🧪Tomorrow I’m going to cast an SDS-PAGE gel. Not a heroic gel. Not a “for publication” gel. Just a gel. The kind you cast because you need bands.🙂
I also suspect this might be my last hand-cast gel of grad school, which feels oddly symbolic. Like closing a chapter by doing free-radical polymerization with shaking hands and pretending oxygen is not actively plotting against me. So I’m writing this protocol blog for future me, the version of me who will stand at the bench tomorrow and think, wait, why is the resolving gel pH different from the stacking gel, 😂 and why does glycine suddenly become a main character.
What this gel is actually doing
In SDS-PAGE, SDS denatures proteins and coats them with negative charge so that migration depends mostly on size rather than native shape or charge. The gel itself is a polyacrylamide mesh, a molecular sieve. Smaller proteins navigate the pores more easily and migrate faster; larger proteins get slowed down.
The part that makes Laemmli feel “clean” is the stacking step. This is not just tradition. It’s an engineered ion trick called discontinuous electrophoresis, where the buffer system creates a moving boundary that compresses proteins into thin starting bands before they enter the resolving gel. Without stacking, many lanes would enter the resolving gel already smeared, like a watercolor painting of your motivation.
The buffer logic in Laemmli (Tris-glycine) and why pH matters
The Laemmli system uses two gel layers with different pH values, plus a running buffer that supplies glycine.
👀The stacking gel is typically buffered with Tris-HCl around pH 6.8. In this environment, glycine (from the running buffer) is mostly in a low net-charge form and migrates relatively slowly. Chloride (Cl⁻) from Tris-HCl moves fast.
Chloride acts as the leading ion and glycine acts as the trailing ion, creating a narrow zone of high electric field between them. Proteins get squeezed into this zone, stacking into a tight band. 😛This is why your lanes look like lanes instead of sadness.
👀The resolving gel is typically buffered with Tris-HCl around pH 8.8. When proteins enter this higher pH region, glycine becomes more negatively charged and migrates faster, the stacking boundary behavior changes, and proteins begin separating by size through the polyacrylamide matrix. The pH shift is part of the design, not a random historical artifact.
The running buffer in classic Laemmli is Tris-glycine-SDS. If you change this buffer system, you can disrupt stacking and resolution even if your gel percentage is perfect. In other words, the buffer is not just “the liquid you had available.” The buffer is half of the electrophoresis.
What each casting component does (so troubleshooting has a target)
Acrylamide is the monomer that polymerizes into chains. Bis-acrylamide is the crosslinker that ties chains together into a mesh. The total acrylamide percentage and the crosslink density control pore size.
APS (ammonium persulfate) generates free radicals that start polymerization. TEMED accelerates that radical formation. Without fresh APS and adequate TEMED, your gel can polymerize slowly, unevenly, or not at all.
Oxygen quenches free radicals and inhibits polymerization, especially at the gel surface. This is why people overlay the resolving gel surface with water or alcohol during polymerization. Oxygen is quiet, invisible, and highly committed to ruining your timeline.
Gel percentage and what a gradient actually is
A single-percentage gel has one uniform pore size. A higher percentage gel has smaller pores and generally resolves smaller proteins better. A lower percentage gel has larger pores and generally allows larger proteins to migrate and separate more effectively.
A gradient gel is a gel where the acrylamide percentage increases from top to bottom (for example, low percent near the wells and higher percent deeper in the gel). That means pore size gradually gets smaller as proteins migrate. The practical benefit is range: big proteins can enter and separate in the larger pores near the top, while small proteins get strong resolution deeper where the pores are tighter. A gradient is useful when your sample contains proteins across a wide molecular weight range, or when you are not sure where the “important” band will land and you want one gel to cover more territory.
The classic Tris-glycine hand-cast workflow (Laemmli)
First, assemble clean glass plates with spacers in a casting stand. The least glamorous trick is the most powerful: test for leaks with water first. If it leaks with water, it will leak with gel, except gel leaks faster and leaves permanent evidence.
Next you pour the resolving gel. In Laemmli, the resolving gel uses Tris-HCl pH ~8.8 and the acrylamide percentage you chose for your protein size range. You mix everything except APS and TEMED, because adding APS and TEMED starts polymerization. Once you add them, you have entered the phase of existence where time is no longer linear.😛
Pour the resolving gel to the correct height, leaving room for stacking gel and wells. Overlay the surface with water or alcohol to exclude oxygen and level the interface, then let it polymerize fully.
After the resolving gel sets, pour off the overlay and rinse gently. Then pour the stacking gel, typically buffered with Tris-HCl pH ~6.8 and lower acrylamide percentage. Add APS and TEMED last, pour, insert the comb slowly, and remove bubbles before they fossilize into permanent lane sabotage.
Let the stacking gel polymerize, then remove the comb slowly and straight. If you remove it like you are pulling a sword from a stone, you will tear the wells and the gel will remember.
Quick reality check: how Bis-Tris is different
Bis-Tris is not just “Tris but fancier.” It is a different buffer system designed to run at lower pH in the gel, and it is typically paired with specific running buffers like MES-SDS or MOPS-SDS rather than Tris-glycine-SDS.
In the Tris-glycine (Laemmli) system, stacking and resolution rely heavily on how glycine’s charge state changes with pH and how it behaves as a trailing ion. In Bis-Tris systems, the ion environment and pH regime are different, which often gives sharper bands and more consistent migration when everything is matched correctly.
Closing notes for tomorrow’s me
If tomorrow’s gel polymerizes cleanly, stacks like a polite little ion highway, and gives me crisp bands, I will enjoy a brief, endangered-species moment of self-confidence. It will last about six minutes, max. Then I’ll move on to the next experiment: Western blotting, the craft where you carefully transfer your beautiful bands onto a membrane and then spend the rest of the day proving they ever existed. 👀If it goes the way it usually goes, I might end up writing a whole troubleshooting post titled “Western Blotting: A Love Story Between Me and a Very Unreasonable Antibody,” because yes, I’ve had issues, and no, the membrane has never once apologized.😂
But the bigger thought is this: we are living in the precast era. Gels arrive in plastic like they were grown in a clean room by well-paid robots, and nobody has to smell TEMED or wonder if acrylamide is quietly plotting against their nervous system. One day, hand-casting will become a lost art. Not because it wasn’t useful, but because it was inconvenient, slightly hazardous, and required patience, which is a resource that modern science budgets do not cover.
I can already picture myself years from now telling a new generation of scientists about it the way people talk about ancient lab folklore. “We used to make the gel ourselves,” I’ll say, staring dramatically into the middle distance. “Two hours. Toxic monomer. Free-radical polymerization. Overlay to fight oxygen inhibition. And we did it all… just to separate proteins by size.” They’ll blink at me from behind their automated capillary system and say something gentle like, “Wow. That sounds unnecessary.” And I’ll nod, because yes. It was unnecessary. But it was also the moment you understood what electrophoresis actually is: pH, ions, polymer chemistry, and the constant reminder that invisible variables like oxygen and temperature are always in the room with you.
So yes, I hope tomorrow’s gel works. Not because science needs my gel😂, but because I want one final victory before hand-casting becomes a bedtime story: “Once upon a time, I mixed toxic chemicals for two hours to make a wet rectangle, and I acted like it was normal.” 🖤
