© Copyright 2001 American Chemical Society
SEPTEMBER 4, 2001 VOLUME 17, NUMBER 18
Letters Van der Waals Interaction during Protein Adsorption on a Solid Covered by a Thin Film V. P. Zhdanov*,†,‡ and B. Kasemo† Department of Applied Physics, Chalmers University of Technology, S-412 96 Go¨ teborg, Sweden, and Boreskov Institute of Catalysis, Russian Academy of Sciences, Novosibirsk 630090, Russia Received March 21, 2001. In Final Form: May 29, 2001 We calculate the contribution of van der Waals forces to the energy of protein adsorption on the solid covered by a thin film. Specifically, adsorption is considered to occur on the hydrocarbon layer deposited above the metal. Protein and hydrocarbon layers are separated by an ultrathin water layer. In a particular case when the hydrocarbon overlayer is absent, the van der Waals protein-substrate interaction is found to be appreciable, =(0.5-1.5) × 10-20 J/Å2. Covering the metal by hydrocarbon is demonstrated to result in decrease of the van der Waals interaction. Specifically, the interaction drops down to about 10-21 J/Å2 when the hydrocarbon layer becomes thicker than 4 Å.
Adsorption of proteins at the solid-liquid interface is a challenging and practically important interdisciplinary field of natural science.1-3 Its practical significance includes such diverse situations as growth of bacteria and cells in culture, implantation and function of biomaterials (medical implantants) in soft and hard tissue and in the blood stream, and a variety of biomedical sensors and diagnostic tools. The field is also interesting from a purely academic perspective, because it contains nontrivial problems related to surface, statistical, and mesoscopic soft-matter physics. One of these problems concerns the nature and strength of the protein-surface interaction in different situations and especially how they are affected by surface properties. In general, this interaction includes van der Waals forces and the formation of ionic, hydrogen, and/or covalent bonds. The balance between these con* To whom correspondence may be addressed. E-mail: zhdanov@ catalysis.nsk.su or
[email protected]. † Chalmers University of Technology. ‡ Boreskov Institute of Catalysis, Russian Academy of Sciences. (1) Norde, W. In Biopolymers at Interface; Malmsten, M., Ed.; Marcel Dekker: New York, 1998; p 27. (2) Kasemo, B. Curr. Opin. Solid State Mater. Sci. 1998, 3, 451. (3) Hlady, V.; Ho, C.-H.; Britt, D. W. In Interfacial Dynamics; Kallay, N., Ed.; Marcel Dekker: New York, 2000; p 405.
tributions is still poorly understood, mainly due to the diversity of the interactions and the metastable (and multiple state) properties of proteins and also a subtle role of surface and protein water shells. In this Letter, we discuss the contribution of van der Waals forces to the protein adsorption energy. Customarily, this contribution is estimated4 by assuming the substrate to be uniform and using the Hamaker approach. Real surfaces are however often heterogeneous. A practically important example (Figure 1a), treated in our study, is when protein adsorption occurs on a nanometer-thick film or overlayer formed or deposited on a metal surface (protein and the overlayer are assumed to be separated by an ultrathin water layer). Such substrates are often formed spontaneously (e.g., metals are usually covered by a thin oxide overlayer) or may be fabricated in order to have adsorbate-substrate systems with desirable properties. A common example, studied extensively in the past years is self-assembled monolayers of alkanethiolates on gold, which serve as a model substrate for studies of protein adsorption2 and cell adhesion.5 (4) Oberholzer, M. R.; Wagner, N. J.; Lenhoff, A. M. J. Chem. Phys. 1997, 107, 9157. (5) Mrksich, M. Chem. Soc. Rev. 2000, 29, 267.
