MES | CAS 4432-31-9 | ≥98%

2-(N-Morpholino)ethanesulfonic acid (MES) is a zwitterionic Good's buffer used to maintain controlled pH in mildly acidic biochemical, molecular-biology, cell-system, and analytical workflows. Protonation/deprotonation at the morpholinium nitrogen gives a pKa of approximately 6.1 at 25 °C and an effective buffering range of approximately pH 5.5–6.7. MES also has one of the smallest temperature coefficients among common Good's buffers (d(pKa)/dT ≈ −0.011 pH unit/°C), markedly lower than Tris and slightly lower than HEPES, so operational pH drifts only modestly between bench, chromatography, and refrigerated incubation conditions. The morpholine-ethanesulfonate scaffold combines high water solubility, very low UV absorbance at nucleic-acid/protein quantitation wavelengths, and absence of primary amines or free carboxylates that would interfere with downstream chemistry. As a metal-binding ligand, MES is one of the cleanest, effectively non-coordinating Good's buffers in its operational pH range: it shows no measurable complexation of Mg2+, Ca2+, Mn2+, Co2+, Ni2+, Zn2+, or Cd2+, and only imperceptible to very weak interaction with Cu2+ and Pb2+, in contrast to HEPES-family buffers that can form strong Cu2+-binding species. As the free acid (CAS 4432-31-9), MES is normally dissolved and titrated to the working pH with NaOH or KOH; the corresponding sodium salt is a separate commercial form for direct dissolution.

Bis-Tris / MES SDS-PAGE for Small-to-Medium Proteins

In commercial Bis-Tris discontinuous SDS-PAGE systems, MES functions as the trailing-ion component of the running buffer, paired with a Bis-Tris/Cl− gel buffer. Because MES has a lower pKa than MOPS (approximately 6.1 vs 7.2), MES-based running buffer migrates faster, shifts the stacking/resolving boundary, and improves resolution of smaller proteins relative to MOPS on the same Bis-Tris gel chemistry. Practical separation windows are approximately 1–200 kDa for MES, with strongest utility below ~50 kDa, versus approximately 14–260 kDa for MOPS, with stronger performance for larger proteins. MES running buffer is therefore the preferred choice when low-molecular-weight protein resolution, Western blot input quality near the dye front, or excision of smaller bands matters more than separation of high-molecular-weight proteins.

EDC / Sulfo-NHS Carbodiimide Bioconjugation

MES is the canonical activation buffer for EDC and EDC/sulfo-NHS-mediated coupling of carboxyl groups to primary amines. EDC activates a carboxyl group to an amine-reactive O-acylisourea; sulfo-NHS trapping converts that transient intermediate into a more stable amine-reactive ester. Activation is most efficient in the mildly acidic range, and standard two-step workflows perform carboxyl activation in MES at approximately pH 5–6 before raising the pH to near-neutral conditions for reaction with the amine partner. MES is preferred because it satisfies the chemical requirements of the coupling: it contains no primary amine that would consume EDC, no free carboxylate that would be activated as a competing substrate, and minimal metal coordination that could distort metal-sensitive conjugation mixtures. Tris, TBS, and glycine systems are incompatible because their amine functionality competes in the coupling chemistry, while phosphate is usable but often less efficient.

Plant Tissue Culture and Defined Biological Media

MES is a standard pH stabiliser in plant tissue culture media, including Murashige-Skoog and Arabidopsis-style formulations, where it helps maintain the mildly acidic pH range used for seedling, callus, organ, and plant-cell culture. Low-millimolar MES supplementation is common in defined plant media adjusted around pH 5.6–5.8, including formulations supplied with MES already incorporated. Its value is that pH buffering can be separated from mineral-salt composition, carbon source, growth-regulator formulation, and chelation-sensitive components such as iron-EDTA. Unlike phosphate or organic-acid buffers, MES offers pH control with minimal metal complexation and low biochemical reactivity. The same defined-buffer character is useful in microbial, archaeal, and selected mammalian-cell-system media where pH control must be separated from nutrient composition and metal-containing supplements.

Other Applications

• Cation-exchange chromatography running and elution buffers in the mildly acidic pH window for protein and peptide purification.
• Ion-exchange chromatography of acidic proteins, virus-particle preparations, and selected bioprocess samples.
• Reaction buffer for metalloenzyme and metal-sensitive enzyme assays where Tris, phosphate, or HEPES-family buffers may distort metal speciation.
• SPR and surface-functionalisation workflows requiring mildly acidic, amine-free activation conditions.