Colloid Journal, Vol. 64, No. 4, 2002, pp. 482–487. Translated from Kolloidnyi Zhurnal, Vol. 64, No. 4, 2002, pp. 535–540. Original Russian Text Copyright © 2002 by Tarasevich, Monakhova.

Interaction between Globular Proteins and Silica Surfaces Yu. I. Tarasevich and L. I. Monakhova Dumanskii Institute of Colloid and Water Chemistry, National Academy of Sciences of Ukraine, pr. Vernadskogo 42, Kiev-142, 03689 Ukraine Received January 26, 2001

Abstract—The adsorption of bovine serum albumin on silica of three different types, i.e., aerosil, macroporous silica gel, and ground natural quartz, was studied. The IR spectra of this protein adsorbed on aerosil were measured and analyzed. It was shown that the carbonyl groups of albumin macromolecules interact with vicinal hydroxyl groups while imido groups, with individual hydroxyl groups of silica surface. The geminal hydroxyl groups of the surface behaved as single adsorption sites with respect to albumin. The number or such sites on quartz surface was estimated. The IR spectra indicated that the adsorption of albumin macromolecules caused the dehydration of aerosil surface and the appearance of a small amount (to 10%) of unfolded β-regions in the secondary structure of the adsorbed protein, while the α-helical macromolecular structure remains preserved as a whole. Changes in the tertiary structure of the protein resulted from the adsorption were discussed. Protein macromolecules folded into globules were shown to be tilted with respect to the adsorbent surface.

Various types of disperse silica are used as efficient carriers for the immobilization of enzymes and drugs [1, 2]. In connection to this, the interaction between proteins and the silica surface is currently extensively being studied [3–5]. However, comparative studies of quartz, silica gel, and aerosil adsorbability with respect to globular protein macromolecules have not been performed. Moreover, there are only few published data of the IR spectroscopic studies of the interaction between the proteins and silica. In this work, we studied the adsorption of bovine serum albumin (BSA) with the molecular mass M = 68 000 and the isoelectric point corresponding to pH 4.9 on silica of three aforementioned types. The mechanism of the interaction between the globular proteins and silica was considered with the reference to the results of IR spectroscopic study of the aerosil–adsorbed albumin system. The objects for the study were a nonporous silica aerosil A-175 (Production Enterprise Chlorvinyl, Kalush, Ukraine); fine spherical macroporous silica gels for chromatography MSA-750 (pore radius r = 44 nm), MSA-1500 (r = 51 nm), and MSA-2500 (r = 100 nm) (Nizhnii Novgorod Pilot Plant of Research Institute for Oil Processing, Russia); and the samples of crystalline powder of natural α-quartz ground with a TEMA tungsten carbide mill (Germany), which provides only the cleavage, but not the abrasion of the quartz particles during milling. First, a fraction of quartz powder with particle sizes of 10–20 µm and specific surface area S = 1.2 m2/g was prepared. Then, this fraction was additionally milled for 4 and 8 min resulting in the samples with S = 3.2 and 3.5 m2/g, respectively. An X-ray anal-

ysis showed that such a milling does not change the quartz α-structure and does not contaminate the samples with impurities. The specific heats of wetting q of α-quartz samples with the dispersities close to those of the samples used in this work were measured earlier [6, 7] as functions of temperature of their preliminary evacuation, composition, and pH of electrolyte solutions. Albumin adsorption on two samples of aerosil, namely, on mesoporous silica gels KSK-1 and KSK-2 with r = 10–8 and 7.5–6.0 nm, respectively, and silochrom SCh-3 with r = 25–40 nm was studied in [8]. The time required to establish adsorption equilibrium was 48 h (most of the protein is adsorbed during initial 5–6 h), the maximal BSA adsorption is observed in the vicinity of its isoelectric point at pH 4.9–5.0. At this pH value, the numbers of ionized amino and carboxy groups in the protein macromolecules are minimal and equal to each other. The penetration of BSA macromolecules into the mesopores with r = 6.0–10.0 nm is hindered, and only the pores with r > 25 nm provide an unhindered albumin diffusion into the silica structure [8]. Therefore, macroporous silica gels, for chromatography grade, which are characterized by a high purity and uniform large pores with r > 25 nm, were selected as the objects for this study. As to aerosils used in [8], their S values were inaccurate due to false specification data, therefore the maximal values of specific adsorption Γ∞ for albumin were slightly underestimated. This is the reason for the repeated study of A-175 aerosil. The specific surface area S of the silica samples was determined by a low-temperature nitrogen adsorption using the BET equation. The integral heats Q of water wetting of the studied silica samples subjected to pre-

