Effect of Structural Stability on the Characteristics ofAdsorbed Layers of T4 Lysozyme

Johan C. Froberg,*,† Thomas Arnebrant,‡,§ Joseph McGuire,| andPer M. Claesson†

Laboratory for Chemical Surface Science, Department of Chemistry, Physical Chemistry,Royal Institute of Technology, S-100 44 Stockholm, Sweden, and Institute for SurfaceChemistry, Box 5607, S-114 86 Stockholm, Sweden, Department of Food Technology,University of Lund, Box 124, S-221 00 Lund, Sweden, and Department of BioresourceEngineering, Oregon State University, Gilmore Hall 116, Corvallis, Oregon 97331-3906

Received June 24, 1997. In Final Form: October 30, 1997

The interferometric surface force technique has been employed to determine how the structural stabilityof globular proteins affects their adsorption and the interactions between adsorbed protein layers. Thesystem consisted of positively charged bacteriophage T4 lysozyme and negatively charged mica surfaces.The wild type and one synthetic mutant of the protein, Ile3 f Trp, differing in structural stability whilethe total charge and tertiary structure are the same, were studied. The adsorption leads to a nearlycompleteneutralization of thenegative surface charge ofmica, reducing the long-range electrostatic double-layer interaction acting between mica surfaces. The thickness of the adsorbed layer is for the wild typeconsistent with the dimensions of the protein, while the Ile3 f Trp mutant gives a layer with a thicknesssmaller than any of its native dimensions. Another consequence of the difference in structural stabilityis that the short range attraction between one protein layer and one bare mica surface is an order ofmagnitude larger for the Ile3 f Trp mutant than for the wild type. The results demonstrate that the lessstable mutant loses its tertiary structure upon adsorption, whereas the wild type retains its globularshape. Thesedifferencesprovideanunderstanding for thedifferences inadsorbedamountandcomplementsthe information about changes in secondary structure upon adsorption observed with other methods.

1. Introduction

Conformational changesundergonebyglobularproteinsupon adsorption have been inferred from indirect obser-vations including isotherm shape,1,2 titration data,3 calo-rimetric measurements,4-6 and more direct observationsvia antibody binding to epitopes hidden in the nativeglobular structure.7 Norde and co-workers showed thatsuchchangescouldcontribute to theadsorption freeenergyby increasing the entropy of the polypeptide chain5,8 andthus contribute to the driving force for adsorption. Theincrease in entropy upon adsorption may at first seemcounterintuitive, and it is opposite to the decrease inentropy occurring upon adsorption of synthetic polymers.One should remember, however, the very compact struc-ture of globular proteins, with significantly less free

volume than bulk water, and secondly that their functionrequires a specific globular conformation. Furthermore,mostaminoacid residues in thenativeproteinare involvedin hydrogen bonding which restricts their rotationalfreedom. A change in conformation of the protein due toadsorption may therefore lead to a large increase inentropy. This is in agreement with experimental quan-tification of the loss in secondary structure in desorbedproteins,2,9 aswell asuponadsorption tonanoparticles.10,11The tendency for structural alteration upon adsorption isrelated to the conformational stability of theprotein.5,8,12,13This has been inferred froma certaindegree of correlationbetween ∆Gdenat and surface tension or foamability andbetween adiabatic compresssibility and content of struc-tural units asmeasured by circular dichroism. However,no quantitative correlation has emerged while there isqualitative support that structurally less stable proteins(small ∆Gdenat) lose their structure more easily uponadsorption.Structural changes experienced by proteins upon ad-

sorption are of great importance in areas ranging frombiomaterial applications and immunological assays, toemulsification and foam stabilization. Several groupshave studied the effect of amino acid substitutions onprotein or peptide behavior at interfaces.13-15 Kato andYutani13 used this approach to show that the structural

* Corresponding author: e-mail, emgion@physchem.kth.se; fax,+46-8-20 89 98.

† Royal Institute of Technology and Institute for Surface Chem-istry.

‡ University of Lund.§ Present address: Department of Prosthetic Dentistry, Centre

for Dental Health Sciences, Carl Gustavs vag 34, 214 21 Malmo,Sweden.

| Oregon State University.(1) Norde, W.; Lyklema, J. J. Colloid Interface Sci. 1978, 66, 257.(2) Soderquist, M. E.; Walton, A. G. J. Colloid Interface Sci. 1980,

75, 386.(3) Norde, W.; Lyklema, J. J. Colloid Interface Sci. 1978, 66, 266.(4) Norde, W.; Lyklema, J. J. Colloid Interface Sci. 1978, 66, 295.(5) Haynes, C. A.; Norde, W. Colloids Surf., B: Biointerfaces 1994,

2, 517.(6) Yan, G.; Li, J.-T.; Huang, S.-C.; Caldwell, K. D. Calorimetric

Observations of Protein Conformation at Solid-Liquid Interfaces. InProteins at Interfaces II; Horbett, T. A., Brash, J., Eds.; AmericanChemical Society: Washington, DC, 1995.

