Reaction Buffers & Hybridization Stringency¶
Every hybridization mix in this workflow is built on a ligase buffer — Ampligase buffer on the direct-RNA route, the ligase's own buffer on the cDNA/SNP routes — so that the downstream ligation shares one buffer system. That is convenient, but it means swapping the ligase silently changes the salt the probes anneal in. This page records what is in each buffer and how much that actually matters.
The short answer
Less than you would guess. At 10 mM Mg²⁺ — which every buffer here carries — the divalent term dominates ionic strength so completely that even a 2.7-fold change in monovalent salt moves arm Tm by only ≈2 °C. The stringency of this workflow is set by temperature and formamide, not by which ligase buffer you picked.
Buffer Compositions (at 1X)¶
| Buffer | Buffering / pH | Monovalent | Mg²⁺ | Cofactor | Other |
|---|---|---|---|---|---|
| Ampligase (Lucigen/Biosearch) | 20 mM Tris-HCl, pH 8.3 | 25 mM KCl | 10 mM MgCl₂ | 0.5 mM NAD⁺ | 0.01% Triton X-100 |
| SplintR (NEB M0375) | 50 mM Tris-HCl, pH 7.5 | — | 10 mM MgCl₂ | 1 mM ATP | 10 mM DTT |
| T4 DNA Ligase (NEB B0202) | 50 mM Tris-HCl, pH 7.5 | — | 10 mM MgCl₂ | 1 mM ATP | 10 mM DTT |
| HiFi Taq DNA Ligase (NEB M0647) | 20 mM Tris-HCl, pH 8.5 | 150 mM KCl | 10 mM MgCl₂ | 1 mM NAD⁺ | 10 mM DTT, 0.1% Triton X-100 |
| Taq DNA Ligase (NEB B0208) | 20 mM Tris-HCl, pH 7.3 | 25 mM KOAc | 10 mM Mg(OAc)₂ | 1 mM NAD⁺ | 10 mM DTT, 0.1% Triton X-100 |
| Tth DNA Ligase | 20 mM Tris-HCl, pH 7.6 | ~100 mM KCl | 10 mM MgCl₂ | 1 mM NAD⁺ | 10 mM DTT |
| 5X RT Buffer (Maxima H Minus) | 50 mM Tris-HCl, pH 8.3 | 75 mM KCl | 3 mM MgCl₂ | — | 10 mM DTT |
Tth row is literature-typical, not a vendor sheet
Thermo does not publish the 10X composition for its Tth DNA Ligase (EL07981); the row above is the standard Tth reaction condition from the literature. Verify against the lot datasheet before relying on it for a Tm calculation.
Two structural points worth noting:
- Cofactor splits the ligases cleanly. SplintR and T4 are ATP-dependent; Ampligase, Taq, HiFi Taq and Tth are NAD⁺-dependent. You cannot cross buffers between the two classes and expect ligation — the cofactor simply is not there.
- Ampligase buffer already carries NAD⁺ (0.5 mM at 1X). That is why the direct-RNA hybridization buffer is Ampligase buffer even though SplintR does the ligation — it is a historical carry-over from a thermostable-ligase protocol, not a requirement.
