Langmuir 2009, 25, 4021-4026
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Cooperative Tuneable Interactions between a Designed Peptide Biosurfactant and Positional Isomers of SDOBS at the Air-Water Interface† Lizhong He,‡ Andrew S. Malcolm,‡ Mirjana Dimitrijev,‡ Sagheer A. Onaizi,‡ Hsin-Hui Shen,§ Stephen A. Holt,| Annette F. Dexter,‡ Robert K. Thomas,§ and Anton P. J. Middelberg*,‡ Centre for Biomolecular Engineering, Australian Institute for Bioengineering and Nanotechnology and School of Engineering, The UniVersity of Queensland, St. Lucia QLD 4072, Australia, Department of Physical and Theoretical Chemistry, Oxford UniVersity, South Parks Road, Oxford OX1 SQZ, U.K., and ISIS, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, U.K. ReceiVed August 28, 2008. ReVised Manuscript ReceiVed NoVember 28, 2008 Rationally designed peptide biosurfactant AM1 was mixed with sodium dodecyl benzene sulfonate (SDOBS) to self-assemble a mixed surfactant-biosurfactant layer at the air-water interface. Under optimal conditions in the presence of Zn2+, the interfacial elasticity of the mixed layer was approximately 5-fold higher than for biosurfactant alone. Two head positional isomers, SDOBS-2 and SDOBS-6, were compared for their ability to enhance interfacial film strength. SDOBS-6 forms a stronger layer with AM1 than does SDOBS-2. The highest interfacial elasticity of the AM1/SDOBS-6 layer was 640 mN m-1 whereas the maximum value for the AM1/SDOBS-2 layer was 440 mN m-1. Neutron reflection was used to investigate the structure of AM1/SDOBS films at varied bulk SDOBS concentrations. Both deuterated and nondeuterated SDOBS-2 and SDOBS-6 were used to provide contrast variation. It was shown that there is cooperative interaction between AM1 and SDOBS at low SDOBS concentration in the presence of 100 µM Zn2+, promoting AM1 adsorption at the interface to form a two-layered structure of AM1 resulting in a mechanically strong interfacial film. In the presence of EDTA, only a single AM1 layer was formed at the same SDOBS concentration, and the film did not show lateral force transmission capability. Further increasing the SDOBS concentration to a molar excess of >10× decreased the peptide population at the interface and resulted in a mechanically weak layer. Compared to SDOBS-6, SDOBS-2 depletes AM1 at a lower bulk concentration. These results demonstrate that the film strength of a self-assembled surfactant-biosurfactant mixed layer can be fine tuned by changing the isomer type and concentration of surfactant and by adding or removing metal ions.
Introduction Stimuli-responsive interfaces can dynamically alter their physicochemical properties, such as wettability and hydrophobicity, in response to changes in environmental conditions.1 These reversibly switching interfaces open great opportunities in interfacial engineering across diverse fields.2,3 The study of stimuli-responsive interfaces is in its infancy, and most work has been focused on solid-liquid and/or solid-air interfaces. For example, a self-assembled monolayer (SAM)-coated gold surface can reversibly switch between hydrophilic and hydrophobic states in response to an electric potential.4 Reversible wetting of a surface can also be realized using photoresponsive pyrimidineterminated molecules.5 Studies of stimuli-responsive fluid-fluid interfaces (liquidliquid or liquid-air interfaces) are comparatively limited, although there is a great need for temporary emulsions or foams in industries ranging from pharmaceuticals and foods to oil and minerals. † Part of the Neutron Reflectivity special issue. * Author to whom correspondence should be addressed. Phone: +617-3346-4189. Fax: +61-7-3346-4197. E-mail:
[email protected]. ‡ The University of Queensland. § Oxford University. | ISIS.
(1) Liu, Y.; Mu, L.; Liu, B. H.; Kong, J. L. Chem.sEur. J. 2005, 11, 2622. (2) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (3) Nath, N.; Chilkoti, A. AdV. Mater. 2002, 14, 1243. (4) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16. (5) Abbott, S.; Ralston, J.; Reynolds, G.; Hayes, R. Langmuir 1999, 15, 8923.