(7) Elwing, H.; Nilsson, B.; Svensson, K.-E.; Askendahl, A.; Nilsson,U. R.; Lundstrom, I. J. Colloid Interface Sci. 1988, 125, 139.

(8) Norde, W. Adv. Colloid Interface Sci. 1986, 25, 267.

(9) Chan,B.M.C.; Brash, J. L.J.Colloid Interface Sci.1981, 84, 263.(10) Kondo,A.;Oku,S.;Higashitani,K.J.Colloid InterfaceSci.1991,

143, 214.(11) Norde, W.; Favier, J. P. Colloids Surf. 1992, 64, 87.(12) Kondo, A.; Higashitani, K. J. Colloid Interface Sci. 1992, 150,

344.(13) Kato, A.; Yutani, K. Prot. Eng. 1988, 2, 153.(14) Elbaum, D.; Harrington, J.; Roth, E. F. J.; Nagel, R. L.Biochim.

Biophys. Acta 1976, 427, 57.(15) Arnebrant, T.; Ericsson, B. J. Colloid Interface Sci. 1992, 150,

428.

456 Langmuir 1998, 14, 456-462

S0743-7463(97)00674-4 CCC: $15.00 © 1998 American Chemical SocietyPublished on Web 01/06/1998

stabilities of tryptophansynthaseR-subunitsandmutantsthereof strongly influenced their adsorption at air/waterandoil/water interfaces. Thiswasevidencedby theabilityof the proteins with higher conformational stability todecrease surface tension and stabilize foams and emul-sions less efficiently than the less stable wild type.Using single-site mutations, Matsumura et al.16 dem-

onstrated that hydrophobic interactions at the site of Ile3in bacteriophage T4 lysozyme, cf. Figure 1, contributedirectly to its overall thermal stability. This was quanti-fied as the difference in free energy change upon thermalunfolding between each mutant and the wild type, ∆∆G(∆∆G < 0, corresponding to a less stable mutant). TheIle3mutants are fully functional lysozymesand the three-dimensional structures for the crystallized Ile3fTyr andIle3 f Val mutants are similar to that of the wild type.16With a set of these mutants, remarkable differences inadsorption behavior and interactions with surfactantswere recently demonstrated for mutants with relativelylow (Ile3fTrp,∆∆G) -2.8 kcalmol-1, pH 6.5) and high(Ile3 f Cys, ∆∆G ) 1.2 kcal mol-1, pH 6.5) structuralstability.17,18 On adsorption to silica, lower plateauamounts, i.e., higher area permolecule, and significantlyhigher and faster loss of R-helix structure implied morepronounced conformational changes for the Ile3 f Trpmutant than for the wild type.In order to obtain information on the interactions

between adsorbed layers of bacteriophage T4 lysozymeand to elucidate the difference in interfacial behavior ofthe wild type T4 lysozyme and the less stable Ile3 f Trpmutant (∆∆G) -2.8kcal/mol), the interferometric surface

force technique19-21 has been employed. This techniqueallows a determination of the interaction between twomacroscopic surfaces (here negatively chargedmuscovitemica) as a function of their absolute separation. In thisreport we demonstrate how the interaction betweensurfacesbearingadsorbed layers ofprotein, combinedwiththe known dimensions of the protein can be used to gaininformation about the orientation and conformation ofproteins on the surface and to demonstrate that largescale structural changes occur upon adsorption.21

2. Experimental Section

Both the wild type bacteriophage T4 lysozyme and the Ile3 fTrp mutant were produced using individual bacterium strainscontaining the mutant lysozyme expression vectors kindlyprovided by Professor Brian Matthews and co-workers.22 Theexpression and purification followed established proce-dures.16,17,23-25 The bacteriophage T4 lysozyme has 164 aminoacid residues and a molecular weight26 of 18 700 g/mol, and itsglobular shape canbedescribedasanellipsoidwithdimensions27(3 × 3 × 5) nm. Its isoelectric point is above pH 9 and it carriesa net positive charge of +9 at neutral pH.The ionic strength was adjusted to 1 mM by use of sodium

chloride, suprapure grade used as received fromMerck, and thepHwas that of unbuffered salt solutions, 5.6. The solutionswereprepared from deaereated water taken fresh from a two-stepMilliporeunit: aMilli-RO10PLUSpretreatmentunitwithdepthfiltration, carbon adsorption, and decalcination preceedingreverse osmosis, and aMilli-Q PLUS185 unit consisting of a UVsource (185 and 254 nm) treating the feed water to the Q-pack,an active carbon unit followed by amixed bed ion exchanger andan Organex cartridge. The water is then passed through a final0.22 µm Millipak 40 filter.The surface forcemeasurementswereperformedusingaMkIV

