Bis-Tris Gels: How They Work, MES vs MOPS and When to Use Them

A Bis-Tris gel is a polyacrylamide gel made with a Bis-Tris buffer so that it runs near neutral pH, and it is used for denaturing SDS-PAGE of proteins. Its running buffer is usually MES for small proteins or MOPS for mid-size proteins. Below: what the chemistry is, how the two buffers differ, and a checklist of compatibility points taken from the manufacturer’s own documentation.

Short answer

Bis-Tris gels separate denatured proteins at close to neutral pH, where the common alternative, the Tris-glycine system, runs at about pH 9.5. The best-documented Bis-Tris products are Thermo Fisher’s NuPAGE and Bolt gels, so the figures below are vendor-stated and should be checked against the instructions for use (IFU) of the lot in your hands.

Definition and scope

Thermo Fisher describes its NuPAGE Bis-Tris gels as using a Bis-Tris HCl gel buffer at pH 6.4, with a running buffer at pH 7.3 to 7.7 and an operating gel pH of about 7.0 (Thermo Fisher, NuPAGE Bis-Tris gels, checked 2026-10-29). The vendor says the neutral pH of the Bis-Tris chemistry “helps minimize protein modifications and reduce protein degradation.” That is a manufacturer claim, not an independent standard.

The gels are built for denaturing electrophoresis. The same vendor lists gel percentages of 8%, 10%, 12% and 4-12%, though not every percentage is offered in every well format. Storage is stated as 4 to 25 °C, with shelf life of up to 16 months at room temperature depending on format.

How Bis-Tris gels are used in practice

Why neutral pH matters

According to the vendor, Tris-glycine gels run at about pH 9.5. At that basic pH, residual acrylamide can react with cysteine and lysine side chains, and the vendor states that this reaction is slower at neutral pH. The vendor also notes that conventional Laemmli sample buffer drops from pH 6.8 to about 5.2 on heating, which can cleave Asp-Pro bonds, whereas its NuPAGE LDS sample buffer stays above pH 7.0, and that adding NuPAGE Antioxidant to the running buffer reduces protein oxidation during the run. These are the reasons given for choosing the neutral system when protein integrity matters, for example before a western blot or other downstream analysis.

MES versus MOPS running buffer

The vendor’s protocol page says MES is used “to resolve small molecular weight proteins” and MOPS “to resolve mid-size proteins”, and that MES is the faster-running buffer (Thermo Fisher, one-dimensional SDS gel electrophoresis protocol, checked 2026-10-29).

Item	MES	MOPS
Intended use (vendor protocol)	Small proteins	Mid-size proteins
1X running buffer pH	7.3	7.7
Resolving range, vendor spec table	3.5 to 160 kDa	15 to 260 kDa
NuPAGE run time at 200 V	about 35 min	about 50 min
Bolt Bis-Tris Plus run time	20 min	35 min

The vendor’s own pages do not agree with each other on range: the product page text gives 1 to 200 kDa (MES) and 14 to 260 kDa (MOPS), while the specification table gives the figures above, and the protocol page cites six separation ranges across 1 to 200 kDa. Treat any single range as dependent on gel percentage and buffer, and confirm it for your specific gel in its IFU.

Types and related concepts

Two vendor product families use the chemistry: NuPAGE Bis-Tris gels and Bolt Bis-Tris Plus gels, which differ in the run times shown above. Bis-Tris gels sit alongside Tris-glycine gels as one of the standard precast formats for SDS-PAGE; the vendor positions Tris-Acetate gels, not Bis-Tris, for native work. For a broader orientation, see the general guide to gel electrophoresis.

Where it matters for labs: a compatibility checklist

These points come from the vendor documents cited above and are the places where Bis-Tris runs most often go wrong.

• Denaturing use only. The protocol states the gels contain no SDS but are designed for denaturing electrophoresis.
• Never run without an SDS-containing buffer. The vendor warns: “Do not use the NuPAGE Bis-Tris Gels with NuPAGE MOPS or MES Running Buffer without SDS.” Native work belongs on Tris-Acetate gels.
• Match the sample buffer. The vendor’s LDS sample buffer is the one it states keeps pH above 7.0 on heating.
• Choose the running buffer for the size range you need to resolve, and plan for the longer MOPS run.
• Keep the system consistent. Ladders and transfer conditions should be chosen for the gel chemistry in use; see the guides on protein markers, western blot transfer buffer and western blot kits.

Common misunderstandings

• “Bis-Tris means native.” No. The neutral pH is not a native condition; the gels are intended for SDS denaturing runs.
• “Neutral pH guarantees intact proteins.” The benefit is a vendor claim about reducing modification and degradation, not a guarantee for every sample.
• “MES and MOPS are interchangeable.” They run at different speeds and favour different size ranges.
• “One published kDa range applies to every gel.” Ranges depend on the gel percentage and buffer, and even the vendor’s pages differ.

FAQ

What is a Bis-Tris gel used for?

Denaturing SDS-PAGE of proteins at near-neutral pH, typically before staining or a western blot.

Should I use MES or MOPS?

The vendor recommends MES for small proteins and MOPS for mid-size proteins. MES runs faster.

Can I run native samples on a Bis-Tris gel?

The vendor says not to use these gels with MOPS or MES running buffer lacking SDS, and points native work to Tris-Acetate gels.

Sources

• Thermo Fisher Scientific, NuPAGE Bis-Tris gels product page, checked 2026-10-29.
• Thermo Fisher Scientific, One-dimensional SDS gel electrophoresis of proteins (pre-cast gels), checked 2026-10-29.