Why DNA and RNA Co-Purify With Your Protein: Fixing High A260/A280 and Viscous Lysates

Your lysate pulls like syrup, the affinity column pressure climbs, and the “pure” elution has an A260/A280 ratio close to 1.0. The gel may show one dominant protein band, but the sample is carrying DNA, RNA, or both. That contamination can inflate concentration estimates, broaden SEC peaks, disrupt binding assays, and make an otherwise soluble protein appear aggregated.

Nucleic-acid contamination is not one problem with one cure. Genomic DNA released during lysis creates viscosity. RNA is more abundant by mass in rapidly growing bacteria and often survives clarification. Basic or nucleic-acid-binding proteins can retain either species through every chromatography step. The right fix depends on whether the contaminant is merely trapped in the lysate, bound to the target, or binding the resin independently.

Key Takeaways

• Diagnose before treating. Viscosity, absorbance ratios, nuclease response, salt response, and an agarose gel distinguish bulk carryover from a stable protein–nucleic-acid complex.
• Shear is not digestion. Mechanical shearing lowers viscosity but leaves short polyanions that can remain bound to basic proteins.
• Nuclease requires the right chemistry. Benzonase-like nucleases need a compatible divalent cation and lose effectiveness when added after chelators, denaturants, or high salt.
• Salt breaks many electrostatic complexes. A 0.5–1.0 M NaCl extraction or wash is often the fastest test, but some proteins destabilize or require nucleic acid for function.
• Polyethyleneimine can remove nucleic acids at scale. It is powerful but empirical: excess polymer also precipitates acidic proteins and can be hard to reverse.
• Use orthogonal cleanup when the target truly binds nucleic acid. Heparin, ion exchange, and carefully staged nuclease treatment are usually more reliable than repeating IMAC.

First, Identify Which Failure You Have

Failure A: The lysate is physically viscous

Long genomic DNA is the usual cause. Pipette tips clog, clarification is poor, and the sample forms strings between the pipette and tube. The danger is mechanical as well as chemical: viscous feed distributes badly through a column and can trap host proteins and particulates.

Failure B: The clarified lysate flows, but A260 remains high

Short DNA fragments and RNA contribute strongly at 260 nm without obvious viscosity. A low A260/A280 ratio does not prove purity, and a high ratio does not identify the exact species, but the trend is useful. For a purified protein with aromatic residues, A260/A280 is usually well below the values typical of isolated nucleic acid. Interpret the ratio against a buffer blank and the protein’s sequence-derived extinction coefficient.

Failure C: Nucleic acid follows the target through purification

This is common for DNA/RNA-binding proteins, positively charged proteins, ribosomal proteins, and proteins with basic patches. If nuclease treatment shifts the SEC profile or releases the protein from a high-molecular-weight peak, you were purifying a complex rather than two independent contaminants.

Failure D: The affinity resin enriches nucleic acid indirectly

Nucleic acid can bind host proteins that also bind IMAC resin, or it can bridge the target to contaminants. A clean Coomassie lane therefore does not establish that the sample is chemically homogeneous.

A Small Diagnostic Panel Saves a Large Purification

Split 200–500 µL of clarified lysate or first-step eluate into matched aliquots:

1. untreated control;
2. nuclease plus the required Mg²⁺;
3. 500 mM NaCl;
4. nuclease plus Mg²⁺ followed by 500 mM NaCl;
5. optional EDTA control after nuclease digestion is complete.

Incubate under conditions your target tolerates, then compare viscosity, A260/A280, agarose-gel signal, soluble recovery, and analytical SEC. This matrix answers two questions: can the nucleic acid be cut, and does electrostatic screening release the fragments from the protein?

If salt alone collapses the high-molecular-weight SEC shoulder, binding is largely electrostatic. If nuclease changes viscosity but not the target peak, short fragments still bind. If neither treatment changes the profile, investigate oligomerization, membrane association, or irreversible aggregation instead.

Why Mechanical Shearing Only Solves Half the Problem

Sonication, homogenization, and repeated passage through a narrow needle reduce the length of genomic DNA. That can restore manageable viscosity and improve clarification. It does not remove the phosphate backbone.

A 20 kb DNA molecule and a collection of 100 bp fragments contain the same total negative charge. The fragments diffuse faster, enter resin pores more readily, and may stay bound to a positively charged target. Aggressive sonication can also heat the sample, introduce air–liquid interfaces, and damage a fragile protein.

Use mechanical disruption to achieve complete lysis and modest shearing. Use enzymatic or chromatographic chemistry to remove the nucleic acid.

Nuclease Treatment That Actually Works

Add it early

Treat the lysate before clarification when possible. Nuclease has access to the long polymers, viscosity falls before centrifugation or filtration, and debris releases trapped protein.

Check the cofactor

Many nonspecific nucleases require Mg²⁺. If the lysis buffer contains EDTA or another chelator, the enzyme may be present but inactive. Add the manufacturer-recommended free Mg²⁺ concentration, accounting for chelators and phosphate.