SFA20utilizing interferometricdetectionof thesurfaceseparation,D, via fringes of equal chromatic order (FECO).28 The force iscalculated according to Hooke’s law, using the force constant ofthe springontowhich the lower surface ismountedand the springdeflection. This latter quantity is calculated as the differencebetween the observed change in D and the expansion of thepiezoelectric tube which holds the upper surface. The piezoexpansion isdeterminedprior to each force run,while thesurfacesare kept far apart for no interaction to be present. The two silicadisks, onto which the silvered side of themica sheets (A) 1 cm2)are glued using an Epoxy resin (Epon 1004), are mounted in acrossed-cylinder configuration.A droplet of approximately 0.1 mL protein solution of

concentration0.02mg/mLin1mMNaClwas thenplacedbetweenthe two surfaces. After allowing the adsorption to take place for2 h or more, when the adsorption was found to have reached aplateau, the force-distance relationship was monitored. Thebox was then filled with protein-free 1 mM sodium chloridesolution, an effective dilution by a factor of 3000, and left for atleast12htoallow foranypossibledesorption. The force-distancerelationship was then determined in this diluted system beforeand after one of the surfaces was replaced with a clean micasurface.

(16) Matsumura,M.;Becktel,W.J.;Matthews,B.W.Nature (London)1988, 334, 406.

(17) McGuire, J.;Wahlgren,M.C.; Arnebrant, T.J. Colloid InterfaceSci. 1995, 170, 182.

(18) Billsten,P.;Wahlgren,M.C.;Arnebrant,T.;McGuire, J.;Elwing,H. J. Colloid Interface Sci. 1995, 175, 77.

(19) Israelachvili, J. N.; Adams, G. E. J. Chem. Soc., Faraday Trans.1 1978, 74, 975.

(20) Parker, J. L.; Christensson, H. K.; Ninham, B. W. Rev. Sci.Instrum. 1989, 60, 3135.

(21) Claesson, P. M.; Blomberg, E.; Froberg, J. C.; Nylander, T.;Arnebrant, T. Adv. Colloid Interface Sci. 1995, 57, 161.

(22) Institute of Molecular Biology, University of Oregon, Eugene,OR.

(23) Alber, T.; Matthews, B. W. Methods Enzymol. 1987, 154, 511.(24) Muchmore, D. C.; McIntosh, L. P.; Russell, C. B.; Andersson, D.

E.; Dahlquist, F. W. Methods Enzymol. 1989, 177, 44.(25) Poteete, A. R.; Dao-Pin, S.; Nicholson, H.; Matthews, B. W.

Biochemistry 1991, 30, 1425.(26) Matthews, B.W.; Dahlquist, F. W.; Maynard, A. Y. J.Mol. Biol.

1973, 78, 575.(27) Matthews, B.W.; Remington, S. J. Proc. Natl. Acad. Sci. U.S.A.

1974, 71, 4178.(28) Israelachvili, J. N. J. Colloid Interface Sci. 1973, 44, 259.

Figure 1. The backbone of bacteriophage T4 lysozyme (whichcan be described as an ellipsoid with dimensions 3× 3× 5 nm).The site of the Ile3fTrp substitution is indicated by the arrow.Figure is based on an original provided by BrianW.Matthews.

Stability of Adsorbed Protein Layers Langmuir, Vol. 14, No. 2, 1998 457

The results, based on severalmeasurements at each conditionand position, are presented with the force normalized by thelocal geometric mean radius of the interacting surfaces, F/R, asa function of surface separation,D. This in turn allows the forceto be converted to the free energy per unit area of interacting flatsurfaces, Gf(D), according to the Derjaguin approximation29

and therefore makes a comparison between experiments andtheory possible. This relationship is valid as long asD , R andinsofarasRdoesnotvarywithD; bothrequirementswere fulfilledfor the results presented here with the exception of the highlyattractive forces between bare mica and mica covered with theIle3fTrpmutant. The resolution of the interferometric surfaceforce technique, and thus the numbers for the distance given,are within 0.2 nm, implying a detection limit of (0.01 mN‚m-1

for the normalized force. A mechanical instability occurswhenever the gradient of the measured force exceeds the springconstant,30 which is observed as a jump to the closest separationwhere the system again is mechanically stable.To facilitate further analysis of the data, a comparison of the

observed forces with those calculated using DLVO theory, i.e.,additive contributions from thenonretarded vanderWaals forceand the electrostatic double-layer force, was carried out. Theelectrostatic contribution was calculated assuming interactionat constant charge and constant potential in the nonlinearPoisson-Boltzmannapproximation, using thealgorithmofChanet al.31 for the symmetrical case of two protein-covered surfaces.In the asymmetric case, i.e., with one bare mica surface and oneprotein-covered surface, the electrostatic contribution wascalculated using the algorithm of Bell and Peterson32 for solvingthe nonlinear Poisson-Boltzmann equation in the case ofconstant charge boundary conditions, while the approach ofDeveraux and deBruyn33 was used to solve the same equationfor the case of constant potential.

