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Accelerated Exchange in Polyelectrolyte Multilayers by “Catalytic” Polyvalent Ion Pairing Houssam W. Jomaa and Joseph B. Schlenoff* Department of Chemistry & Biochemistry and Center for Materials Research and Technology (MARTECH), The Florida State University, Tallahassee, Florida 32306 Received April 26, 2005. In Final Form: June 20, 2005 Self-exchange of isotopically labeled polycarboxylic acid within a polyelectrolyte multilayer proceeds to completion and is reversible. Similar exchange with poly(styrene sulfonate), which forms nonlabile polyelectrolyte complexes, is slow and irreversible but is facilitated by polyvalent ion pairing interventions of a third polyelectrolyte. This is an example of accelerated kinetics in “sticky” synthetic systems associated by nonspecific polyvalent interactions.
Introduction Interactions among molecules, particles, and surfaces can be substantially enhanced if multiple connection points are made simultaneously.1 Such “polyvalent” binding relies on a roughly additive effect of the free energy of each interaction site, ∆Gsite, over N such sites.1 Countless examples of polyvalent interactions are found in nature1 and include base pairing of DNA over very long correlation lengths and the attachment of viruses to their target hosts.1 Cooperative interactions are also driving forces for the formation of synthetic polyelectrolyte complexes, such as those formed by the layer-by-layer sequential adsorption method (“polyelectrolyte multilayers”, PEMUs).2 Although polyvalency enhances interactions between molecules that might otherwise be weakly associated, the resulting structures are sometimes rugged enough to be kinetically irreversible.1,3 Irreversible association may not be desirable for “living” or responsive systems. Reversibility is a delicate balance between the number of sites and the energy for each site. Nowhere is this balance more exquisitely demonstrated than in the binding of complimentary strands of DNA, where individual strands separate at sufficiently low salt concentration.4 Alternatively, highly designed molecular machinery, such as helicases, must be recruited to prize apart interlocked strands of polynucleic acids.5 Polyvalent interactions in other biomolecules, such as transcription factors, also assist in binding to DNA.6 “Unzipping” of complexed synthetic polyelectrolytes is similarly challenging if the interactions are strong enough but the specialized molecular apparatus for assisting in the dissociation is lacking. We report here an example of a synthetic macromolecule assisting, in a catalytic sense, in the * Corresponding author. E-mail:
[email protected]. Fax: (850) 644-3810. (1) (a) Mammen, M.; Chio, S.; Whitesides, G. M. Angew. Chem., Int. Ed. 1998, 37, 2754. (b) Misevic, G. Trends Glycosci. Glycotechnol. 1991, 3, 400. (2) (a) Multilayer Thin Films-Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003. (b) Decher, G. Science 1997, 277, 1232. (c) Decher, G. In Comprehesive Macromolecular Chemistry; Sauvage, J. P., Hosseini, M. W., Eds.; Pergamon Press: Oxford, U.K., 1996; Vol. 9, Chapter 14. (d) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (3) Schlenoff, J. B.; Ly, H.; Li, M. J. Am. Chem. Soc. 1998, 120, 7626. (4) (a) Rouzina, I.; Bloomfield, V. A. Biophys. J. 2001, 80, 894. (b) Gotoh, O.; Wada, A.; Yakubi, S. Biopolymers 1979, 18, 805. (5) Patel, S. S.; Picha, K. M. Annu. Rev. Biochem. 2000, 69, 651. (6) Maher, L. J. Curr. Opin. Chem. Biol. 1998, 2, 688.