Control salt, pH, and temperature

Very high salt can slow some nucleases, while room-temperature incubation can accelerate target proteolysis. A practical compromise is a short treatment at cool temperature with enough activity to reduce viscosity, followed immediately by clarification.

Do not overload the digestion with impossible substrate

If the lysate is extremely dense, dilute it or add nuclease in stages. Confirm the endpoint by loss of stringiness and by running extracted material on an agarose gel. “We added nuclease” is not the same as “the nucleic acid was digested.”

Remember that digestion produces binding-competent fragments

Nuclease often needs a partner intervention: salt, a polyanion competitor, heparin chromatography, or ion exchange. For a strong nucleic-acid-binding protein, digestion reduces the complex from huge to merely heterogeneous.

High-Salt Extraction and Washing

Electrostatic protein–nucleic-acid interactions weaken as ionic strength increases. A useful screening series is 150, 300, 500, 750, and 1,000 mM NaCl in a small lysate aliquot.

Monitor three outcomes:

• target recovery in the soluble fraction;
• A260/A280 after capture;
• monodispersity after returning to the final buffer.

For His-tagged proteins, a 500–750 mM NaCl IMAC wash can remove nucleic acid and associated host proteins without changing the imidazole program. If your protein fails to bind the resin, diagnose that separately using our His-tagged protein and Ni-NTA troubleshooting guide.

Salt is not universally benign. It can screen favorable intramolecular contacts, dissociate required subunits, or precipitate some intrinsically disordered proteins. Dialyze or desalt gradually, and compare activity before and after the excursion.

Polyethyleneimine: Powerful, Cheap, and Easy to Overtitrate

Polyethyleneimine (PEI) is a positively charged polymer that complexes with negatively charged nucleic acids and precipitates them. Classic studies established selective nucleic-acid precipitation with PEI as a scalable biochemical operation (Atkinson and Jack, 1973; Cordes et al., 1990).

The operating window depends on lysate composition, pH, ionic strength, PEI molecular weight, and target charge. Never transfer a literature percentage directly to a large batch.

Run a titration

1. Clarify gross debris while keeping the lysate cold.
2. Prepare a defined PEI stock at a measured pH.
3. Add increasing final concentrations to equal lysate aliquots.
4. Mix gently for a fixed time.
5. Centrifuge and assay both supernatant and pellet for target, A260, and total protein.
6. Choose the lowest concentration that removes most nucleic acid while retaining target recovery.

If the target itself is acidic or exists in a nucleoprotein complex, it may precipitate with the nucleic acid. Recovery can sometimes be tuned with salt, but the correct answer may be heparin or ion exchange instead.

Heparin Chromatography for Persistent Complexes

Heparin is a dense polyanion and behaves as an affinity-like resin for many nucleic-acid-binding proteins. It can separate the target from free nucleic acid and from host proteins based on differences in binding strength. Load at moderate salt and elute with a salt gradient.

This is especially useful when:

• the target has a genuine nucleic-acid-binding surface;
• nuclease leaves short fragments attached;
• IMAC produces a clean gel but poor A260/A280;
• the protein’s functional assay requires a nucleic-acid-free starting state.

Heparin is not proof of biological nucleic-acid binding. Many positively charged proteins bind it nonspecifically. Treat it as a purification behavior, then test function with defined ligands.

Ion Exchange as a Separation, Not Just a Polishing Step

Ion exchange can exploit the large charge difference between nucleic acid and most proteins. At neutral pH, DNA and RNA are strongly anionic and bind anion exchangers. A target that is positively charged under the same condition may flow through or bind a cation exchanger in a separate step.

Three configurations are worth screening:

• anion-exchange flow-through: nucleic acid binds while the target passes;
• anion-exchange bind-and-elute: useful when target and contaminant have resolvable salt dependence;
• cation-exchange capture: a basic target binds while nucleic acid remains on a different path.

The choice between anion and cation exchange deserves its own decision screen; the same pH, charge, and conductivity principles determine whether nucleic acid or target should bind.

A Step-by-Step Rescue Workflow

Step 1: Measure the input honestly

Record wet cell mass, resuspension volume, OD-equivalent, viscosity, and A260/A280 after clarification. Save aliquots before and after every intervention.

Step 2: Optimize lysis at small scale

Compare nuclease timing, Mg²⁺, salt, and mechanical disruption. Keep target concentration low enough that aggregation does not masquerade as nucleic-acid removal.

Step 3: Clarify completely

Residual debris and long DNA overload filters and columns. If filtration is required, centrifuge first and track target loss to the pellet.

Step 4: Use one discriminating capture wash

For IMAC, compare 150 versus 500 mM NaCl at the same imidazole concentration. For an affinity step sensitive to salt, move the salt intervention before loading.

Step 5: Add an orthogonal charge step

Use PEI for bulk precipitation, heparin for persistent nucleic-acid-binding targets, or ion exchange for a controllable chromatographic separation.