3. ResultsThe observed interactions between the two negatively

charged mica surfaces in a solution of 0.02 mg/mL of thewild type bacteriophage T4 lysozyme in 1mMNaCl at pH5.6 are presented inFigure 2. Averyweak repulsive forceis observed on approach, which contrasts to the situationin the electrolyte in absence of protein where the long-range interaction is dominated by a strong electrostaticdouble-layer repulsion decaying in agreement with Pois-son-Boltzmann theory (decay length, κ-1 )9.6 nm).Withthe wild type protein in solution a hard wall is present ata separation of D ) 9.5 nm, originating from contactbetween the two adsorbed layers. We note that unlikethe previously studied hen egg white lysozyme, we do notsee any evidence of surface dimerization of the T4 wildtype.Also evident from Figure 2 is that a dilution by a factor

of 3000 does not alter the interaction on approach. Thereis however a small increase in the forceneeded to separatethe two surfaces from the adhesive minimum at 9.5 nm,leading to a sudden jump apart to separations where noforce acts between the surfaces. The magnitude of thispull-off force increases from 0.3 before to 1 mN‚m-1 afterdilution.Figure 3 shows the force curves measured when the

single-site Ile3 f Trp mutant of bacteriophage T4lysozyme is present in 1 mM NaCl to a concentration of

0.02 mg/mL in the droplet between the surfaces. Theforces measured on approach with protein present in thebulk solutiondisplayno repulsivedouble-layer interactionshowing that the surfaces are uncharged. At 12 nmseparation an attraction sets in, leading to a sudden jumpinto contact between the adsorbed layers at a separationof 3.5 nm. After dilution with 1 mM NaCl by a factor of3000 a weak double-layer interaction is observed duringthe first approach, and as a consequence no jump intocontact occurs. The stericwall at 3.5nmremains. Duringsubsequent approaches the double-layer force has nearlydisappeared and the attraction giving rise to an inwardsudden jump, now from8.5 nm, has reappeared. Another

(29) Derjaguin, B. Kolloid Z. 1934, 69, 155.(30) Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1981, 75, 1400-

11.(31) Chan, D. Y. C.; Pashley, R. M.; White, L. R. J. Colloid Interface

Sci. 1980, 77, 283.(32) Bell, G. M.; Peterson, G. C. J. Colloid Interface Sci. 1972, 41,

542.(33) Devereux, O. F.; de Bruyn, P. L. Interaction of Plane-Parallel

Double Layers; MIT Press: Cambridge, MA, 1963.

Figure 2. Interaction betweenmica surfaces after adsorption(squares) from a solution of 0.02 mg/mL bacteriophage T4lysozyme wild type in 1 × 10-3 M NaCl solution at pH 5.6 andT ) 20 °C. The triangles show the interaction after a dilutionwith pure buffer by a factor of 3000. Filled andunfilled symbolsrepresent the interaction on approach and separation, respec-tively.

Figure 3. Interaction betweenmica surfaces after adsorption(squares) of the Ile3fTrpmutant of bacteriophageT4 lysozymefrom a solution of 0.02 mg/mL in 1 × 10-3 M NaCl solution atpH 5.6 and T ) 20 °C. The triangles show the interaction afteradilutionwithpurebufferbya factor of 3000: trianglespointingupward are the first approach while those pointing downwardrepresent subsequent ones. Filled and unfilled symbols rep-resent the interactiononapproachandseparation, respectively.The solid line is thenonretarded vanderWaals force calculatedaccording to the triple layer film model using A232 ) 2.34 ×10-21 J as the Hamaker constant for the protein-solution-protein interface.

F(D)/R ) 2π Gf(D) (1)

458 Langmuir, Vol. 14, No. 2, 1998 Froberg et al.

difference in this systemcompared to that of thewild typeis that the pull-off force is of equal strength before andafter dilution, with a magnitude (1 mN‚m-1) comparableto that for the wild type after dilution in Figure 2.In order to determine the interactions between the

protein molecules and the negatively charged substratesurface, one of the protein-covered surfaces was in eachexperiment replaced with a bare mica surface after thedilution was performed. This procedure led for bothproteins to a halving of the respective distance of closestapproach and an increase in the repulsive double-layerinteraction as displayed in Figures 4 and 5 and sum-marized in Table 1.Figures 4 and 5 show the interactions after one surface

has been replaced, for the wild type and the Ile3 f Trpmutant, respectively. The insets in these figures showthe interaction on approach for both the symmetric (twoprotein-covered surfaces in buffer) and the asymmetricsystems (one protein-covered and one bare mica surfaceinbuffer) onasemilog scale to elucidate thedecaybehaviorof the forces. The solid lines represent best fits of DLVOtheory to the measured interactions using both theboundary conditions of constant charge and constantpotential, corresponding to the upper and lower curves ineach pair, respectively. The electrostatic double-layerinteraction is assumed to have its origin at the adsorbedlayer/solution interface. The parameters obtained fromthe fitting procedure are presented in Table 1.