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The bands of the stretching vibrations of OH groups are observed in the aerosil spectra (Fig. 1) at 3750 and 3640 cm–1 corresponded to the individual and hydrogenbonded (vicinal) surface OH groups, respectively [11]. Quite a large half-width and clearly pronounced asymmetry of the latter band are explained by the overlap with the band of stretching vibrations of OH groups of residual molecules of adsorbed water. After albumin adsorption, the bands at 3750 and 3640 cm–1 disappeared in the spectrum. This fact directly indicates the involvement of both individual and vicinal surface OH groups of silica in hydrogen bonding with adsorbed protein molecules. The spectral changes for the adsorbed albumin macromolecules resultant from their hydrogen bonding with the surface are harder to determine, because the peptide groups of protein form sufficiently strong intramolecular (α-helix) or intermolecular (stretched β-structure) hydrogen bonds. As a result, the bands of stretching vibrations of NH and CO groups in the albumin spectrum are shifted to a low-frequency region (to 3300 and 1650 cm–1, respectively) compared with the bands typical of the vibrations of hydrogen-unbonded COLLOID JOURNAL

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For the spectroscopic measurements, BSA was adsorbed on aerosil from aqueous 0.4% solution at pH 5 and the solid-to-liquid ratio of 1 g/100 cm3. During this experiment, protein was almost completely adsorbed on aerosil. The adsorption value was a = 0.35 g/g. To record the IR spectra, oriented films of a pure albumin and albumin adsorbed on an A-175 aerosil were prepared on a fluorite substrate. The amount of the substance deposited onto the transparent substrate was m = 0.4 and 0.5 mg/cm2, respectively. The spectra of pure aerosil were measured using its pellets (m = 13 mg/cm2) pressed at 10 t/cm2. The spectra were measured with a Perkin-Elmer-325 spectrometer in the region from 4000 to 1300 cm–1. The apperture width in this region amounted to 1.20–0.96 cm–1. After the samples were placed into the cell compartment of the spectrometer, they were dried using a zeolite desiccant that made it possible to substantially reduce the effect of adsorbed water on the spectra.

Absorption, %

The adsorption of BSA on aerosil and silica gels was performed from aqueous 0.2–1.2% solutions; the adsorption on the ground quartz, from 0.01–0.1% solutions at t = 5°ë and pH 5. The solution-to-adsorbent ratio was constant and amounted to 100 cm3/g. The duration of the contact between the adsorbents and the solutions was equal to 48 h. The values of protein adsorption were calculated from the difference between its concentrations in the initial and equilibrium solutions, which were determined by the Lowry method [10].

3750

liminary evacuation at 110°C were measured with a Calvet microcalorimeter as described in [9]. Then, the obtained values were related to unit surface area of the adsorbent, and the specific heats of wetting q = Q/S were determined.

