Why Most Pulldowns Cost You the One Thing You Wanted to Keep

Team Dynamic Matrices | 26 November 2025

Ask any biochemist what happens the moment a protein leaves solution and hits a bead, and you'll get a resigned shrug. Pulldown and affinity purification are supposed to answer a simple question: what does this protein bind, and how active is it once isolated? But the sample preparation step designed to answer that question is often the very thing that destroys the answer.

Streptavidin-biotin pulldowns require harsh elution conditions to break the near-covalent biotin-streptavidin bond. Antibody-based immunoprecipitation exposes targets to low-pH glycine elution or SDS, both of which unfold protein structure. Even "gentle" competitive elution schemes leave residual denaturant behind, or take so long that enzymes lose activity through prolonged handling at room temperature. By the time a researcher gets to run an activity assay, they're often measuring what survived the purification, not what the cell actually produced.

This is more than an inconvenience. In enzymology, structural biology, and interactome mapping, the whole point of pulling a protein out of a complex mixture is to study it in a state that still reflects its biological function. If the capture method itself denatures, aggregates, or chemically modifies the target, downstream data (kinetics, binding affinities, cryo-EM structures) can be systematically biased without anyone realizing it until results fail to reproduce.

The core tension in protein sample preparation

Every affinity capture method has to solve two conflicting problems at once: bind the target tightly enough to survive washing away everything else, and then release it gently enough that its native fold survives. Traditional chemistries push hard on the first problem and treat the second as an afterthought.

Biotin-streptavidin is the extreme case, one of the strongest non-covalent interactions in biology, with a dissociation constant around 10⁻¹⁵ M. That strength is exactly why elution is so punishing: breaking that bond typically requires boiling in SDS, free biotin competition over hours, or enzymatic cleavage of an engineered linker, none of which are compatible with keeping a folded, catalytically competent protein.

Antibody-based capture trades some of that binding strength for specificity, but pays for it in a different way. Elution generally means acid, chaotropes, or high-salt conditions that can irreversibly denature sensitive targets, particularly multi-domain enzymes or complexes held together by weak interactions that fall apart outside physiological buffer conditions.

Magnetic bead-based pulldowns inherit both problems, plus a mechanical one: repeated bead washing and vortexing introduces shear stress that can dissociate labile protein-protein interactions researchers are often trying to preserve in the first place.

A different approach: capture through phase change, not chemical bonding

LASSO's dynamic crosslinker platform sidesteps this tension by changing what "capture" means at a physical level. Instead of relying on a single high-affinity chemical bond that then has to be forcibly broken, LASSO uses DNA crosslinker libraries to trigger a controlled phase change in solution. The crosslinkers self-assemble into a soft polymer network that selectively engulfs the target biomolecule, whether DNA, RNA, or protein, while excluding everything that isn't recognized.

Because the "keys" that trigger this phase transition are DNA sequences rather than covalent chemistry, release is equally controlled: adding the complementary strand or adjusting the crosslinker composition reverses the phase change and liberates the captured target back into solution under the same mild, physiological buffer conditions used for capture. There is no acid wash, no SDS, no prolonged incubation in denaturant. The protein experiences a gentle change in its local environment rather than a chemical assault.

This matters most for exactly the targets that traditional pulldown chemistry handles worst: multi-subunit enzyme complexes, allosterically regulated proteins, and any target whose function depends on a fold that's only marginally stable outside its native context. When the isolation method doesn't force denaturation as a side effect of achieving specificity, activity assays performed immediately after pulldown reflect the protein's actual behavior in the cell, not an artifact of how it was extracted.

What this looks like in practice

Consider a common sample preparation workflow: isolating a tagged enzyme from lysate to measure its kinetic parameters. With a conventional biotin-based pulldown, a researcher faces a choice between an elution step harsh enough to disrupt activity, or an on-bead assay that introduces its own artifacts from steric hindrance and non-specific bead interactions. Neither option is ideal, and comparing results across different bead lots or wash protocols becomes a persistent source of variability.

With a dynamic-crosslinker approach, the same workflow separates cleanly into two mild steps. Capture happens through selective network formation around the target, and release happens through a change in crosslinker state, both compatible with buffer conditions the enzyme already tolerates. The result is an isolated protein pool that can go directly into a kinetic assay, a binding study, or even structural work, without an intervening recovery period to let activity "come back," because it was never lost.

This same logic extends beyond single-protein isolation. In interactome and complex-pulldown experiments, where the goal is often to capture a bait protein along with its native binding partners, gentle capture and release conditions are what determine whether weak or transient interactions survive the purification at all. Aggressive elution chemistries systematically under-represent exactly the transient interactions that are often biologically most interesting: kinase-substrate contacts, regulatory complexes, and signaling scaffolds that exist only briefly in vivo.

Reproducibility is a sample preparation problem

It's worth stepping back to note why this matters beyond any single experiment. A large share of the reproducibility challenges reported across structural biology and enzymology traces back not to the biology itself, but to variability introduced during sample preparation: different labs using different elution stringency, different incubation times, or different bead chemistries that each perturb the target to a different degree. When the isolation chemistry is inherently gentler and more consistent, that source of inter-lab variability shrinks considerably.

There's also a practical cost dimension. Traditional affinity resins and antibody-based capture reagents are expensive, have limited shelf lives, and often require cold-chain storage and handling. A polymer-based system built on stable DNA crosslinkers is comparatively robust, remains stable over long storage periods, and avoids the batch-to-batch variability that plagues antibody production. For labs running pulldowns routinely, whether for enzyme characterization, interactome studies, or diagnostic target enrichment, that translates into more predictable budgets and fewer failed runs traced back to reagent quality rather than biology.

Rethinking what "capture" should cost the sample

The underlying message here isn't that every existing pulldown method is broken. Biotin-streptavidin and antibody-based approaches remain the right tool for many applications, particularly where downstream analysis doesn't depend on retained activity. But for the growing set of experiments where the whole point is to study a protein's function after isolation, sample preparation chemistry that treats denaturation as an acceptable cost is quietly limiting what can be learned.

Approaches built on reversible, DNA-programmable phase separation, like LASSO, represent a shift in how that trade-off is framed. Selectivity no longer has to come at the price of activity. As more labs run functional assays directly on pulldown products rather than treating isolation and characterization as separate problems, sample preparation methods that preserve native protein state will matter more, not less.