For both proteins the exchange of one surface for a baremica surface results in an increase in repulsive forcecaused by the introduction of the strongly charged micasurface. However for the wild type protein an attractionoccurs, followed by a sudden jump, at a separationcomparable to that of the steric wall in the symmetriccase. A similar attraction is present in the case of themutant overcoming the repulsion at 4.5 nm on the firstapproach, while on subsequent approaches the rangeincreases and the jump occurs at 7.5 nm. This is aconsequence of the reduced double-layer force. As will bediscussed below, this is likely due to some transfer ofproteins onto the bare mica surface.Another effect of replacing one surfacewith a baremica

surface is a change in the measured pull-off force. Theeffect is clearly more evident for the single-site mutantwhere this force is -10 mN‚m-1 following the firstapproachwhile increasing in strength on subsequent onesto -30 mN‚m-1. In comparison, the pull-off force for thewild type is reduced from -3 to -2 mN‚m-1 for the firstand subsequent cycles, respectively.

4. Discussion

The mica surfaces used as substrate in these experi-ments carry a net negative charge of typically 0.043 C/m2

under these conditions (equivalent to one charge per 3.7nm2), due to dissolution of potassium ions from the crystalplane facing the solution which are in part replaced by

Table 1. Parameters (K-1, Debye Length; Ψ∞, Apparent Interfacial Potential; σ, Surface Charge; A, Hamaker Constant)Obtained When Fitting DLVO Theory at 1 mM NaCl/ to the Experimental Results As Described in the Text, Together

with Measured Layer Thicknesses, L, and Pull-Off forces, F0/R

systemκ-1

(nm)Ψ1,∞(mV)

σ1× 103(C/m2)

Ψ2,∞(mV)

σ2× 103(C/m2)

L(nm)

A × 1020(J)

F0/R(mM/m)

buffera 9.6 -120 -43.5 -120 -43.5 0 2.2 <-40wild type symmetric 9.6 -10 -0.423 -10 -0.423 4.5 2.2 -1wild type asymmetric 9.6 -120 -43.5 -25 -0.924 4.5 2.2 -3mutant symmetric 9.6 -11 -0.450 -11 -0.450 1.5 1.4 -3mutant asymmetric 9.6 -120 -43.5 -55 -3.25 1.5 2.2 -30a Literature values, e.g., ref 35.

Figure 4. Interaction between one of the surfaces in Figure2, i.e., with adsorbed wild type T4 lysozyme, and a bare micasurface in 1 × 10-3 M NaCl. The circles are the interactionduring the first approach while the diamonds are for thesubsequent ones. Filled and unfilled symbols represent theinteraction onapproachand separation, respectively. The insetshows the interaction on approach on a semilog scale togetherwith the interactionbetween twoprotein-covered surfaces afterdilution (triangles). The solid lines represent best fits of DLVOtheory as described in the text.

Figure 5. Interaction between one of the surfaces in Figure3, i.e., with adsorbed Ile3 f Trp mutant T4 lysozyme, and abare mica surface in 1 × 10-3 M NaCl. The diamonds are theinteraction during the first approach while the circles are forthe subsequent ones. Filled and unfilled symbols represent theinteraction onapproachand separation, respectively. The insetshows the interaction on approach on a semilog scale togetherwith the interactionbetween twoprotein-covered surfaces afterdilution (triangles). The solid lines represent best fits of DLVOtheory as described in the text.

Stability of Adsorbed Protein Layers Langmuir, Vol. 14, No. 2, 1998 459

adsorbing protons. The interaction of two mica surfacesin 1mMNaCl is, therefore, due to thehigh surface charge,characterized by an electrostatic double-layer repulsionwith considerable magnitude, well described with DLVOtheory. Acomparisonwith the longrange interactionafterallowing bacteriophage T4 lysozyme to adsorb, which inboth cases studied here is absent or of low magnitude,indicates that the adsorption led to a nearly completeneutralization of thenegative surface charge ofmica. Thischarge neutralization is a combined effect of the chargesbrought to the interface by the proteins and that of thesmall ions, the concentration of which is necessarilyregulated in this region of low dielectric constant so as toreduce the amount of free charges.Following Norde and co-workers5 the extent of such an

ion adsorption regulation can be estimated by taking intoaccount the adsorbed amount and the apparent surfacecharge with and without adsorbed protein. The inves-tigationpresentedheredoesnot allow for suchananalysissince nomethod to directlymeasure the amount adsorbedto themica surfaces is readilyavailable. Wehavehoweverfor hen egg white lysozyme estimated the ion regulationusing the adsorbed amount obtained through ESCAmeasurements and indeed found that such a regulationtakesplace. Theadsorptionofpositively charged lysozymeis accompanied by a desorption of small positive ions.34The apparent similarity between hen egg white andbacteriophage T4 lysozyme suggests that the samewouldbe true for T4 lysozyme. A similar analysis in the caseof bacteriophage T4 lysozyme in which values fromellipsometry on hydrophilic silica were used for theadsorbed mass (most likely yielding underestimatedamounts for the adsorption to mica21) also showed thesame tendency.The charge neutralization following adsorption points

