Binding Affinity of Thermoresponsive Polyelectrolyte Hydrogels for

Mar 21, 2014 - Binding Affinity of Thermoresponsive Polyelectrolyte Hydrogels for. Charged Amphiphilic Ligands. A DSC Approach. Valerij Y. Grinberg,*...
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Binding Affinity of Thermoresponsive Polyelectrolyte Hydrogels for Charged Amphiphilic Ligands. A DSC Approach Valerij Y. Grinberg,*,† Tatiana V. Burova,‡ Natalia V. Grinberg,‡ Alexander S. Dubovik,† Angel Concheiro,§ and Carmen Alvarez-Lorenzo§ †

N.M. Emanuel Institute of Biochemical Physics, Russian Academy of Sciences, Kosygin St. 4, 119334 Moscow, Russia A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov St. 28, 119991 Moscow, Russia § Department of Pharmacy and Pharmaceutical Technology, University of Santiago de Compostela, Santiago de Compostela 15782, Spain ‡

ABSTRACT: Controlled drug binding and release stand among top requirements postulated for targeted drug delivery systems of the new generations. “Smart” polymers and gels are highly suitable for the controlled delivery due to their structural sensitivity to minor environmental variations. The aim of this work was to study thermoresponsive polyanionic and polycationic hydrogels of N-isopropylacrylamide copolymers with acrylic acid and N-aminopropylmethacrylamide in terms of their interaction with two widely used drugs, propranolol and ibuprofen. Binding energetics of these drugs by the gels in swollen and collapsed state was estimated by means of high-sensitivity differential scanning calorimetry. Thermodynamic parameters of the gel collapse (transition temperature, enthalpy, heat capacity increment, and width) were determined as a dependence of the drug concentrations. From these data the excess free energy of collapse was calculated as a function of drug concentration. Deconvolution of this function resulted in the evaluation of binding parameters and contributions from interactions of various types to the free energy of binding. The binding mechanism of both drugs to the swollen and collapsed gels was elucidated. Its main features are the cooperative character of the drug binding by the collapsed gel and the predominant role of the hydrophobicity of drugs in their affinity for the swollen gel.

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INTRODUCTION Polymer systems capable of changes in their structure and properties in response to minor changes in physicochemical characteristics of physiologic medium are considered to be highly promising for controlled drug delivery.1 From this point of view, “smart” polymer gels undergoing reversible volume phase transitions are of a great importance, especially since the gel transitions can be triggered by small variations in temperature. Hydrogels of poly(N-isopropylacrylamide), PNIPA, represent a class of the most studied thermoresponsive polymer gels.2,3 They swell at temperatures below 33 °C and shrink at higher temperatures. The problem of all “smart” gels, proposed for controlled drug delivery, lies in their inability to interact effectively with drugs as they mostly serve just as inert containers.4 Introduction of a comonomer, capable of formation of specific reversible bonds with a drug, seems to be the key factor providing a control over drug release.5 The hydrogels based on the NIPA copolymers with functional groups suitable for formation of ionic, hydrogen, or hydrophobic bonds demonstrate promising results for design of advanced drug delivery systems.6,7 Combination of the temperature and pH sensitivity may allow one to optimize the controlled drug release.8 A reversible molecular absorption caused by multipoint interactions of a ligand with the functional gel component as a result of the volume phase transition was demonstrated.9−13 The functional groups of subchains reveal a high affinity for the © 2014 American Chemical Society

target molecule if they come so close together that can form a specific three-dimensional binding site (quasireceptor). Such a quasireceptor can be formed in the collapsed state of the gel. In contrast, the affinity decreases notably in the swollen gel as a result of spatial separation of the network functional groups. It means that the gel affinity for the target ligand can be modulated by the reversible volume phase transition. Such a conformation-dependent binding of ligands allows one to consider a gel as a simplest protein-like polymer system which by analogy with the protein catalyst is able to perform reversible binding−release cycles of the target ligands. This aspect implies an important fundamental significance of energetics of the controlled binding in thermoresponsive polymer gel−ligand systems.14 There is inacceptable information gap regarding the effects of the gel subchain flexibility, comonomer distribution, or presence of counterions in the medium on the drug binding ability of the gel. Little is known to date on hydrophobic and electrostatic contributions to the binding mechanisms as well as entropic events favoring the in-gel formation of spatial quasireceptors with high affinity for target ligand. In the meantime, all these phenomena have fundamental analogy with the binding processes in biopolymers, for example, in globular proteins.15−17 The fact that a thermoresponsive gel in Received: February 17, 2014 Revised: March 21, 2014 Published: March 21, 2014 4165