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2500 1800

1600

3

ν, cm–1

Fig. 1. Infrared spectra of (1) aerosil, (2) albumin film, and (3) albumin adsorbed on aerosil.

imido and carbonyl groups (3460 and 1700 cm–1, respectively [12]). There is no difference between the absorptions related to the stretching vibrations of NH groups in the adsorbed protein and in the albumin films. However, the band of C=O stretching vibrations (the amide I band) of the adsorbed protein has a shoulder on the side of lower frequencies at 1635 cm–1. This is a result of hydrogen bonding between the carbonyl groups of adsorbed protein and the surface OH groups of aerosil. A decrease in the frequency of stretching vibrations of CO groups in the adsorbed protein reflects a lowered multiplicity of C=O bond resulted from a shift of the bond-forming π-electrons to the oxygen atom under the effect of electron-acceptor surface OH groups. Vicinal hydroxyl groups of silica surface have pronounced electron-acceptor properties [13]. Carbonyl groups of albumin form hydrogen bonds precisely with these surface OH groups. The comparison of the spectroscopic data obtained in this work with the spectra of acetone interacting with the silica vicinal OH groups [14] shows that hydrogen bonding between BSA carbonyl groups and the surface vicinal OH groups is stronger than similar bonding in the acetone–silica system. Hence, better conditions are created in a polypeptide chain for πelectron shift to the oxygen atoms of carbonyl groups under the action of electron-acceptor partners. Only a small part of the protein peptide groups (18%, as estimated in [15]) is involved in the specific interaction with silica hydroxyl groups. Most probably they are the peptide groups located near the typical of albumin disulfide bridges bonding together remote parts of one macromolecule or two neighboring macromolecules of the protein. In the vicinity of disulfide bridges, the helical (secondary) structure of BSA is dis-

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torted, and the polymer becomes amorphous in these regions. It is these regions whose carbonyl groups are involved in hydrogen bonding with the vicinal hydroxyl groups of aerosil surface. In addition to the electron-donor carbonyl group, the peptide unit contains the proton-donor imido group NH, which is also capable of hydrogen bonding with the hydroxyl groups of silica surface. The analysis of the properties of OH groups of SiO2 demonstrates that the individual and geminal hydroxyl groups responsible for the band at 3750 cm–1 have basic rather than acidic properties and can exchange, for example, for fluorine anions [16]. Quantum-mechanical calculations [13] suggest the interaction between polar hydrogencontaining structures and oxygen atoms of these groups. Hence, in our opinion, NH groups of the disordered regions of globular proteins are most likely to form hydrogen bonds with oxygen atoms of silica hydroxyl groups responsible for the band at 3750 cm–1 in the IR spectrum. The bands corresponding to the stretching vibrations of OH groups of adsorbed water (ν = 3620–3400 cm–1) are absent in the spectrum of aerosil with adsorbed protein. Hence, BSA adsorption on aerosil surface is accompanied by the adsorbent dehydration. As was shown in [17], this suggests a strong bonding between the protein and the solid surface. The aforementioned suggestion is confirmed by the fact that the fundamental band of amid I at 1655 cm–1 related to α-helical secondary structure of the protein has a low-frequency shoulder at 1635 cm–1 corresponding to the unfolded β-form of albumin [18]. The analysis of the amid II band at 1540 cm–1, which is mainly determined by the interaction between the C–N stretching vibration and the N–H deformation vibration, where the latter predominates (40 and 60%, respectively), also confirms the formation of a small amount of unfolded fragments in the albumin polypeptide chain as a result of the interaction between the corresponding peptide groups and silica surface. A lowfrequency peak at 1530 cm–1 characteristic of the unfolded β-structure is observed in the spectrum of adsorbed BSA. The band at 1530 cm–1 can be attributed to the formation of a hydrogen bond NH…OH, where OH is an individual hydroxyl group of silica. This bond is weaker compared with the intramolecular hydrogen bond NH…O=C typical of the secondary helical structure of protein. The shoulder at 1520 cm–1 observed in the spectra of both pure and adsorbed BSA (Fig. 1) is assigned to the parallel component of the amid II band typical of the protein α-structure [19]. This peak is characteristic of the α-helix and serves to identify the latter [19]. A lowintense peak at 3070 cm–1 observed in the spectra of solid and adsorbed albumin (Fig. 1) is attributed to the first overtone of the amid II band, 2ν = 2 × 1535 = 3070 cm–1 [19]. The bands at 2960, 2930, and 2875 cm–1