to the importance of electrostatic interactions as one ofthe driving forces for adsorption of bacteriophage T4lysozyme. This is also in agreement with other measure-ments of the interactions in systems containing proteinswhich are positively charged at the conditions used, e.g.,lysozyme,35 cytochrome c,36,37 and ribonuclease A.38 Thedecay lengths obtained from fitting DLVO theory to theexperimental results agree well with that expected forthe ionic strength used; cf. Table 1. The contribution ofthe protein to the ionic strength is unimportant due to thelow protein concentration used (≈1 µM) relative to thebackground electrolyte, in agreementwith the discussionofKekicheff andNinhamon the importance ofmultivalentions to the decay length.36Two differences between the wild type and the mutant

bacteriophageT4 lysozymesareobvious fromacomparisonof Figures 2 and 3: (i) the presence of a long rangeattraction and (ii) a smaller distance of closest approach,in the case of the mutant protein. The appearence of along rangeattractionbetween themutant layers is simplydue to theabsence of anydouble-layer force for this systemunlike for the wild type where a small double-layer forceis observed. The most probable origin of this long rangeattraction is a van der Waals force between the adsorbedfilms. The solid line inFigure3 represents the interactioncalculated according to the triple-layer film model of

Ninham and Parsegian.39 The system is considered to becomposed of two semi-infinite slabs of material 1, eachcovered with a thin film of material 2 of thickness L,interacting across medium 3 at distances D between thetwo slabs. The Hamaker constant for the mica-water-mica interface,A131, is known, while that for the protein-water-protein interface, A232, is assumed to give a set ofHamaker constants for the system according to thefollowing combining rules40

An effective Hamaker constant Aeff is then calculatedaccording to

to give the correct gradient of the attractive force at thedistance where the surfaces jump into contact,Djump, thisjump being due to the mechanical instability occurringwhen thegradient of the force exceeds the spring constant.For the present case A232 ) 2.35 × 10-21 J, giving aneffectiveHamaker constant of 1.40× 10-20 J, gives a goodfit to thedata. ThevalueofA232 is lower than that reportedfor proteins, e.g., for bovine serum albumin41 6.6 × 10-21

J, which indicates a high water content in the proteinlayer. This is realistic considering the presence ofionizable groups and other hydrophilic entities at theinterface. Hence it seems as one does not need to invokeany bridging mechanism or hydrophobic interaction toexplain the observed attraction.Perhaps the most remarkable difference between the

two proteins studied is their layer thicknesses. Figures2 and 3 show a distance of closest approach of 9.5 nm forthe wild type and 3.5 nm for themutant in the case of twoprotein-covered surfaces. Since this is a measure of thethickness of two layers in contact, the thickness of onelayer is half this value, i.e., 4.8 and 1.8 nm, respectively.However, thedistanceof closestapproachmightalso resultfrom interpenetration of the two layers in contact. Toassure that what is measured is the thickness of oneadsorbed layer, one of the surfaces was exchanged for abaremica surface. This resulted, as canbe seen inFigures4 and 5, in a distance of closest approach of 5.0 nm for thewild type and 1.5 nm for the Ile3 f Trp mutant. Theselayer thicknesses were not altered during a sequence ofapproach-separation cycles.The tertiary structure for both the wild type and the

Ile3 f Trp bacteriophage T4 lysozyme in solution, cf.Figure 1, can be described as an ellipsoidwith dimensions3 × 3 × 5 (nm). A comparison of these dimensions withthe layer thicknesses indicates that thewild type adsorbsin a monolayer with enough molecules having the longaxis perpendicular to the surface, i.e., end-on, to preventthe surfaces from approaching further. It does not,however, show that all molecules are adsorbed end-on.

(34) Blomberg, E.; Claesson, P. M.; Froberg, J. C. Biomaterials, inpress.

(35) Blomberg, E.; Claesson, P. M.; Froberg, J. C.; Tilton, R. D.Langmuir 1994, 10, 2325.

(36) Kekicheff, P.; Ninham, B. W. Europhys. Lett. 1990, 12, 471.(37) Afshar-Rad, T.; Bailey, A. I.; Luckham, P. F.; MacNaughtan,

W.; Chapman, D. Biochim. Biophys. Acta 1987, 915, 101.(38) Belfort, G.; Lee, C. S. Proc. Natl. Acad. Sci. U.S.A. 1991, 88,

9146.

(39) Ninham, B.W.; Parsegian, V. A. J. Chem. Phys. 1970, 52, 4578.(40) Israelachvili, J. N.; Tabor, D. Prog. Surf. Membr. Sci. 1973, 7,

1.(41) Nir, S. J. Theor. Biol. 1975, 53, 83.