10.1021/la0104222 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/31/2001
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water. This model is a special case of a more general slab model treated by Barash and Ginzburg.9 Their analysis takes into account the retardation effect related to a finite value of the light velocity. In the case under consideration, this effect may be significant only when the proteinsubstrate interaction is negligibly weak (e.g., when the water layer separating proteins and the substrate is thick). Taking into account that such situations are not of practical interest, we neglect retardation. Following then the conventional prescriptions8,9 consisting of calculation of frequencies of surface modes and using the analytical properties of the dielectric susceptibilities, we represent the van der Waals protein-substrate interaction as (cf. eqs 66, 78, and 79 in ref 9)
U)
pS 16π2
∫0∞∫0∞ x ln D(iω,x) dx dω
(1)
where
D(iω,x) ) 1 + F1(iω,x) F2(iω,x) exp(-xh2) F1(iω,x) ) (m + l)(l - w) + (m - l)(l + w) exp(-xh1) (m + l)(l + w) + (m - l)(l - w) exp(-xh1) F2(iω,x) ) (w - p)(p + w) + (w + p)(p - w) exp(-xh3) Figure 1. (a) Schematic arrangement of proteins adsorbed on the layered substrate and (b) the corresponding slab model (h1, h2, and h3 are the slab thicknesses).
Calculation of the contribution of van der Waals forces to the energy of the protein-substrate interaction is complicated because these forces are not additive (in particular, the conventional Hamaker approach implying the additivity of the interactions is strictly speaking not applicable in this case). In addition, protein adsorption is often accompanied by conformational changes resulting in denaturation of the native protein structure. Such changes may modify van der Waals interaction between amino acid residues and accordingly provide “indirect” van der Waals contribution to the energy of the proteinsubstrate interaction. Physically, it is clear that the latter contribution can hardly be evaluated phenomenologically because it strongly depends on the specifics of the protein structure. Our goal is to scrutinize the former contribution. A general phenomenological theory making it possible to tackle the problem under consideration was developed by Lifshitz.6,7 The original version of this theory was rather cumbersome. Later on, it was reformulated by van Kampen, Nijboer, and Schram.8 The latter approach discussed in detail by Barash and Ginzburg9 is much more straightforward. For an arbitrary shape of adsorbed proteins (Figure 1a), it can however also hardly be realized. To simplify our treatment, we use a slab model (Figure 1b); i.e., the metal is assumed to be covered by an oxide or organic (e.g., hydrocarbon) overlayer, on top of which there are a very thin water later, a protein layer, and bulk (6) Lifshitz, E. M. J. Exp. Teor. Fiz. 1955, 29, 94 (in Russian). (7) Dzyaloshinskii, I. E.; Lifshitz, E. M.; Pitaevskii, L. P. Usp. Fiz. Nauk 1961, 73, 381 (in Russian); Engl. transl.: Sov. Phys. Usp. 1961, 4, 153. (8) van Kampen, N. G.; Nijboer, B. R. A.; Schram, K. Phys. Lett. 1968, 26A, 307. (9) Barash, Yu. S.; Ginzburg, V. L. Usp. Fiz. Nauk 1975, 116, 5 (in Russian); Engl. transl.: Sov. Phys. Usp. 1975, 18, 305.
(w + p)(p + w) + (w - p)(p - w) exp(-xh3) In these equations, S is the contact area, h1, h2, and h3 are the slab thicknesses, and m(iω), l(iω), w(iω), and p(iω) are the metal, overlayer, water, and protein dielectric susceptibilities calculated at imaginary frequencies iω. To illustrate the relation between the equations above and the other results available in the literature, it is instructive to treat two formal limits which do not directly correspond to protein adsorption. For h1 ) 0 and h3 f ∞, the equations above can easily be reduced to
U)
pS 32π2h22
∫0∞∫0∞ x2G(iω,x) dx dω
(2)
where
[
G(iω,x) ) 1 +
(m + w)(w + p) (m - w)(w - p)
]
-1
exp(xh2)
In the limit h1 f ∞ and h3 f ∞, eq 1 is also reduced to eq 2 but with
[
G(iω,x) ) 1 +
(l + w)(w + p) (l - w)(w - p)
]
-1
exp(xh2)
In these two limits, describing interaction of two semiinfinite media separated by a film, eq 2 coincides with that derived by Lifshitz6 and van Kampen et al.8 Our goal was to calculate the contribution of van der Waals forces to the energy of the protein-substrate interaction in the case when the metal is covered by a hydrocarbon layer. To employ eq 1 in this case, we need explicit expressions for the metal, hydrocarbon, water, and protein dielectric susceptibilities in the UV region, because the main contribution to the interaction under consideration results from the UV frequencies. For
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Figure 2. Van der Waals protein-substrate interaction as a function of the hydrocarbon-layer thickness for the water-layer thickness of h2 ) 0.5 (solid line) and 1 Å (dashed line).