separation and exchange of two other highly associated polyelectrolytes. Experimental Section Materials. Poly(diallyldimethylammonium chloride), PDADMAC, and poly(styrene sulfonate), PSS, were used as received from Sigma-Aldrich, and poly(tert-butyl methacrylate), PBMA, and deuterated poly(tert-butyl methacrylate), d8-PBMA, were from Polymer Source Inc. and were hydrolyzed to produce the corresponding acids. The corresponding polyester (0.1 g, 7.69 × 10-4 mole for PBMA and 6.66 × 10-4 mole for d8-PBMA) was dissolved in 10 mL of 1,4-dioxane to which excess trifluoroacetic acid was added. The solution was then heated at 105 °C for 13 h. At this point, the acid derivative had precipitated from solution. The solution/solid mixture was then dried under vacuum to produce the crude product, which was redissolved in water and dialyzed extensively against pure water. The polycarboxylic acid was isolated using a Heto vacuum centrifuge in series with a cooling trap. Infrared spectra of both PMA and d5-PMA reveal complete (>95%) hydrolysis of the corresponding ester into an acid. IR bands at 1370 and 1030 cm-1 for PBMA and d8-PBMA disappeared from the spectra of the protonated and deuterated acids, respectively. Properties and structures of all polyelectrolytes used for multilayer buildup and exchange are presented in Table 1 and Scheme 1. Sodium chloride and sodium phosphate were from Fisher. Single-stranded salmon sperm DNA was a generous gift from Dr. Scott Olenych of the Department of Biological Sciences at The Florida State University. Multilayer Buildup on ATR-FTIR. Our system starts with a 12-layer PEMU of PDADMA and deuterated poly(methacrylic acid), d5-PMA, or poly(methacrylic acid), PMA, built on a germanium attenuated total internal reflectance (ATR) crystal. Such a setup, used extensively by Granick,7 Sukhishvili,8a and others,9 allows the composition of the thin film to be monitored by FTIR in situ as the PEMU is exposed to other polyelectrolytes (the solution concentration of these is too low to be detected). ATR-FTIR measurements were performed with a Nicolet Nexus 470 FTIR fitted with a 0.5 mL flow-through ATR assembly housing a 70 × 10 × 6 mm3 45° germanium crystal (Specac Benchmark). Multilayer buildup was carried out from 3 mM polymer solutions in 0.2 M NaCl and 0.1 M phosphate buffer at pH 8.5 for 10 min. Between alternating depositions, the crystal was washed with 60 mL of phosphate buffer at the same pH. The pH was maintained at 8.5 (with 0.1 M sodium phosphate buffer) to ensure complete ionization of the carboxylate groups, and the salt concentration was 0.2 M, which creates conditions for rather weak (i.e., labile) interactions between PDADMA and poly(7) Frantz, P.; Granick, S. Macromolecules 1994, 27, 2553. (8) (a) Izumrudov, V.; Kharlampieva, E.; Sukhishvili, S. A. Macrmolecules 2004, 37, 8400. (b) Kovacevic, D.; van der Burgh, S.; de Keizer, A.; Cohen Stuart, M. A. Langmuir 2002, 18, 5607. (9) Shibayama, M.; Morimoto, M.; Nomura, S. Macromolecules 1994, 27, 5060.
10.1021/la051117+ CCC: $30.25 © 2005 American Chemical Society Published on Web 08/06/2005
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Table 1. Characteristics of Polyelectrolytes Used for Buildup and Exchange polyelectrolyte poly(diallyldimethylammonium chloride) poly(styrene sulfonate) poly(methacrylic acid)a deuterated poly(methacrylic acid)a a
Mw
Mw/Mn
PDADMAC PSS PMA d5-PMA
3.69 × 6.89 × 104 5.2 × 104 4.3 × 104 105
2.09 1.06 1.05 1.08
Mw is after hydrolysis.
Scheme 1. Structure of Polyelectrolytesa
a
abbreviation
Left to right: PDADMAC, PSS, PMA, and d5-PMA. Scheme 2
(A) Negative polyelectrolyte within a complex exchanging with another polyanion and (B) polyelectrolyte exchange assisted by a chaperone molecule. (carboxylic acids).10 A 12-layer multilayer was also built from PDADMA and sheared single-stranded salmon sperm DNA (PDADMA/DNA)6. Polyelectrolyte Exchange and Data Collection. The multilayer was annealed in buffer for 6 h. The exchange between (PDADMA/d5-PMA)6 and PSS or PMA and between (PDADMA/ DNA)6 and PSS was observed by exposing the multilayer to a 3 mM solution of the exchanging polyanion. Spectra were collected from 32 scans at 4 cm-1 resolution against a background of the crystal, the multilayer, and the solution at time t < 0 (i.e., just prior to the exchanging solution being passed over the multilayer). In this manner, only changes to the multilayer were observed. Because (PDADMA/d5-PMA) has a unique band at 1381 cm-1, it was used to probe the amount of d5-PMA in the multilayer. Complete exchange was marked by the complete disappearance of this band. Exchange data between different multilayers was correlated by normalizing to the carboxyl peak at ∼1650 cm-1 present in both PMA and d5-PMA. The exchange was initiated by passing a 2 mL aliquot of the exchanging solution (4 times the ATR flow cell volume) over the multilayer and stopping the flow. When the IR spectrum stabilized, another 2 mL aliquot was passed over the multilayer. This process was repeated several times until the peak area at 1381 cm-1 showed no further change.