What Actually Moves Arm Tm¶
Four terms, in descending order of how much they matter here:
| Term | Effect | Notes |
|---|---|---|
| Mg²⁺ | Dominant | 10 mM Mg²⁺ ≈ 380 mM Na⁺-equivalent via von Ahsen's [Na⁺]eq = [Na⁺] + 120·√([Mg²⁺] − [dNTP]). This is why the monovalent differences above barely register |
| Temperature | Direct | The 45 °C anneal is the stringency-defining condition |
| Formamide | −0.5 °C per % | 20% formamide ≈ −10 °C. The designer uses 0.5 °C/% (literature range 0.6--0.72; 0.5 is the conservative in-house value) |
| Monovalent (K⁺/Na⁺/Tris) | Weak here | All monovalent cations are equivalent — only the total ionic strength matters, not which salt |
| Probe concentration | Few °C per 10x | Tm carries an R·ln(C_T/4) term, so lowering probe concentration lowers Tm |
Worked Comparison: Does Swapping the Ligase Buffer Matter?¶
The hybridization mix is 1X ligase buffer + 50 mM added KCl + 20% formamide:
| Hybridization built on | Monovalent total | Mg²⁺ | Na⁺-equivalent | ΔTm vs default |
|---|---|---|---|---|
| Ampligase — direct-RNA, FFPE, iLock (default) | 75 mM | 10 mM | ~454 mM | — (baseline) |
| HiFi Taq — SNP record | 200 mM | 10 mM | ~579 mM | ≈ +2 °C |
| Tth (literature-typical) | 150 mM | 10 mM | ~529 mM | ≈ +1 °C |
The trap: don't compute this from monovalent salt alone
Using the familiar monovalent-only rule ΔTm ≈ 16.6 × log₁₀([M⁺]₂/[M⁺]₁) gives
+7 °C for the Ampligase → HiFi Taq switch, which would be a serious change in
stringency. That number is wrong here, because it ignores the 10 mM Mg²⁺ common to
both buffers. Fold the Mg²⁺ in as Na⁺-equivalent first and the same switch is
≈ +2 °C — a shift small enough to ignore against a Tm model whose own error is a
few °C.
Practical consequence: the SNP route's use of HiFi Taq buffer during hybridization does not require re-tuning the formamide or the anneal temperature. If you ever want to compensate exactly, ~2 °C is about 4% formamide.
The Ligation Step Is Not the Stringent One¶
It is natural to worry that the padlock arms fall off during ligation, since the SplintR and T4 buffers have no added monovalent salt at all. They do not, for two reasons that both point the same way:
- Ligation runs at 37 °C, well below the 45 °C anneal.
- The ligation mix has no formamide — removing 20% formamide is worth about +10 °C of duplex stability, which swamps the ~1 °C lost to the lower monovalent.
So arms are substantially more stable during ligation than during hybridization. Hybridization and the stringent wash are where specificity is won or lost; the ligation buffer is permissive by comparison. This is also why the ligase's own fidelity — not the buffer — is what determines whether single-nucleotide discrimination is possible.
Computing This Properly¶
Don't hand-calculate for real design work. The lab's probe designer models all of this
explicitly (probe_design/designer/probe_designer/chemistry.py, ReactionConditions):
monovalent, Mg²⁺, dNTP chelation, formamide, and strand concentration, with the
SantaLucia 1998 salt correction plus the von Ahsen Mg²⁺ conversion. It also picks the
right nearest-neighbour table per chemistry, which matters more than the buffer:
| Duplex | NN table | Used by |
|---|---|---|
| DNA probe : mRNA | Sugimoto 1995 (R_DNA_NN1) |
Direct RNA, iLock |
| DNA probe : cDNA | SantaLucia & Hicks 2004 (DNA_NN4) |
cDNA, SNP genotyping |
Full provenance for every parameter, including the sources behind the numbers on this page,
is in probe_design/designer/docs/thermodynamics_parameters.md.
Keep the designer's defaults in sync with these protocols
ReactionConditions defaults are pinned to a specific protocol version. When a
hybridization condition changes here — probe concentration, formamide, anneal
temperature, or the buffer — the designer's defaults need the same change, or probe
panels get designed against conditions the bench no longer runs.
References¶
- von Ahsen N, Wittwer CT, Schütz E. Oligonucleotide melting temperatures under PCR conditions. Clinical Chemistry 47, 1956--1961 (2001).
- SantaLucia J. A unified view of polymer, dumbbell, and oligonucleotide DNA nearest-neighbor thermodynamics. PNAS 95, 1460--1465 (1998).
- Owczarzy R, et al. Predicting stability of DNA duplexes in solutions containing magnesium and monovalent cations. Biochemistry 47, 5336--5353 (2008).
- McConaughy BL, Laird CD, McCarthy BJ. Nucleic acid reassociation in formamide. Biochemistry 8, 3289--3295 (1969).
- NEB — Taq DNA Ligase Reaction Buffer (B0208) · SplintR Ligase (M0375) · HiFi Taq DNA Ligase (M0647)
- Ampligase 10X Reaction Buffer — LGC Biosearch Technologies
- Thermo Fisher Scientific. Maxima H Minus Reverse Transcriptase user guide, Pub. No. MAN0012047 Rev. C.00 (2024).