There are several switchable systems reported in the literature.6-11 These include redox-active surfactants with controlled surface activities,7,8 polymer-based emulsifiers with the reversible formation of hydrophobic units,9 surfactants with interfacial tension sensitive to UV light,11 and other photoresponsive surfactants.10 A recent example is a switchable oil-water interface based on selectively populating and depopulating a long-chain amidine surfactant by its reversible reaction with CO2, reported by Liu et al.12 Depending on whether it is in a charged, surfaceactive state or an uncharged, surface-inactive state, the surfactant can migrate between the bulk oil phase and the interface, leading to phase coalescence. Although this behavior can control phase coalescence, the method requires hours for the interface to switch from one state to another, and in some cases the switch is incomplete. A new class of reversibly switchable designed biosurfactants, enabling rapid and reversible stimuli-responsive changes in interfacial properties at fluid-fluid interfaces, has recently been demonstrated.13,14 The film strength of the switchable peptide (6) Aydogan, N.; Rosslee, C. A.; Abbott, N. L. Colloids Surf., A 2002, 201, 101. (7) Gallardo, B. S.; Hwa, M. J.; Abbott, N. L. Langmuir 1995, 11, 4209. (8) Gallardo, B. S.; Metcalfe, K. L.; Abbott, N. L. Langmuir 1996, 12, 4116. (9) Mathur, A. M.; Drescher, B.; Scranton, A. B.; Klier, J. Nature 1998, 392, 367. (10) Shang, T. G.; Smith, K. A.; Hatton, T. A. Langmuir 2003, 19, 10764. (11) Shin, J. Y.; Abbott, N. L. Langmuir 1999, 15, 4404. (12) Liu, Y. X.; Jessop, P. G.; Cunningham, M.; Eckert, C. A.; Liotta, C. L. Science 2006, 313, 958. (13) Dexter, A. F.; Malcolm, A. S.; Middelberg, A. P. J. Nat. Mater. 2006, 5, 502.
10.1021/la802825c CCC: $40.75 2009 American Chemical Society Published on Web 01/13/2009
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biosurfactant AM1 at fluid-fluid interfaces responds to the addition and removal of metal ions. Consequently, the AM1coated interface can be quickly switched between a mechanically strong film state and a mobile detergent state. One can thus reversibly form, stabilize, and break emulsions and foams by using this peptide and appropriate facile bulk triggers.13,14 Using neutron reflection, the structure of an AM1 film at the air-water interface has been elucidated, showing that the film comprises a thin layer that is 15 Å thick in both the mechanically strong and weak states.15 Polymer-surfactant mixtures in solution and at interfaces have been extensively studied because of their broad application in, for example, fabric and hair conditioners, paints, coatings, cosmetics, and drug delivery.16-18 These mixed systems can be either uncharged polymers with charged surfactants19 or charged polymers with oppositely charged surfactants.17,18,20 The mixed systems often show cooperative effects on physicochemical properties, enhancing their intended application.21 Designed biosurfactants such as AM1 can also be mixed with either polymers or surfactants to form new and interesting systems possessing attractive properties. For example, we recently reported that a formulation of AM1 with an oppositely charged polymer (Eudragit S-100) can allow control over interfacial elasticity by varying the solution composition, inducing enhanced foaming behavior.22 In this work, we investigate a mixed system of biosurfactant AM1 and the conventional surfactant sodium dodecyl benzene sulfonate (SDOBS) at the air-water interface. SDOBS is a member of the class of linear alkyl benzene sulfonates (ABS), the most important group of synthetic surfactants in terms of industrial-scale manufacture because of widespread use in laundry detergents.23 We expect that anionic surfactants will strongly interact with AM1 because AM1 is cationic and was designed to bind divalent metal ions.24 We show that the addition of SDOBS to AM1 self-assembled layers significantly alters both the mechanical strength of the mixed film at the air-water interface and its structure. At a suitable concentration of peptide and SDOBS, force transmission of the AM1 film can be increased 5-fold by adding SDOBS. In contrast, SDOBS alone is incapable of transmitting a force laterally in the plane of the interface. This behavior clearly shows that there are cooperative interactions between SDOBS and AM1 at the interface. Industrial production processes for ABS usually result in a mixture of alkyl chain homologues with a range of headgroup positional isomers, and varying the ratio of isomers in the synthesis processes can dramatically change the properties of the final products.25 For example, positional isomers of SDOBS differ in their solubility and interfacial activities.25 In this work, two representative isomers of SDOBS, SDOBS-6 and SDOBS-2 (14) Malcolm, A. S.; Dexter, A. F.; Middelberg, A. P. J. Soft Matter 2006, 2, 1057. (15) Middelberg, A. P. J.; He, L.; Dexter, A. F.; Shen, H. H.; Holt, S. A.; Thomas, R. K. J. R. Soc. Interface 2008, 5, 47. (16) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (17) Penfold, J.; Taylor, D. J. F.; Thomas, R. K.; Tucker, I.; Thompson, L. J. Langmuir 2003, 19, 7740. (18) Penfold, J.; Tucker, I.; Thomas, R. K.; Zhang, J. Langmuir 2005, 21, 10061. (19) Cooke, D. J.; Dong, C. C.; Lu, J. R.; Thomas, R. K.; Simister, E. A.; Penfold, J. J. Phys. Chem. B 1998, 102, 4912. (20) Penfold, J.; Tucker, I.; Thomas, R. K.; Taylor, D. J. F.; Zhang, X. L. Langmuir 2006, 22, 7617. (21) Lee, L. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 205. (22) Dexter, A. F.; Malcolm, A. S.; Zeng, B.; Kennedy, D.; Middelberg, A. P. J. Langmuir 2008, 24, 3045. (23) Kosswig, K., Surfactants. In Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH Verlag GmbH: Weinheim, 2000 (electronic formats). (24) Dexter, A. F.; Middelberg, A. P. J. J. Phys. Chem. C 2007, 111, 10484. (25) Ma, J. G.; Boyd, B. J.; Drummond, C. J. Langmuir 2006, 22, 8646.