A132 ) (xA131A232

A121 ) A131 + A232 - 2A132 (2)

A323 ) A232

Aeff(D)

6(D - 2L)2) 16 ( A131

(D - 2L)2-

2A123

(D - L)2+A121

D2 ) )

-Fvdw(D)

R(3)

460 Langmuir, Vol. 14, No. 2, 1998 Froberg et al.

The layer thickness of the Ile3fTrpmutant on the otherhand is smaller than any of the solution dimensions andthis demonstrates that the protein has lost its tertiarystructure upon adsorption. The smallest solution dimen-sion is 3.0 nm and the layer thickness is only 1.5 nmsuggesting that the changes in conformation are consid-erable. Previously, Billsten et al. studied circular dichro-ism spectra from wild type and Ile3 f Trp mutantlysozyme in solution and adsorbed to nanometer-sizedsilica particles.18 They reported a considerably higherloss in R-helix content upon adsorption for the mutantcompared with the wild type. Hence, the mutant losesmore of its secondary structureuponadsorption thandoesthewild type. The surface force datapresentedhere showthat this is manifested in considerable changes in thetertiary structure of themutantuponadsorption,whereasthe tertiary structure of thewild type is largelyunaffected.An unfolding of the protein is well-known to lead to anincrease in conformational entropy for the protein andthe importance of this for the adsorption process has beenconsideredearlier;5,8 an increase inconformational entropyis thus a plausible reason as to why the structure of themutant changes dramatically upon adsorption.The fact that the Ile3 f Trp mutant unfolds upon

adsorptionalsohasconsequences for theadsorbedamount.When the protein unfolds, it occupies a larger area on thesurface compared to if it retains its structure. Previousellipsometric studies have shown that the adsorbedamount of the mutant on silica indeed is less than thatof the wild type. From the data presented here it seemsvery likely that this is a consequence of the unfolding ofthe mutant on the surface whereas the wild type keepsits tertiary structure.The occurrence of a repulsive force after one of the

protein-covered surfaces has been replaced with a baremica surface is expected for an asymmetric system of onehighly charged surface and one nearly uncharged. Theinsets to Figures 4 and 5 show the measured forces on alog scale together with the results of theoretical calcula-tions within the DLVO approximation performed asdescribed above; cf. Table 1. The lower curve in each pairrepresents interaction at constant potential while theupper is for constant charge. The relative differencebetween the two boundary conditions ismore pronouncedin the asymmetric case compared to the symmetric. Thuscharge regulation is a more important factor whenconsidering asymmetric systems. The two theoreticalcurves should be considered to be limiting cases for theinteraction. For the symmetric case the low magnitudeof the experimentally observed repulsion after dilutionmakes a fit of theoretically calculated forces to themeasured forces difficult. Only at separations less than20 nm is the magnitude of the force larger than theexperimental scatter.The inset to Figure 5 shows the agreement between

measurements and calculated DLVO forces for the in-teraction on a first approach after dilution in the case ofthe Ile3 f Trp mutant. It is also clear that this firstapproachalters theproperties of theadsorbed layers sinceon subsequent approaches no repulsion can bemeasured;cf. the different triangles in Figure 5. Onemay thereforeconclude that the act of bringing the surfaces together thefirst time leads to formationof completelyneutral surfaces,even if the observed interaction during the first approachalready is very weak. The reason for this is most likelythat the unfolded protein resembles a flexible polyam-pholyte and that the layer consists of loops, trains, andtails. When the surfaces are pushed together, the layerbecomes more compact with smaller loops and tails, a

structural changewhich is only slowly reversible. Similardifferences between first and subsequent approaches, butwith larger magnitudes, have been observed for surfacescoated with synthetic polyelectrolytes.42,43 For the asym-metric case, the apparent potential for the bare surfacewas set to -120 mV, a value within the range obtainedfor mica in 1 mMNaCl.35 The result of the fitting for theprotein-covered surfacewas a potential of-25mV for thewild type and -55 mV for the Ile3 f Trp mutant.Compared with the results for the symmetric case wherethe fitting resulted in -10 and -11 mV, respectively, thevalues appear surprisingly high. This could be an effectof material transfer occurring when the surfaces areinitially brought together prior to measuring the long-range forces. Thehigherpotential for themutant indicatesa higher transfer ofmaterial than for thewild type,whichis consistent with the higher adhesion between themutant-coatedsurfaceandmica. The interactionbetweenonebaremica surface andone coveredwithmutant agreeswith that predicted from theory down to a separation of4.5 nm on the first approach, where an attractiveinteractionpulls the surfaces into contact. Onsubsequentapproaches this jumpoccursata larger separation, around7.5 nm. The increase in the range of the attractive forceis a consequence of the reduced double-layer force, andthis difference between subsequent approaches againindicates material transfer.Despite the similarity in apparent potential obtained