Results and Discussions The exchange process is depicted in Scheme 2, which shows a negative polyelectrolyte in a multilayer exchanging with a solution polyanion (A). The polycation remains and preserves overall electroneutrality within the thin film. In studies of polyelectrolyte movement or exchange (10) Dubas, S. T.; Schlenoff, J. B. Langmuir 2001, 17, 7725.
on surfaces11 or from thin films3 and colloidal suspensions of complexes,12 it is generally found that polyelectrolytes are irreversibly associated, unless interactions are particularly weak.13 For example, hydrophilic polyanions, such as polycarboxylates, form labile complexes with select polycations.14 As a rule, salt must be added to solution to loosen the polymer-polymer ion pairing further.10,15 Sufficiently weakly bound polyelectrolytes move within the complex, exchange with solution polymer13 (yielding thermodynamically controlled compositions8), or dissociate completely at the extreme.16 Polyelectrolyte exchange between solution and the “quasi-soluble” complex was extensively evaluated by Kabanov et al.12 Polymer exchange was found to occur more rapidly with complexes formed from polymers differing in molecular weight. In all cases, however, factors such as pH, ionic strength, and electrostatic affinity between interacting polyelectrolytes played a major role in determining the efficiency of the exchange and the position of the reaction equilibrium.12 For example, it was found that exchange reactions involving polyelectrolytes and complexes formed by poly(vinylpyridine), PVP, PMA, and DNA chains occur reversibly. However, when sulfonate groups were involved, as in poly(ethylene sulfonate), heparins, or poly(styrene sulfonate), the displacement of more “labile” polycarboxylates was irreversible.12 In the context of the present discussion, reversible (or labile) complexes can be diagnosed only by exchange (or self-exchange) experiments. As pointed out by Fleer et al.,17 interactions between polymers are generally of the high-affinity type when probed by adsorption or association isotherms. In fact, it is commonly found that polymers will not spontaneously desorb from surfaces under any physically attainable solution concentrations because of multiple interactions between the polymers and the surface.17 The mechanism of polyelectrolyte exchange was previously found to occur through short-lived intermediates formed between the exchanging polyelectrolyte and a polyelectrolyte complex resulting from the interpenetration of the original complex and the incoming polyelectrolyte as a result of random collisions in solution.12 This complex acts as a transition state in a bimolecular reaction mechanism. Because of similarities between solution complexes and polyelectrolyte multilayers, we conclude that the reaction mechanism follows a similar pathway. The introduction of three 2 mL aliquots of unlabeled PMA (Figure 1A) leads to complete (self-) exchange of (11) Hoogeveen, N. G.; Cohen Stuart, M. A.; Fleer, G. J. J. Colloid Interface Sci. 1996, 182, 146. (12) Multilayer Thin Films-Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff J. B., Eds.; Wiley-VCH: Weinheim, Germany, 2003; Chapter 2. (13) (a) Lavelle, P.; Vivet, V.; Jessel, N.; Decher, G.; Voegel, J.-C.; Mesini, P. J.; Schaaf, P. Macromolecules 2004, 37, 1159. (b) Huebsch, E.; Ball, V.; Senger, B.; Decher, G.; Voegel, J.-C.; Schaaf, P. Langmuir 2004, 20, 1980. (14) Kabanov, V. A.; Zezin, A. B. Pure Appl. Chem. 1984, 56, 343. (15) Farhat, T. R.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 4627. (16) Dubas, S. T.; Schlenoff, J. B. Macromolecules 2001, 34, 3736. (17) Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993.
