A bottom-up approach to understanding protein layer formation at solid–liquid interfaces

Abstract

A common goal across different fields (e.g. separations, biosensors, biomaterials, pharmaceuticals) is to understand how protein behavior at solid-liquid interfaces is affected by environmental conditions. Temperature, pH, ionic strength, and the chemical and physical properties of the solid surface, among many factors, can control microscopic protein dynamics (e.g. adsorption, desorption, diffusion, aggregation) that contribute to macroscopic properties like time-dependent total protein surface coverage and protein structure. These relationships are typically studied through a top-down approach in which macroscopic observations are explained using analytical models that are based upon reasonable, but not universally true, simplifying assumptions about microscopic protein dynamics. Conclusions connecting microscopic dynamics to environmental factors can be heavily biased by potentially incorrect assumptions. In contrast, more complicated models avoid several of the common assumptions but require many parameters that have overlapping effects on predictions of macroscopic, average protein properties. Consequently, these models are poorly suited for the top-down approach. Because the sophistication incorporated into these models may ultimately prove essential to understanding interfacial protein behavior, this article proposes a bottom-up approach in which direct observations of microscopic protein dynamics specify parameters in complicated models, which then generate macroscopic predictions to compare with experiment. In this framework, single-molecule tracking has proven capable of making direct measurements of microscopic protein dynamics, but must be complemented by modeling to combine and extrapolate many independent microscopic observations to the macro-scale. The bottom-up approach is expected to better connect environmental factors to macroscopic protein behavior, thereby guiding rational choices that promote desirable protein behaviors.

Experimental characterization of adsorbed protein orientation, conformation, and bioactivity

“…15,17 Some of the methods that can provide information on adsorbed protein orientation and tertiary (and quaternary) structure include fluorescence, [18][19][20][21] time-of-flight secondary-ion mass spectrometry, [22][23][24] nuclear magnetic resonance spectroscopy (NMR), 25,26 and amino acid labeling/mass spectrometry (AAL/MS). [27][28][29][30][31] Methods for the determination of secondary structure of adsorbed proteins include Fourier transform infrared spectroscopy, 32,33 surface enhanced Raman scattering, 34,35 and circular dichroism spectropolarimetry (CD).…”

Comment on “Tuning the bioactivity of bone morphogenetic protein-2 with surface immobilization strategies” by Chen et al.

“…The results vary with the experimental setups (variable surface area and chemistry), the timescales, and the sensitivities of the respective techniques, and it remains challenging to compare different studies. For proteins, there are different models describing their adsorption/desorption behavior at interfaces, 31 the most widely known being the Langmuir model and the random sequential adsorption model. However, these models have two main limitations: 31 first, the equilibrium hypothesis does not apply as desorption experiments generally lead to a residual, irreversibly bound protein fraction and desorbed proteins can potentially readsorb at the surface.…”

“…For proteins, there are different models describing their adsorption/desorption behavior at interfaces, 31 the most widely known being the Langmuir model and the random sequential adsorption model. However, these models have two main limitations: 31 first, the equilibrium hypothesis does not apply as desorption experiments generally lead to a residual, irreversibly bound protein fraction and desorbed proteins can potentially readsorb at the surface. Second, these models do not account for interactions between surface-adsorbed molecules such as rearrangements with increasing surface coverage.…”

Reversible Adsorption Kinetics of Near Surface Dimer Colloids

“…For instance, single molecule tracking experiments have given more detailed insights into the kinetics of proteins at interfaces. 10 Individual particle tracking has shown that protein-protein interactions and protein conformation can change the residence time of proteins on the interfaces and enhance clustering on the interface. 11,12 The effect of particle shape supports previous studies that fit protein surface coverage experiments to kinetic models of non-spherical molecules.…”