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Figure 1. Structures of two representative isomers of sodium dodecyl benzene sulfonate (SDOBS).
(chemical structures shown in Figure 1), have been compared for their ability to enhance the strength of biosurfactant AM1 films. These two positional isomers show remarkable differences in their ability to alter the interfacial elasticity of the film. To understand the mechanism behind these differences, we employed neutron reflection to investigate the structure of these mixed layers at the interface. Neutron reflection has been extensively used to study adsorption from mixed systems (e.g., proteins/surfactants and polymers/ surfactants) at the air-liquid interface.16,26,27 Important information such as the surface excess of each species and the interfacial structure of the adsorbed layer can be provided by neutron reflectivity measurements. Interesting results such as the cooperative adsorption of polymers and surfactants have been reported.17,28 In this work, we have used both deuterated and nondeuterated SDOBS isomers to provide contrast variation for AM1 and SDOBS. The populations of individual components at the interface were thus determined, providing valuable information about the interaction of the chemical surfactant and biosurfactant at the interface.
Materials and Methods Materials. AM1 (Ac-MKQLADS LHQLARQ VSRLEHACONH2) was from GenScript Corporation (Piscataway, NJ), and its purity was >95%. The peptide content was determined by quantitative amino acid analysis (Australian Proteome Analysis Facility, Sydney, Australia). AM1 experiments were carried out at a bulk peptide concentration of 5 µM, at which concentration a thin peptide layer forms,15 in the presence of 100 µM ZnSO4 or 100 µM Na+ ethylenediamine tetraacetate (EDTA, to chelate adventitious metal ions). Buffer solutions contained 25 mM Na+ 4-(2-hydroxyethyl)1-piperazine ethanesulfonate (HEPES, ICN Biomedical), pH 7.4. Two representative isomers of SDOBS, SDOBS-2 and SDOBS-6, were prepared by chemical synthesis, and alkyl-chain-deuterated SDOBS-2 and SDOBS-6 were also prepared. GC-MS was used to characterize SDOBS isomers and their precursors, confirming their high purity. Interfacial Tension. Axisymmetric bubble shape analysis used a DSA10 tensiometer (Kru¨ss GmbH, Hamburg, Germany). Air bubbles of ∼8 µL were formed inside a quartz cuvette containing the solution of interest (8 mL). Measurement of the interfacial tension started 2 > 3 > 4 > 5 > 6. Thus, SDOBS-2 tends to adsorb strongly at the interface at a lower concentrations than SDOBS6. This may partially explain why the optimal concentration
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of SDOBS-2 for the maximum adsorption of AM1 is lower than that of SDOBS-6. The sequence of AM1 is based on its parent peptide, Lac21, but with two internal sites substituted with metal-binding histidine residues.13,24 Metal ions, such as Zn2+ used in this work, can bind AM1 histidine residues at the air-water interface. Besides promoting intermolecular cross-linking between two AM1 molecules,24 it is also possible that negatively charged SDOBS can be attracted to AM1-bound Zn2+, enhancing the interaction between SDOBS and AM1. The enrichment of SDOBS at the interface, in turn, may attract more SDOBS and/or AM1 molecules to the interface by nonelectrostatic interactions. Thus, two-layer structures can be formed at an appropriate SDOBS concentration, and because the second layer is less dense than the first, the interaction is likely not strong enough to promote the formation of more layers. The critical role of Zn2+ was further confirmed by the absence of a second layer when EDTA was present in the buffer, instead of Zn2+.
Conclusions We have demonstrated that there is cooperative and synergistic interaction between peptide biosurfactant AM1 and chemical surfactant SDOBS at the air-water interface. In the presence of Zn2+, the inclusion of SDOBS in AM1 solutions can promote the adsorption of AM1 and SDOBS, resulting in stronger
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interfacial films having increased interfacial elasticity. Two positional isomers of SDOBS, SDOBS-2 and SDOBS-6, show different film-enhancing effects. SDOBS-2 requires a lower concentration to reach a maximum interfacial elasticity. However, the highest interfacial elasticity (440 mN m-1) of AM1/SDOBS-2 is lower than that of AM1/SDOBS-6 (640 mN m-1). At very high relative SDOBS concentrations, AM1 is depleted from the interface, resulting in a decrease in interfacial elasticity. Neutron reflection reveals that AM1 exists as two layers at the interface at the optimal SDOBS concentration and that the highest interfacial elasticity occurs when a two-layered structure is observed. The cooperative behavior requires the presence of Zn2+, and adding EDTA can convert the two AM1 layers back to a monolayer, dramatically switching a strong film to a weak film, as reported for an interfacial film composed only of AM1. Acknowledgment. This investigation was conducted with the financial support of the Australian Research Council (grants FF0348465 and DP0771910) and the Access to Major Research Facilities Programme (AMRFP grant 07/08-N-33). L.H acknowledges the receipt of an AINSE Research Fellowship. A.P.J.M. acknowledges the support of the Australian Research Council through the award of a Federation Fellowship. LA802825C