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Figure 2. Exchange of d5-PMA within (PDADMA/d5-PMA)6 with PSS (open circles) and a 2:8 (mole ratio) mixture of PSS/ PMA (closed circles). Exchange in 0.1 M phosphate buffer at pH 8.5 with 0.2 M NaCl added. The arrow indicates the point at which the PSS solution was replaced by a d5-PMA solution to attempt reintroduction of d5-PMA. Table 2. Half-Time Estimates for the Exchange of Different Polyanions with (PDADMA/d5-PMA)6 exchanging polyelectrolytea
t1/2 (min)
PMA PSS 8:2 PMA/PSS
20 60 3
a All polyelectrolyte exchange solutions had a concentration of 3 mM.
Figure 1. (A) Spectra for the exchange of (PDADMA/d5-PMA)6 ca. 512 Å with 3 mM PMA in 0.1 mM phosphate buffer at pH 8.5 with 0.2 M NaCl added. The peak at 1381 cm-1 decreases, and a peak at 1196 cm-1 increases (labeled). (B) Plot of % d5PMA in the multilayer vs time for the same exchange with successive 2 mL aliquots of PMA. Squares indicate release of d5-PMA; circles indicate reintroduction. Arrows indicate at which point a new aliquot was passed over the PEMU. Note that every two data points were averaged for clarity.
d5-PMA from the film. Approximately 54% of this d5-PMA could be reintroduced following the addition of five 2 mL aliquots of d5-PMA. The process is completely reversible, as shown in Figure 1B. The process of introducing and releasing PMA and d5-PMA was performed for three cycles (the first two are shown in Figure 1 and the last one is shown in Supporting Information). Figure 1B shows a preference of PMA over d5-PMA for the multilayer (it takes more aliquots of the latter to displace the former) resulting from the higher molecular weight of the PMA. In a study of the exchange/competition of deuterated and nondeuterated polystyrene on silica, Granick7 and co-workers found, in contrast, a preference for deuterated material of similar molecular weights. For adsorption driven by van der Waals and other weak interactions, it appears that substituting D for H changes the net energy balance to favor the former.7 Polyelectrolyte complexation, however, appears to be driven almost exclusively by entropy considerations.18 Theoretical treatments of polymer adsorption19 predict a preference for molecules of higher molecular weight in the absence of specific polymer-surface interactions. The kinetics for displacement, taken from the initial slopes following each addition, are similar and relatively faststhe half-time for exchange is ca. 20 min. When the experiment was repeated using poly(styrene sulfonate), PSS, as the displacer, different behavior was (18) Michaels, A. S. Ind. Eng. Chem. 1965, 57, 32. (19) (a) Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Fleer, G. J. J. Polym. Sci., Polym. Phys. Ed. 1980, 18, 559. (b) Roe, R. J. J. Chem. Phys. 1974, 60, 4192. (c) Karibyants, N.; Dautzenberg, H. Langmuir 1998, 14, 4427.