correspond to protein C–H stretching vibrations and the peak at 1450 cm–1, to C–H deformation vibrations. The comparison of the spectrum of BSA adsorbed on aerosil with the spectra of α- and β-forms of proteins and polypeptides [18, 19] directly shows that the adsorption causes a relatively small part of the polypeptide chain of BSA macromolecule to be transformed into the β-form. Comparison of the IR spectra presented in Fig. 1 and the spectra of BSA absorbed in the interlayer spacings of montmorillonite with a width ∆d = 1.4 nm [20] also confirms the appearance of the unfolded β-structure in a protein adsorbed layer. During the adsorption in the thin slit-like pores of this mineral with a width of ≈1.4 nm, BSA macromolecules whose diameter in an aqueous solution is d = 6.2 nm [21] are forced to drastically change their secondary structure also due to a substantial rise in the fraction of planar β-fragments of the polypeptide chain. As a result, the amide I band in a spectrum of BSA adsorbed by montmorillonite is transformed into a doublet of the components at 1658 and 1638 cm–1 (α-helix and β-structure, respectively). In the range of amid II unit vibrations, in addition to the high-frequency component at 1554 cm–1 (α-helix), a lower-frequency peak with an equal intensity is observed at 1538–1532 cm–1 (β-structure). The peak at 1532 cm–1 becomes dominating in the IR spectrum upon heating. The spectroscopic data published in [20] and considered here demonstrate once more that BSA macromolecules adsorbed in the interlayer spacings of montmorillonite have a structure containing approximately equal amounts of α- and β-fragments. The layer of the protein adsorbed on a nonporous aerosil surface is characterized by substantially smaller amount of unfolded βfragments. However, the characteristic peaks at 1635 and 1530 cm–1 observed in its spectrum unambiguously witness their presence. As was shown in [21], the helix content in BSA macromolecules in an aqueous solution amounts to 66%. When BSA interacts with molecules of anionic or cationic surfactants, it decreases to approximately 50%. Similar 15–30% decrease in the content of the helical fragments in bovine and human serum albumins desorbed from SiO2 surface was observed in [22, 23] by the method of UV circular dichroism. Note that a mean decrease in the amount of helical fragments of adsorbed albumin ∆ = 20% [22] coincides with the fraction of carbonyl groups bonded with SiO2 surface amounting to 18% [15]. This coincidence is not accidental. A strong bonding between the fragments of BSA macromolecules and silica surface causes an evident decrease in the amount of α-helices in adsorbed macromolecule. Our spectroscopic data do not allow us to quantitatively determine the content of α- and β-structures in BSA adsorbed on aerosil. However, the graphical division and the subsequent measurement of relative inteCOLLOID JOURNAL

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gral intensities of the bands at 1655 and 1635 cm–1 indicate that the fraction of β-structure in the adsorbed protein does not exceed 10%. Macromolecule of BSA is known to be a “soft protein” [23]; i.e., a protein, which quite readily and without distortions in the globular structure as a whole, adapts itself for interactions with partners, whether it be surfactant molecules in solutions or an adsorbent surface. A small amount of β-fragments appearing in the adsorbed biopolymer macromolecule does not break, but only slightly modify its globular structure. Morrissey and Stromberg [15] also concluded that the interaction of 18% carbonyl groups of BSA with silica surface does no markedly change the globular structure of the adsorbed protein. Protein adsorption under conditions close to its iso+ electric point ensures minimal amount of charged NH 3 and ëéé– groups in the side units of the polypeptide chain. Some of the peaks characteristic of the charged + NH 3 groups (the band at 3225 cm–1 assigned to asymmetric stretching vibrations of NH bonds) and ëéé– groups (the band at 1396 cm–1 assigned to symmetric stretching vibrations of ëéé– group [18]) are observed in the presented IR spectrum. + NH 3