through the fitting procedure, distinct differences can befound in the measured force curves in Figures 4 and 5.Themeasured double-layer forces between baremica andmica coated with the wild type have the character ofinteractionunderchargeregulation,as they fall inbetweenthe limitsgivenby theboundary conditions for separationsof 15-9.5 nm where they jump into contact. The double-layer forces between bare mica andmica coated with Ile3f Trp mutant on the other hand more closely follow theinteraction at constant charge. The reason for thisdifference between the two proteins ought to be relatedto the fact that the Ile3fTrpmutantunfoldsat thesurfacewhereas the wild type does not. As a result the mutantexposes more hydrophobic groups, with no charge regu-lating abiblities toward the solution.When the force between a baremica surface and amica

surface with adsorbed Ile3 f Trp mutant was measuredrepeatedly on the same position, it was noted that theposition of the inward jump changed from 4.5 to about 7.5nm. This indicates transfer of material from the protein-coated surface to the bare mica surface. We note thatmaterial transfer between bare surfaces and surfacescoatedwith cationicpolyelectrolytehasbeenreported,44-46

and in these cases the pull-off forces are even strongerthan those found between mica and the adsorbed Ile3 fTrp mutant layer. No similar change in jump positionwas observedwhenabaremica surface andamica surfacewith adsorbed wild type lysozyme were brought togetherrepeatedly. Hence, in this case no, or only limited,material transfer appears to occur.The strength of the attraction giving rise to the jump

into contact is a measure of the strength of the protein-surface interactions, one important origin being interac-

(42) Dahlgren, M. A.; Waltermo, Å.; Blomberg, E.; Claesson, P. M.;Sjostrom, L.; Åkesson, T.; Jonsson, B. J. Phys. Chem. 1993, 97, 11769.

(43) Luckham, P.; Klein, J. J. Chem. Soc., Faraday Trans. 1 1984,80, 865.

(44) Claesson, P. M.; Paulson, O. E. H.; Blomberg, E.; Burns, N. L.Colloids Surf., A: Physicochem. Eng. Apects 1997, 123-124, 341.

(45) Dahlgren,M. A. G.; Claesson, P.; Audebert, R.Nordic Pulp Pap.Res. J. 1993, 8, 62-67.

(46) Dahlgren,M. A. G. J. Colloid Interface Sci. 1996, 181, 654-656.

Stability of Adsorbed Protein Layers Langmuir, Vol. 14, No. 2, 1998 461

tion between charges on the bare surface and ionizablegroups in the adsorbed layer. In the case of the wild typethe interaction can be explained by a van der Waalsattractionandelectrostaticdouble-layer interactionundercharge regulation as can be seen in Figure 3. For themutant on theotherhand, themagnitudeof theattraction,which is enough to overcome the electrostatic double-layerinteraction under constant charge, indicates that it hasanother origin.The reason for the difference between the mutant and

the wild type is that the mutant unfolds on the surfacewhich increases its flexibility and facilitates strongercontacts with the bare surface, which is also apparentfrom the stronger pull-off force and the formation of loopsand tails. The long range attraction seen for the mutantis thus likely due to tails that give rise to a bridgingattraction like that observed for polyelectrolytes.42The adhesion forces give further insight into the

protein-surface interaction. These forces are strongerfor the interactions between one protein layer and a baresurface than between two protein layers, which we alsoobserved for hen egg white lysozyme and human serumalbumin in earlier studies.35,47 This is also more pro-nounced for themutantwhere themagnitude of this forceis very strong, reaching a value 10 times that recorded forthe two protein-covered surfaces. It has previously beenobserved that the adhesion forces between a layer ofhuman serum albumin, a rather “soft” globular protein,and mica are strong and that the magnitude of theadhesion is strongly dependent on the history of thesystem.47 In contrast, the adhesion between a layer of

the more compact hen egg white lysozyme and mica ismuch smaller, comparable to that found between thewildtype T4 lysozyme and mica in this study.

5. Conclusions

From differences in layer thickness recorded for thetwo proteins, differences in forcesmeasured on approach,and much stronger adhesion toward bare mica for themutant,weconclude that the less stable Ile3fTrpmutantof bacteriophage T4 lysozyme loses its tertiary structureupon adsorption while the wild type retains the globularstructure. The fact that adsorption of these proteinsresults in a neutralization of the mica surface chargeclearly shows that electrostatic interactions are animportant driving force for adsorption of the proteins. Themuchstrongeradhesion forceandrangeof attraction showthat the Ile3 f Trp mutant interacts more strongly withthe mica surface than does the more stable wild typebacteriophage T4 lysozyme. This is a consequence of theunfolding of the protein which also appears to give riseto a bridging attraction.

Acknowledgment. We acknowledge the members inProfessor Joseph McGuire’s group in Oregon who per-formed the expression and isolation of the proteins.Professor Brian Matthews and co-workers are thankedfor providing the original bacterial strains containing theexpression vector and the original, on which Figure 1 isbased. The Swedish Research Council for EngineeringSciences (TFR) is acknowledged for financial support.

LA970674W(47) Blomberg, E.; Claesson, P. M.; Tilton, R. D. J. Colloid Interface

Sci. 1994, 166, 427.

462 Langmuir, Vol. 14, No. 2, 1998 Froberg et al.