seen (Figure 2). Interactions between the sulfonate groups and the positive repeat units on PDADMA are much stronger.10 Although the surface d5-PMA is rapidly displaced (Figure 2), the process slows down as PSS penetrates the PEMU. The PSS exchange seems to proceed in two phases. We believe the initial exchange is much faster because of the higher concentration of extrinsic charge at the surface.15 The exchange in Figure 2 is also irreversible. Substituting a large excess of d5-PMA for PSS in the solution (even when exposing the multilayer to multiple fresh aliquots of d5-PMA for 3 days) at the point indicated in Figure 2 yielded no reincorporation of d5-PMA in the PEMU, as a result of unfavorable thermodynamic factors (the adsorption energy of PSS is much greater10). When a mixture of PMA (2.4 mM) and PSS (0.6 mM) was applied (Figure 2), exchange was rapid and complete, but no PMA was observed in the PEMU, even though the solution concentration of PMA was much higher. The latter observation clearly results from the greater affinity of PSS for PDADMA,10 but the enhancement of the exchange kinetics was unexpected.20 Simply put, the exchange process was accelerated by the PMA in solution, but no PMA remained in the multilayer. Referring to Scheme 2, the upper pathway (A) through route i, which was relatively rapid for the polycarboxylate self-exchange, is much slower in the case of PSS exchange (t1/2 ≈ 60 min). An alternative pathway, shown in Scheme 2B, passing through routes ii-iv suggests a possible mechanism for the acceleration of PSS uptake by a third polyelectrolyte. Estimates of the half times for the exchange with PMA, PSS, and 8:2 PSS/PMA are given in Table 2. In our hypothesized mechanism (Scheme 2, lower), polycarboxylate from solution serves as an intermediary, as shown. The solution polycarboxylate actually loosens the interactions between PSS and PDADMA, making them more labile and allowing the PSS to diffuse into the PEMU. This does not contradict the mechanism proposed earlier for polyelectrolyte exchange; it simply involves two exchanging chains instead of only one. A similar effect (20) This discovery was fortuitous. We were simply trying competition experiments between PMA and PSS.
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might be observed with the addition of a large quantity of salt.8,10 The polycarboxylate lowers the binding energy, ∆Gpoly, between PSS and PDADMA, which is also the activation energy needed to detach and move the polyelectrolytes relative to each other. The simultaneous loosening of adjacent polyelectrolyte repeat units, or short runs of repeat units, effectively increases the “off” rate for the polyelectrolyte ion pairing. A reasonable functional form for the enhancement might be
[
koff(cat) ) koff exp -
]
∆Gpoly RT
(1)
where ∆Gpoly ) N∆Gsite. This relationship makes it clear that polyvalent interactions are needed to transform the interacting polyelectrolytes from “frozen” to labile/ exchangeable. Exchange was also demonstrated on (PDADMA/DNA)6 with PSS. In this case too, the process was irreversible as seen in Figure 3. However, the amount exchanged was lower than seen with (PDADMA/d5-PMA)6 with a thickness of 512 Å. The fact that polyelectrolyte exchange in multilayers occurs with a different pair of polyanions suggests that the process should in fact occur for all labile weakly interacting polyelectrolytes. This process of polyelectrolyte exchange has also been observed in exponentially growing multilayers13 (which allow incoming polymers to diffuse all the way through the bulk), where the interaction between the polyelectrolytes is presumably weak. An example of “assisted” macromolecular exchange is found in DNA strand exchange,21 a natural mechanism for repairing or circumventing the transcription of damaged DNA segments. (21) (a) Kowalczykowski, S. C.; Eggleston, A. K. Annu. Rev. Biochem. 1994, 63, 991. (b) Singelton, S. F.; Xiao, J. Biopolymers 2002, 61, 145. (c) Bianco, P. R.; Tracy, R. B.; Kowalczykowski, S. C. Front. Biosci. 1998, 3, 570.
Figure 3. Exchange of DNA within (PDADMA/DNA)6 (ca. 840 Å thick) with PSS (open circles).The first arrow from the left indicates the point at which a new aliquot (2 mL) of PSS was added. The second arrow indicates the point at which the PSS solution was replaced by a DNA solution to attempt reintroduction of DNA.
Strand exchange is usually enhanced in the presence of protein RecA. In short, RecA couples with ssDNA to form a nucleoprotein filament, which aligns with a complimentary part of dsDNA. Within this three-stranded complex, a rapid and partial strand switch occurs, proceeding until the strand exchange is complete.21 Of course, in the PEMU system described herein there are no sequence-specific interactions, but if there were, the “catalytic” polymer would be most effective in promoting exchange if it also had complimentary sequences. Acknowledgment. This work was supported by grant DMR-0309441 from the National Science Foundation. Supporting Information Available: Infrared spectra for complete exchange of (PDADMA/d5-PMA)6 with PMA and PSS and exchange between (PDADMA/DNA)6 with PSS. Exchange between PMA and (PDADMA/d5-PMA)6 at 0.2 M NaCl, cycled three times. This material is available free of charge via the Internet at http://pubs.acs.org. LA051117+