Positions of the peaks related to and ëéé– groups of the side units of the polypeptide chain of serum albumin are similar in the spectra of its solid film and adsorption layer. This means that the interaction of the charged groups of side units in the polypeptide chain of adsorbed protein with the surface is not dominating in the case under consideration. Such groups are mainly involved in electrostatic interaction with each other. This was confirmed in [22], where only carboxyl groups could be titrated in adsorbed albumin in the vicinity of the isoelectric point at pH ≈ 5.5. Hence, these groups do not directly interact with the surface. The interaction between the part of positively charged + NH 3 groups of adsorbed protein and OH groups of silica is undoubtedly more probable. The band of the + stretching vibrations of NH 3 groups H-bonded with the surface is in the region of 3225 cm–1. The length R of the N+–H…O hydrogen bond, where O is an oxygen atom of a hydroxyl group on silica surface, was calculated by the equation [24] ∆ν = 50[(d/R)12 – (d/R)6],

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Fig. 2. Adsorption isotherms of BSA on (1) aerosil, (2) MSA-750, (3) MSA-1500, and (4) MSA-2500 macroporous silica gels; and ground quartz with different specific surface areas: (5) 3.5 and (6) 1.2 m2/g.

of the side units of BSA and hydroxyl groups of silica surface are characterized by moderate strength. Passing to the adsorbed state, protein macromolecule changes not only its secondary, but also tertiary structure. The data on the adsorption give some information about these changes. They are presented in Fig. 2 and the table. Hydroxyl group concentrations on the Physicochemical characteristics of the studied types of silica Silica

where ∆ν is the frequency shift of an XH group stretching vibration due to X–H…Y hydrogen bonding, d is the van der Waals contact distance between X and Y (for the N–H…O bonds it is equal to 0.34 nm), and R is the length of the X–H…Y hydrogen bond. The comparison of the obtained value R = 0.294 nm with the lengths of N+–ç…é hydrogen bonds in various compounds (R = 0.268–0.324 nm [25]) allows us to state + that the hydrogen bonds formed between NH 3 groups

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Aerosil Silica gel MSA-750 Silica gel MSA-1500 Silica gel MSA-2500 Ground quartz Ground quartz Ground quartz