hydrocarbon, water, and proteins, we use10
(iω) ) 1 + (n2 - 1)/(1 + ω2/ω02)
(3)
where n is the refractive index of the medium in the visible region and ω0 is the typical electronic absorption frequency. Specifically, n2 ) 2.0 and ω0 ) 1.9 × 1016 s-1 for hydrocarbons, and n2 ) 1.78 and ω0 ) 1.9 × 1016 s-1 for water.10 In analogy with hydrocarbons, one can expect that for proteins the frequency dependence of the refractive index is not strong and employ the dielectric permittivity instead of n2. Following this guideline, we use for proteins n2 ) 4.0 and ω0 ) 1.9 × 1016. For metals (Au, Ag, Cu), one has10
(iω) ) 1 + ωe2/ω2
(4)
where ωe ) (2-4) × 1016 s-1 is the so-called plasma frequency of the free electron gas. In our calculations, we employ ωe ) 3 × 1016 s-1. The water layer between the substrate and proteins is expected to be very thin (down to one monolayer). Our results have been calculated for h2 ) 0.5 and 1 Å. For the protein slab, we use h3 ) 5 Å. According to our calculation (Figure 2), the van der Waals protein-substrate interaction is appreciable, U/S = (0.5-1.5) × 10-20 J/Å2, when the hydrocarbon overlayer is absent (h1 ) 0), i.e., when the surface closest to the proteins is metallic. If for example the proteinsubstrate contact area is 20 Å2, one has in this case U = (1-3) × 10-19 J (or 60-180 kJ/mol). Over most of this (10) Israelachvili, J. N. Intermolecular and Surface Forces; Academic Press: New York, 1992.
range (g80 kJ/mol), protein adsorption is thus irreversible at room temperature (RT), as also observed in experiments. Covering the metal by a hydrocarbon overlayer results in decrease of the van der Waals protein-substrate interaction due to the lower hydrocarbon polarisibility or, more specifically, due to smaller difference in the polarisibilities between hydrocarbon and protein. In particular, the interaction rapidly drops down to ≈10-21 J/Å2 at h1 g 4 Å. For S ) 20 Å2, this value corresponds to U ≈ 2 × 10-20 J (or ≈10 kJ/mol). In the latter case, adsorption would be strongly reversible at RT. In other words, the surface would be protein resistant if only van der Waals interactions were involved. In addition, Figure 2 illustrates that the water layer between proteins and the substrate may appreciably reduce the protein-substrate interaction. In reality, such a layer may be formed on strongly hydrophilic substrates. One should however remember that due to statistical fluctuations the water layer may be destroyed if the driving force for protein adsorption on the “clean” substrate is significant. In summary, our calculations clearly indicate that the van der Waals protein-metal interaction can be dramatically reduced by covering the metal by a thin (h1 g 4 Å) hydrocarbon layer and that the intervening water layer is also significant. In particular, our results explain why protein adsorption on hydrocarbon (e.g., lipid) overlayers is often negligible while adsorption on the clean metal surfaces is usually strongly irreversible. Finally, we point out that our analysis is basically phenomenological. In particular, our model is focused on the van der Waals protein-metal interaction and does not take into account specific properties of real systems (e.g., the nature of hydrocarbon end groups, etc.). Such details11 and/or conformational changes resulting in denaturation of the native protein structure may of course also be important for the understanding of interaction of proteins with the substrates under consideration. All these factors do not however cancel one another. In particular, the van der Waals interaction we discuss will always be operative. For this reason, our general results and conclusions are expected to be applicable to a wide class of proteins. Acknowledgment. This work was done with financial support from the NUTEK Biomaterials Consortium (Contract 8424-96-09362) and the Engineering Science Research Council (TFR Contract 97-643). LA0104222 (11) Harder, P.; Grunze, M.; Dahint, R.; Whitesides; G. M. Laibinis, P. E. J. Phys. Chem. B 1998, 102, 426.
