Steve
Martin
a,
Hanqing
Wang
b,
Laura
Hartmann
b,
Tilo
Pompe
a and
Stephan
Schmidt
*a
aInstitute of Biochemistry, Leipzig University, Johannisallee 21-23, 04103 Leipzig, Germany. E-mail: stephan.schmidt@uni-leipzig.de; Fax: +49 341 9736939; Tel: +49 341 9732978
bInstitute of Organic and Macromolecular Chemistry, Heinrich Heine University Düsseldorf, Universitätsstraße 1, Düsseldorf, Germany. Fax: +49 211 8115840; Tel: +49 211 8110360
First published on 23rd December 2014
We present a robust and fast method to quantify the adhesion energy of surface anchored proteins on material surfaces using soft colloidal particles as sensors. The results obtained from studying the adhesion of fibronectin on surfaces with different hydrophobicity were in good agreement with theoretical considerations demonstrating the feasibility of the method.
Atomic force microscopy (AFM) or surface force apparatus (SFA) are suitable techniques to study contact phenomena between surface anchored proteins and materials.14–16 These methods have been used to investigate adhesion forces of proteins layers which is an important part of the rational design of biomaterial surfaces. Hence, such force-based techniques can be considered as an alternative to measuring the thermodynamics of protein surface adsorption. However, handling of SFA or AFM is rather difficult and their throughput too low in order to process a significant number of proteins and material surfaces. Therefore, in the present work, we adapted a novel screening method to study interactions of surface anchored proteins in a simplified and rapid fashion.17–19 Our method uses soft protein coated hydrogel particles, also called soft colloidal probes (SCPs), which undergo mechanical deformation when adhering to material surfaces.19,20 The mechanical deformation can be conveniently read out by reflection interference contrast microscopy (RICM) and related to the adhesion energy of the protein layer bound to the SCP (Fig. 1). The underlying theory was developed by Johnson, Kendall and Roberts (JKR model21). The JKR adhesion energy Wadh of an elastomeric particle resting on a surface can be calculated as:
![]() | (1) |
The polymer coated model surfaces were composed of alternating maleic anhydride (MA) copolymers where the co-monomers are varied from ethene, propene, styrene and octadecene. Accordingly, the different MA copolymers are termed PEMA, PPMA, PSMA and POMA (see Fig. 2A). The MA-copolymers surfaces were prepared on amino-silane coated glass coverslips as previously described23 (for details see ESI,† S6). Note that the polymer layer achieves densely packed dangling chains on the surface in mushroom-brush conformation with an overall thickness of some tens of nm.24 Importantly, the different copolymers lead to variations of the surface hydrophobicity. The water contact angle strongly decreases from POMA (100°), PSMA (75°), PPMA (38°) to PEMA (21°) coated surfaces.9,13 As expected, the contact areas of FN coated SCPs resting on MA-copolymer surfaces followed the trend in hydrophobicity of the polymer surfaces, as can be seen in the RICM images (Fig. 2B). A clear increase of the contact area was observed for more hydrophobic surfaces.
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Fig. 2 (A) Chemical structures of the hydrolyzed MA-copolymers; (B) typical RICM images of FN SCPs on the different MA-copolymer surfaces. |
From the RICM images the contact radii of the SCPs were evaluated in order to calculate the adhesion energy per area via the JKR approach. For this purpose the contact radii of a large number of SCPs was measured and plotted vs. the SCP radius (Fig. 3A). Using eqn (1) the data were fitted yielding the adhesion energy Wadh as single fit parameter. The comparison of the obtained adhesion energies for FN on the MA-polymer surfaces show the expected trend, more hydrophobic surfaces resulting in increased adhesion energies (Fig. 3B). Wadh is a measure of the change in free energy when the hydrated polymer and protein surface form a contact. Generally, the two main contributions to Wadh are the interaction free energy of FN and polymer at the interface and the solvation free energy (hydrophobic effect) of FN and the polymer. The solvation free energy of the polymer should lead to larger adhesion energies for more hydrophobic surfaces as less energy is required to overcome water–polymer interactions to form FN–polymer contacts. According to Young's equation, the water contact angle cos(θ) of a material surface is proportional to its solvation free energy. Interestingly, when plotting the adhesion energies versus cos(θ) of the water contact angle of MA-copolymer surfaces (Fig. 4), we found an almost linear trend. This suggests that the large Wadh values on hydrophobic MA-polymers are mostly driven by the entropic gain due to the solvation free energy (hydrophobic effect). The presence of strong entropic effects is supported by the fact that large proteins like FN have the ability to adhere via conformational changes resulting in presentation of hydrophobic sites in the contact zone, while still retaining the beneficial hydration layer facing the bulk solution. It is known that such conformational changes are less strong on polar surfaces with low contact angle; therefore Wadh is reduced in this case. Of course it could be expected that not only hydrophobic effects but also the interaction free energy of FN and polymer changes with the type of MA-copolymer layer. Alternatively it could be argued that changes in the interaction free energy plays only a minor role in this case since both the MA-polymers and FN show a negative net charge at the measurement conditions (pH 7.4) thus reducing attractive electrostatic and dipole contributions.