S, m2/g 170 46 35 20 1.2 3.2 3.5

Γ∞, q, C(OH), mJ/m2 mg/m2 µmol/m2 160 220 – 215 – 425 440

2.23 5.0–6.0 3.26 3.20 7.6–8.1 3.25 3.33 3.43 11.5–12.1 3.42

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surfaces of silica of these three types [11, 26, 27] are also listed in the table. The adsorption isotherms for all studied systems have the Langmuir pattern that makes limiting BSA adsorption easy to determine. Being recalculated per unit surface area of the adsorbent, it determines the specific adsorption Γ∞ of the protein. The Γ∞ values obtained for macroporous silica gel and ground quartz agrees well with published data [2]. As was shown in [28], the Γ∞ and q values are linearly interrelated for various nonporous disperse materials (glass, talc, pyrophyllite, and kaolinite). The Γ∞ and q values listed in the table for aerosil and macroporous silica gel accurately fit the obtained straight line. Because hydroxyl groups are the adsorption sites of these adsorbents, the Γ∞ value must correlate not only with q, but also with the concentration C(OH) of surface hydroxyl groups. Indeed, the ratio of protein specific adsorption to the mean hydroxyl group concentration is practically the same for aerosil and silica gel, being equal to Γ∞/C(OH) = 0.40 and 0.41 mg/µmol, respectively. However, note that the Γ∞ values are practically identical for silica gel and quartz, though the surface concentration C(OH) is by a factor of 1.5 higher for quartz due to the presence of geminal Si(OH)2 groups. Hence, we can conclude that geminal hydroxyl groups act as unit adsorption sites with respect to BSA macromolecules. Such a property of coupled OH groups of silica is typical with respect not only to biopolymers, but also to low-molecular-weight substances, for example, diethyl ether, triethylamine, and acetone, but not with respect to water [14]. Furthermore, the energy of water adsorption on geminal hydroxyl groups is higher than that on two individual OH groups. This observation is confirmed by the fact that the specific heat of wetting for quartz is twofold higher than for silica gel (see table). Assuming that the number of geminal OH groups on silica gel surface can be ignored, we can estimate their concentration on the quartz surface (66–68% of the total OH group content) from the data presented in the table. The size of a globular albumin molecule equals 4 × 4 × 14 nm [29]. The adsorption layer of albumin on methylated silica surface is 4 nm thick, as measured by ellipsometry [26]; i.e., the almumin molecules are adsorbed on a hydrophobic surface by the plane having dimensions of (4 × 14) nm2. Among the studied types of hydrophilic silica, the adsorption of plane-oriented globular albumin molecules can occur only on the aerosil surface, but at their extremely high packing density. At the obtained values of adsorption of serum albumin on silica gel and quartz, the plane orientation of macromolecules in a monolayer on the surface of these adsorbents is unrealistic. At the same time, for the vertical orientation of albumin molecules on silica of these types, the obtained degree of surface coverage is too small (θ ≅ 0.5). Hence, a tilted

orientation of BSA macromolecules on the studied types of silica including aerosil is most probable. It is this orientation that is suggested by the data on the adsorption of albumin on a hydrophilic muscovite mica [30]. It should be kept in mind that the globular structure of albumin is labile. Under the effect of the surface forces, the shape and the sizes of the globules (i.e., tertiary structure of the protein) can deviate from those in an aqueous solution or crystal. This is a consequence of the conformational changes in the secondary structure of the protein resulted from the adsorption. Some structural data concerning this problem are presented in [30] for the albumin–muscovite system. The tertiary structure of globular protein macromolecules adsorbed on silica must be affected by the surface hydroxyl groups of different types inherent to these adsorbents and by their distribution over the surface. Hence, the coexistence of different globules with different types of orientation on hydrophilic silica is most probable. This problem is yet to be studied. ACKNOWLEDGMENT The authors are grateful to E.V. Gribanov for providing the samples of ground α-quartz for the study. REFERENCES 1. Kestner, A.I., Usp. Khim., 1974, vol. 43, no. 8, p. 1480. 2. Izmailova, V.N., Yampol’skaya, G.P., and Summ, B.D., Poverkhnostnye yavleniya v belkovykh sistemakh (Surface Phenomena in Protein Systems), Moscow: Khimiya, 1988. 3. Brusatori, M.A., Van Tassel, P.R., Talbot, J., et al., Fundamentals of Adsorption 6: Proc. 6th Int. Conf., Giens, France, Meunier, F., Ed., Paris: Elsevier, 1998, p. 485. 4. Kazakova, O.A., Gun’ko, V.M., Voronin, E.F., et al., Kolloidn. Zh., 1998, vol. 60, no. 5, p. 613. 5. Davydov, V.Ya. and Khokhlova, T.D., Zh. Fiz. Khim., 2000, vol. 74, no. 7, p. 1292. 6. Gribanova, E.V., Tarasevich, Yu.I., Polyakov, V.E., and Belousov, V.P., Kolloidn. Zh., 1984, vol. 46, no. 2, p. 231. 7. Polyakov, V.E. and Tarasevich, Yu.I., Ukr. Khim. Zh., 1988, vol. 54, no. 11, p. 1134. 8. Tarasevich, Yu.I., Smirnova, V.A., and Monakhova, L.I., Kolloidn. Zh., 1978, vol. 40, no. 6, p. 1214. 9. Polyakov, V.E., Polyakova, I.G., and Tarasevich, Yu.I., Kolloidn. Zh., 1976, vol. 38, no. 1, p. 188. 10. Bailey, J.L., Techniques in Protein Chemistry, Amsterdam: Elsevier, 1962. Translated under the title Metody khimii belkov, Moscow: Mir, 1965, p. 265. 11. Kiselev, A.V. and Lygin, V.I., Infrakrasnye spektry poverkhnostnykh soedinenii (Infrared Spectra of Surface Compounds), Moscow: Nauka, 1972. 12. Bellamy, L.J., The Infra-Red Spectra of Complex Molecules, London: Methuen, 1958. Translated under the title Infrakrasnye spektry slozhnykh molekul, Moscow: Inostrannaya Literatura, 1963. 13. Takahashi, K., J. Colloid Interface Sci., 1982, vol. 88, no. 1, p. 286. COLLOID JOURNAL