Step 6: Remove nuclease and released contaminants

A nuclease is itself a protein contaminant. If it is added in large amounts after capture, confirm it does not co-elute or interfere with downstream assays.

Step 7: Validate the final sample

Measure A260/A280, intact mass, analytical SEC, and the assay-relevant activity. Run an agarose gel or fluorescent nucleic-acid assay if residual contamination matters quantitatively.

Troubleshooting Decision Tree

Viscosity remains after nuclease

Check Mg²⁺ and chelators, verify enzyme activity, dilute the lysate, increase mixing, and treat earlier. Do not simply increase sonication until the target warms or foams.

Viscosity disappears but A260 stays high

Short fragments or RNA remain. Add a salt challenge and compare heparin, anion exchange, or PEI.

The target disappears with PEI

Reduce PEI, raise salt during titration, or abandon precipitation. Assay the pellet: the target may be co-precipitated rather than degraded.

High salt cleans the sample but kills activity

Use a brief high-salt wash rather than prolonged incubation, screen stabilizing cofactors, or perform heparin/ion exchange at lower ionic strength.

A260/A280 is acceptable but SEC is heterogeneous

The remaining problem may be oligomerization or aggregation. Use nuclease-treated and untreated controls, absolute-mass methods, and concentration series before concluding that nucleic acid is still present.

Do Not Turn A260/A280 Into a Purity Specification by Itself

The ratio is convenient because it is immediate, but it compresses several unknowns into one number. Protein absorbance at 280 nm depends heavily on tryptophan, tyrosine, and disulfides. A protein with a low extinction coefficient can show a surprisingly high ratio after a small amount of nucleic-acid carryover. A protein with strong A280 can hide the same contaminant.

Light scattering from aggregates raises absorbance across the spectrum, imperfect buffer matching tilts the baseline, and imidazole or reducing agents can add background. Record the full 220–340 nm spectrum, subtract the exact formulation blank, and inspect whether the baseline returns near zero above 320 nm.

If residual nucleic acid affects an assay or regulated application, use a method with an actual calibration:

• a fluorescent DNA or RNA assay with the correct selectivity;
• agarose gel against a mass standard;
• qPCR when a specific DNA template is the concern;
• analytical anion exchange or capillary electrophoresis;
• A260 combined with an independently measured protein concentration.

Define the acceptance criterion from downstream sensitivity. A structural screen, enzyme kinetic assay, and nucleic-acid-binding experiment do not need the same threshold.

Special Case: The Nucleic Acid Is Part of the Biology

For ribonucleoproteins, DNA repair complexes, transcription factors, and genome-editing systems, aggressive removal can destroy the intended state. Decide whether you need apo protein, a native co-purified complex, or a complex reconstituted with a defined ligand.

If the goal is apo protein, digest the heterogeneous cellular nucleic acid and reconstitute with a known sequence after purification. If the native complex is the object, identify and quantify the ligand rather than calling it contamination. A260, nuclease sensitivity, and SEC shift can then become characterization data.

The crucial distinction is chemical definition: one target bound to one known oligonucleotide is an experimental reagent; an unknown distribution of host DNA and RNA fragments is not.

The Economics of Early Nucleic-Acid Removal

The biggest cost is often not the contaminated batch. It is the false biological conclusion built from it.

Bottom Line

Viscosity is a polymer-length problem; persistent A260 is a chemical-separation problem. Shear long DNA enough to handle the lysate, digest it under conditions where the nuclease is active, and then break or chromatographically resolve the remaining protein–nucleic-acid interactions. Use PEI only after a recovery titration, and validate the final material with more than a clean protein gel.

Building the Workflow in Orbion

Orbion Characterize maps charge, disorder, nucleic-acid-binding features, topology, and structure confidence before a purification route is chosen. Design lets teams compare termini, tags, and construct boundaries that change nonspecific binding. Bench turns the selected route into a literature-grounded protocol with explicit checkpoints for nuclease chemistry, salt tolerance, recovery, and final-sample QC.

The useful outcome is not a generic instruction to “add nuclease.” It is an experiment that reveals why the contaminant follows your target.

References

1. Atkinson, A. and Jack, G. W. Precipitation of nucleic acids with polyethyleneimine and the chromatography of nucleic acids and proteins on immobilised polyethyleneimine. Biochimica et Biophysica Acta 308, 41–52 (1973). doi:10.1016/0005-2787(73)90120-2
2. Cordes, R. M. et al. Precipitation of nucleic acids with poly(ethyleneimine). Biotechnology Progress 6, 283–285 (1990). doi:10.1021/bp00004a009
3. Burgess, R. R. Use of polyethyleneimine in purification of DNA-binding proteins. Methods in Enzymology 463, 331–342 (2009). doi:10.1016/S0076-6879(09)63020-2
4. Bornhorst, J. A. and Falke, J. J. Purification of proteins using polyhistidine affinity tags. Methods in Enzymology 326, 245–254 (2000). doi:10.1016/S0076-6879(00)26058-8