Chelators EDTA and magnesium-dependent enzymes

The polymerase was fine on a control DNA held in water. The same enzyme went quiet when the sample arrived in TE. Start there, because a chelator carried in with the nucleic acid is a common reason a magnesium-dependent enzyme looks dead. This troubleshooting page is that branch: how to tell EDTA carry-over from a bad lot, what the textbook TE composition actually is, and where EGTA is a different tool rather than a synonym.

The water in the enzyme buffer is still a water question, answered in laboratory water types and where they fail. The pH of the Tris in TE is a meter question, answered in preparing a buffer and checking pH. EDTA salts and Tris salts are specified from the reagents and chemicals catalogue. Volumetric make-up belongs with beakers and flasks. The hydrate of the EDTA you need belongs on the quotation request.

Symptoms that point at the chelator

A reaction that works when the template volume is small and fails when the template volume is large, in the same TE, is the classic shape. The enzyme, the master mix, and the instrument did not change. The moles of EDTA did.

A restriction digest that is partial only on DNA from one prep, and complete on a control DNA supplied in a low-EDTA buffer, points the same way. So does a ligase or a kinase that dies when a stopped sample, quenched with an EDTA-containing stop solution, is carried into the next tube without a cleanup. The symptom to distrust is "the enzyme is bad" when a side-by-side control enzyme tube succeeds on a different DNA.

A symptom that does not fit is a failure of every reaction, including a control that contains no EDTA at all. That pattern selects the enzyme lot, the magnesium stock, the water, or the thermal cycler before it selects TE. Do not add chelator theory to a blank that never saw chelator.

What EDTA is doing

EDTA binds divalent metals, including magnesium, calcium, manganese, zinc, and others, and it binds some trivalent metals as well. The binding form is the deprotonated molecule, so chelation is weak in strong acid and much stronger once the solution is near neutral or mildly alkaline. That is why DNA-storage buffers are often set near pH 8, where the chelator is actually awake, and why an acidic EDTA suspension that never dissolved is not the concentration you calculated.

The planning sketch is one-to-one: a mole of EDTA can take a mole of magnesium out of the free pool. The real solution is an equilibrium, and other metals compete, but the sketch is enough to decide whether carry-over is negligible or worth an experiment. Polymerases such as Taq need free magnesium. The enzyme buffer's magnesium was chosen by the method sheet. It was not chosen to mop up an unlimited volume of TE.

A typical TE is about 10 millimolar Tris and 1 millimolar EDTA. Say that plainly: it is a widely used textbook composition, not a rule that overrides the sheet in your hand. Some sheets set the pH at 7.5, some at 8, some drop EDTA to 0.1 millimolar precisely so downstream enzymes survive, and some elute in Tris with no EDTA at all. Copy the sheet. If you are writing the sheet, write all four numbers: Tris, EDTA, pH, and the temperature of that pH.

A worked carry-over, as a sketch

Take a method that states 1.5 millimolar magnesium in a 25 microlitre reaction. That reaction contains 37.5 nanomoles of magnesium. TE at 1 millimolar EDTA contributes 1 nanomole of EDTA per microlitre of template. Ten microlitres of template therefore bring 10 nanomoles of EDTA, a substantial share of the magnesium if you treat binding as one-to-one. Two microlitres bring 2 nanomoles, which is a much smaller share of the same reaction. The arithmetic is a screening tool. Your sheet's magnesium, your volume, and your EDTA concentration replace these figures the moment they differ.

The branch after the sketch says the share is large: repeat the control with less template volume, or precipitate and resuspend in the low-EDTA buffer the method allows, or dilute the DNA in water or in Tris without EDTA if the sheet permits that storage for the few hours of the experiment. The branch after the sketch says the share is tiny: stop blaming TE and return to lot, temperature, and primers. Do not "solve" a large share by pouring in extra magnesium unless the method gives a window. Extra magnesium changes polymerase specificity. You can trade a dead reaction for a dirty one and call it a rescue.

Public protocol libraries such as Addgene and protocols.io show both TE and low-EDTA storage in different workflows. A supplier catalogue such as NEB's shows enzymes arriving with their own buffers and a magnesium expectation. Use those pages to see that the cofactor is specified. Do not copy a microlitre table from them into a different kit.

EGTA is selective, not interchangeable

EGTA also chelates divalent metals, and it prefers calcium over magnesium by a wide margin in the published stability constants. That preference is the reason calcium-control experiments use it: you can pull calcium down while a larger fraction of the magnesium remains free. EDTA does not offer that gap. It holds both tightly enough that a "little EDTA" in a magnesium enzyme is not a calcium-only intervention.

So if the failed enzyme is magnesium-dependent and the contaminant you care about is calcium, EGTA may be the chelator the method names. If the failed enzyme is magnesium-dependent and the contaminant is EDTA carry-over, adding EGTA does not unstick the EDTA-magnesium complex. Match the tool to the metal. Do not put EGTA into TE out of habit and expect DNA storage plus happy Taq.

Dissolve the salt you actually have

EDTA is usually bought as the free acid or as the disodium salt dihydrate. The free acid dissolves poorly. The disodium salt dissolves better and still can leave a slurry if you are in a hurry, and the pH of that slurry is acidic until you adjust it. Undissolved specks that clear an hour later mean the concentration drifted after you made up to the mark. Weigh the hydrate printed on the bottle, dissolve completely, adjust pH with the base the recipe names, then bring the solution to volume. Tetrasodium EDTA is a different sodium load and a different starting pH. It is not a silent substitute.

Water for an EDTA stock should be the grade the downstream assay already demanded. A chelator prepared in dirty water will bind the dirt and still bind your magnesium. Conductivity of the finished TE is a weak check: both Tris and EDTA contribute ions, so a conductivity value only helps if you have a historical band for that exact recipe.

Other failures that imitate chelation

A magnesium stock prepared from the wrong hydrate is a low-magnesium reaction that looks exactly like EDTA carry-over. Read the bottle before you blame the DNA prep. A buffer set at the wrong temperature can put a Tris enzyme buffer outside its working pH once it is in a 37 degree block. That is a pH failure with a similar face. Check the slope and the temperature if the EDTA arithmetic says the carry-over is small.

Nuclease in the water can destroy a template and look like a failed polymerase. The no-enzyme control and the water pillar are the branch. EDTA will not sterilise that water, and it will not replace a nuclease-controlled grade the method already required.

Safety and scope

EDTA is an irritant solid and a persistent chelator in waste. Dispose of solutions as the laboratory chemical procedure requires. Do not tip a concentrated stock into a culture and call it a selection method. This page is not medical advice and not an approval to alter a diagnostic PCR. Enzyme work with infectious material keeps the biosafety rules of that material. Chelating magnesium does not disinfect a tube.

Humidity and the enquiry

EDTA salts pick up water if the lid is left off in a humid room, so the formula weight you trusted becomes optimistic and the stock is weaker than the label you wrote. Weigh with the lid habit you use for other hydrates, and record the hydrate. On a quotation request, ask for the salt form, the hydrate, and a certificate that states them. If you also need a ready TE, specify Tris concentration, EDTA concentration, pH, and the temperature of that pH as separate lines. "TE buffer" on a purchase line is how 1 millimolar and 0.1 millimolar arrive in the same conversation.