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INTERACTION BETWEEN GLOBULAR PROTEINS AND SILICA SURFACES 14. Hair, M.L. and Hertl, W., J. Phys. Chem., 1969, vol. 73, no. 12, p. 4269. 15. Morrissey, B.W. and Stromberg, R.R., J. Colloid Interface Sci., 1974, vol. 46, no. 1, p. 152. 16. Chukin, G.D. and Malevich, V.I., Zh. Prikl. Spektrosk., 1977, vol. 26, no. 2, p. 294. 17. Haynes, Ch.A., Sliwinsky, E., and Norde, W., J. Colloid Interface Sci., 1994, vol. 164, no. 2, p. 394. 18. Suzy, H., Structure and Stability of Bilogical Macromolecules, Timasheff, S.N. and Fasman, G.D., Eds., New York: Dekker, 1969. Translated under the title Struktura i stabil’nost’ biologicheskikh makromolekul, Moscow: Mir, 1973, p. 481. 19. Chirgadze, Yu.N., Infrakrasnye spektry i struktura polipeptidov i belkov (Infrared Spectra and Structure of Polipeptides and Proteins), Moscow: Nauka, 1965. 20. Tarasevich, Yu.I., Smirnova, V.A., and Monakhova, L.I., Kolloidn. Zh., 1975, vol. 37, no. 5, p. 912. 21. Takeda, K., Harada, K., Yamaguchi, K., and Moriyama, Y., J. Colloid Interface Sci., 1994, vol. 164, no. 2, p. 382.

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22. Norde, W., MacRitchie, F., Nowicka, G., and Lyklema, J., J. Colloid Interface Sci., 1986, vol. 112, no. 2, p. 447. 23. Norde, W., Cells Mater., 1995, vol. 5, no. 1, p. 97. 24. Bellamy, L.J. and Owen, A.J., Spectrochim. Acta A, 1969, vol. 25, no. 2, p. 329. 25. Pimentel, G.C. and McClellan, A.L., The Hydrogen Bond, San Francisco: Freeman, 1960. Translated under the title Vodorodnaya svyaz’, Moscow: Mir, 1964. 26. Zhuravlev, L.T., Langmuir, 1987, vol. 3, no. 2, p. 316. 27. Tarasevich, Yu.I., Stroenie i khimiya poverkhnosti sloistykh silikatov (Structure and Surface Chemistry of Layered Silicates), Kiev: Naukova Dumka, 1988. 28. Tarasevich, Yu.I., Monakhova, L.I., and Yurasova, V.A., Ukr. Khim. Zh., 1984, vol. 50, no. 10, p. 1032. 29. Malmsten, M., J. Colloid Interface Sci., 1994, vol. 166, no. 2, p. 333. 30. Blomberg, E., Claesson, M., and Tilton, R.D., J. Colloid Interface Sci., 1994, vol. 166, no. 2, p. 427.