Control of Polymer Brush Morphology, Rheology, and Protein Repulsion by Hydrogen Bond Complexation

Abstract

Polymer brushes are widely used to alter the properties of interfaces. In particular, poly(ethylene glycol) (PEG) and similar polymers can make surfaces inert toward biomolecular adsorption. Neutral hydrophilic brushes are normally considered to have static properties at a given temperature. As an example, PEG is not responsive to pH or ionic strength. Here we show that, by simply introducing a polymeric acid such as poly(methacrylic acid) (PMAA), the highly hydrated brush barrier can change its properties entirely. This is caused by multivalent hydrogen bonds in an extremely pH-sensitive process. Remarkably, it is sufficient to reduce the pH to 5 for complexation to occur at the interface, which is two units higher than in the corresponding bulk systems. Below this critical pH, PMAA starts to bind to PEG in large amounts (comparable to the PEG amount), causing the brush to gradually compact and dehydrate. The brush also undergoes major rheology changes, from viscoelastic to rigid. Furthermore, the protein repelling ability of PEG is lost after reaching a threshold in the amount of PMAA bound. The changes in brush properties are tunable and become more pronounced when more PMAA is bound. The initial brush state is fully recovered when releasing PMAA by returning to physiological pH. Our findings are relevant for many applications involving functional interfaces, such as capture–release of biomolecules.

Accurate Correction of the “Bulk Response” in Surface Plasmon Resonance Sensing Provides New Insights on Interactions Involving Lysozyme and Poly(ethylene glycol)

“…We attribute this to LYZ molecules that reach the underlying gold surface and adsorb irreversibly in between the PEG chains, similarly to what we recently described for polymer–polymer interactions. 16 The signal from irreversible binding after sufficient exposure to LYZ was slightly lower than that from a monolayer formed directly on gold ( Figure S3 ), in agreement with PEG partly preventing primary adsorption to gold. Indeed, the protein is only ∼2 nm in size 23 and should be able to fit in between grafting sites.…”

“… 30 The resulting brushes had a dry thickness of 8.0 ± 1.2 nm and a grafting density of 0.25 ± 0.04 nm –2 determined by SPR spectra in the dry state, as in previous work. 16 , 31 , 32 Figure 1 C shows the typical response in terms of SPR and TIR angles during repeated injections of LYZ to the PEG brushes in ordinary PBS buffer (pH 7.4 and physiological salt). Two things can be concluded immediately: First, the TIR angle responses are comparable to the SPR signals, which is a clear indication that a major part of the SPR response likely originates from LYZ molecules in the bulk liquid.…”

“…The dry thickness and exclusion height of the PEG brushes was determined by Fresnel model fits to the SPR spectra as described previously. 16 In brief, the thicknesses and refractive indexes of metal layers were determined independently using SPR chips without the PEG film. Once the parameters were confirmed and gave a reasonable fit, the same values were used when fitting the spectra after introducing an additional layer representing the PEG.…”

“…This “exclusion height” of the PEG brush is in agreement with our previous work and calculated by Fresnel models. 16 , 17 …”

Pore performance: artificial nanoscale constructs that mimic the biomolecular transport of the nuclear pore complex

“…50 Importantly, PEG is a homopolymer and has no cohesive nature. The degree of hydration for the brushes is above 80% 31,50,55 and still they prevented protein translocation fully (within the uncertainty of the measurements). The FG-Nups are different in their chemical nature compared to synthetic polymers, so this is not exactly contradictive, but still quite opposite to the results in Figure 3B, where the less cohesive construct became leaky for proteins.…”