercoiled form is the product, the open-circular and linear forms are impurities, and they differ from it in shape rather than in chemistry.
Flow-path width matters here in a way it rarely does elsewhere. A study on anion exchange monoliths at 2, 3 and 6 µm found the open-circular isoform captured convectively in the narrower channels, costing yield and building irreversible back-pressure; channels wider than about 3.5 µm were required for plasmids up to 16 kbp, and 6 µm was sufficient (Electrophoresis, DOI 10.1002/elps.202300035). MonoCore™ Q ships with 15 µm channels as standard. Wider is not automatically better — a channel is chosen against the molecule it has to carry, and the one that suits a 16 kbp plasmid is not the one you would pick for a small protein.
The route is usually alkaline lysis, clarification, a capture or intermediate step on anion exchange, and a polishing step — often HIC, which separates the supercoiled form from the open-circular one by the difference in exposed hydrophobic surface.
mRNA — and what we cover
An mRNA process looks different. After in-vitro transcription the product has to be separated from the DNA template, from the enzymes, from truncated transcripts and — the hard one — from double-stranded RNA by-products, which are immunogenic and closely related to the product.
The usual capture step is oligo-dT affinity, which binds the poly(A) tail. We do not make an oligo-dT capsule. Where our chemistries fit is afterwards: anion exchange for template and fragment clearance, and HIC for the dsRNA separation, which exploits the different surface the double-stranded species presents.
We would rather state that boundary plainly than let an application page imply a complete mRNA offering. If oligo-dT capture is what you need, we are not your supplier for that step.
Buffer load
Nucleic-acid processes are buffer-intensive, and at development scale buffer preparation often consumes more time than the chromatography does. Convective transport keeps individual runs short, and a shorter run uses less buffer per cycle. Whether that adds up to a meaningful saving depends on your method — the run planner lets you put your own numbers in rather than take ours.
- Why does channel width matter for plasmid DNA?
- Because a large plasmid is a physically big, flexible molecule — a 16 kbp plasmid extends over hundreds of nanometres. In a study at 2, 3 and 6 µm, flow paths narrower than about 3.5 µm trapped the open-circular isoform, which showed up as lost yield and irreversible back-pressure; 6 µm was sufficient for plasmids up to 16 kbp (Electrophoresis, DOI 10.1002/elps.202300035). MonoCore™ ships with 15 µm channels. That is a channel chosen for large molecules, not a setting that is better in general.
- How is supercoiled plasmid separated from open-circular?
- Most often by hydrophobic interaction chromatography. The two forms have the same chemistry but different shapes, and the relaxed open-circular form exposes more hydrophobic surface — which is what HIC responds to. Anion exchange alone usually does not resolve them well, because their charge is nearly identical.
- How is mRNA purified after in-vitro transcription?
- Typically oligo-dT affinity capture binding the poly(A) tail, then steps to clear the DNA template, the enzymes, truncated transcripts and double-stranded RNA by-products. The dsRNA separation is the demanding one, since those species are closely related to the product and immunogenic.
- Do you offer an oligo-dT capsule for mRNA capture?
- No. We do not make an oligo-dT chemistry. Our chemistries fit the steps after capture — anion exchange for template and fragment clearance, HIC for the double-stranded RNA separation. If oligo-dT capture is your requirement, we are not the supplier for that step.
- Which MonoCore™ chemistry is used for plasmid DNA?
- Anion exchange (MonoCore™ Q) for the capture or intermediate step, since plasmid DNA is strongly negatively charged. HIC is the usual partner for polishing, where the supercoiled and open-circular forms have to be resolved.