From the adhesion energy per area and the estimated SCPs protein density (ESI,† S4) we calculated the adhesion energy per mole of surface bound protein (Fig. 4 right axis, ESI,† S7). The adhesion energy per mol decreases from 51 kJ mol−1 to 8 kJ mol−1 from the most hydrophobic surface (POMA) to the most hydrophilic surface (PEMA). These molecular interaction energies seem very low when compared to literature values. For example ITC measurements revealed almost two orders of magnitude larger adhesion energies (∼1000 kJ mol−1) for BSA adsorbing on SiO2, TiO2 or polystyrene surfaces.6,25 Also theoretical studies7 and kinetic studies on short peptides8 predicted much larger adsorption energies on hydrophobic surfaces. There are several factors which can cause the observed quantitative differences compared to adsorption measurements in solution. At first the densely packed and covalently bound proteins on the SCP cannot undergo conformational changes to the same degree as freely adsorbing proteins.4 Therefore spreading upon adhesion on the polymer surface is reduced, which results in a reduction of the molar adhesion energy compared to adsorption measurements. It is known that entropic and enthalpic contributions via such processes considerably contribute the overall protein–material interaction. Secondly, surface roughness of both interaction surfaces can generally reduce the effective contact points of adhering surfaces and thus the overall adhesion energy.26 This effect could lead to an underestimation of the molar interaction energies. However, surface roughness effects are considered negligible in the presented study. Surface roughness of SCP should be on the order of the size of the protein as the mesh size of the PAA network is on a similar length scale (5.7 nm, see ESI,† S3 and S4). The surface roughness of the MA-copolymer film is known to be of molecular length scale as well.23 Furthermore, deformation of the SCP during contact should lead to a decrease of possible roughness effects.
It is important to note that in the presented setup protein–material interaction is not only constrained by the inhibition of conformational changes of proteins but furthermore by the mechanical forces of the supporting polymeric network of the SCP. Near the edge of the contact zone the attached proteins can be considered to be under a restoring force due to the polymeric linker when the SCP forms a contact with the surface. In contrast, the central contact area is characterized by small compressive forces of the deformed SCP onto the adsorbed protein (see ESI,† S7). While this effect – impacting the quantitative data of the adhesion energy – might be assumed to be disadvantageous, it illustrates the strength of the approach at the same time. The force-based assay allows quantifying adsorption phenomena of large proteins at various surfaces and resembles interaction process occurring at cell culture substrates under physiological conditions. Therein, cells bind via surface receptors, like integrins, to adhesion ligands such as Arg–Gly–Asp motifs presented by FN adsorbed on materials surfaces and apply considerable forces in the process of cell adhesion. In that way, cells can be described as deformable objects binding to surfaces via large adhesion proteins such as FN. Such a setup is nicely modelled by SCP adhesion in our experiments. The exact stress distribution in the contact zone of cells is of course much more complex as is the case for adherent SCP (see ESI,† S7). But again, it is non-homogeneously distributed with tensile and compressive areas within a cell especially during dynamic process like cell migration. Thus, we suggest that the presented assay is able to probe similar interactions and provides meaningful results in a cell adhesion context. Interestingly, a study investigating the molecular reorganisation of the adhesion ligand FN by cell receptor force gave theoretical estimates of the protein–materials interactions in a similar order of magnitude as presented here.27 FN–substrate interactions were estimated in the range of 1–6 kT comparing to 3–20 kT found in our study.
In light of the latter discussion and the nice correspondence of adhesion energies with surface properties of the polymer layers, namely hydrophobicity, the set of FN adhesion energies obtained in this work very well explains the variations of cell adhesion behaviour on the different MA-polymer coatings from earlier studies, which is largely affected by FN–surface interactions.9,11 It was found that the FN–surface interaction can lead to different modes of traction force behaviour, which is based on the fact that cells would sense different adhesion strengths on surfaces with varying hydrophobicity.28 These results highlight the importance to directly measure the adhesion energies between protein layers and material surfaces since pure adsorption measurements do not capture the involved multivalent surface effects.
Footnote |
† Electronic supplementary information (ESI) available: SCP synthesis and functionalization, surface preparation, surface coverage, meshwidh, elastic modulus measurements, stress distribution, RICM protocols. See DOI: 10.1039/c4cp05484a |
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