Protein adsorption to PCR tubes: a simple calculation

How much protein could be lost to surface adsorption in a typical PCR tube? Here's an order of magnitude estimate.

Consider a standard 200 µL PCR tube with dimensions:

• Height: 20.8 mm
• Radius: 2.75 mm

The tube shape lies somewhere between a cylinder and a cone. If it were a cylinder, the surface area would be $2\pi r h \approx 359 \;\text{mm}^2$. For a cone, it would be $\pi r \sqrt{h² + r²} \approx 181 \;\text{mm}^2$. Since we are making a very rough estimate, let's just take the mean of these. Let's also divide by 2, assuming we're only using 100 µL, roughly half the tube. This yields 135 mm² of relevant surface area.

A typical folded protein has a diameter around 5 nm. Assuming a circular footprint, each protein occupies π(2.5 nm)² ≈ 20 nm² of surface area. If proteins formed a perfect monolayer on the surface, this suggests about 7 trillion proteins (6.9×10¹²) could adsorb to the PCR tube.

What does this correspond to in terms of protein concentration in solution? If all these theoretically adsorbed proteins were instead dissolved in the 100 µL sample volume, they would contribute about 114 nM to the concentration.

This calculation suggests surface adsorption could significantly impact biochemical assays involving sticky proteins at hundreds of nanomolar concentration (or less).

All this assumes the protein only forms a monolayer. A nice discussion of multilayer adsorption—with a view to its possible implications for macromolecular condensates in cells—can be found in this perspective paper from Tim Mitchison.