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equilibrium gel concentration was determined by drying the swollen gel samples at 105 °C to constant weight. Stock suspensions of the swollen gel were prepared in water as reported earlier.32 Polymer concentration in the stock suspension was determined by weight after drying of the suspension at 105 °C. Stock solutions of PPN and IBN were prepared in 10 mM sodium phosphate buffer (pH 7.0). Gel suspensions for calorimetric measurements at different concentration of ligands were prepared by adding the stock solution of a ligand to the gel stock suspension. The gel suspensions were incubated in the presence of ligands for 40−42 h at 4 °C prior to calorimetric measurements in order to achieve ionic equilibrium. Calorimetric measurements were carried out with a differential adiabatic scanning microcalorimeter DASM-4 (NPO “Biopribor”, Russia) within temperature range 10−80 °C under excess pressure of 0.25 MPa. The heating rate in all experiments was 1 K min−1. Two or three successive scans were performed, and parameters of the second scan were used for analysis. Primary data processing and transformation of the temperature dependences of the apparent partial heat capacity of polymer network, cp(T), to the excess heat capacity function of the gel collapse transition, cEp (T), were performed by the NAIRTA 2.0 software (A.N. Nesmeyanov Institute of Organoelement Compounds, Moscow, Russia) using the spline-interpolation method for calculation of a baseline of the transition. The temperature of maximum of the excess heat capacity function was considered as a transition temperature, Tt. The transition enthalpy, Δth, was determined by integration of the excess heat capacity function. The transition heat capacity increment, Δtcp, was calculated as a difference in apparent partial heat capacities of polymer in shrunken and in swollen state of the gel at the transition temperature. The transition width, ΔtT, was determined as a ratio of area of the excess heat capacity peak to its height. Based on the obtained experimental dependences of the thermodynamic parameters of the collapse on the ligand concentration, Tt(L), Δth(L), and Δtcp(L), the dependence of the transition free energy on the ligand concentration at the collapse temperature in the absence of ligand, Tt° = Tt(0), was calculated by the integral Gibbs−Helmholtz equation:20

thermodynamic sense can be regarded as a protein-like system makes it possible to investigate binding processes of the target ligands through their effects on the energetics of volume phase transitions of the gels.18−21 A direct and the most robust thermodynamic method of investigation of phase and conformational transitions in biopolymers is high-sensitivity differential scanning calorimetry (HS-DSC).22,23 Application of the HS-DSC methodology to the transitions in interacting systems allowed one to consider their energetics as an appropriate and accessible indicator of molecular recognition that is of high priority for biomedicine and pharmacology.24−27 Investigations of energetics of the volume phase transition in polymer hydrogels by means of HSDSC are rather scarce.18−21,28−31 For instance, HS-DSC was applied to study binding of cationic ligands by a thermoresponsive polyanionic gel of the copolymer of N-isopropylacrylamide with acrylic acid.20 Thermodynamic parameters characterizing affinity of the gel in swollen and collapsed states for cationic ligands of various nature and hydrophobicity were determined, and a preferential electrostatic binding of protons, Ca2+ ions, and cationic surfactants to subchains of the gel in the condensed conformation was detected. In the present work we have investigated energetics of interaction of two ionic drug substances of different pharmacological groups, propranolol (I) and ibuprofen (II) (Scheme 1), with two polyelectrolyte hydrogels of the Scheme 1. Chemical Structure of Propranolol (I) and Ibuprofen (II)

Δt g (L , Tt°) = Δt h(L) × [1 − Tt°/Tt(L)] + Δt cp(L) × [Tt° − Tt(L)] − Tt°Δt cp(L) × ln[Tt°/Tt(L)]

The transition free energy normalized per average molecular weight of the gel subchain (Mc) will be further called the excess transition free energy and denoted ΔtGE. The value Mc (1400 for the NIPA-AAc gel and 1500 for the NIPA-APMA gel) was roughly estimated from the feed composition of the gel network assuming a uniform monomer distribution along the copolymer network.

copolymers of NIPA with acrylic acid (AAc) and Naminopropylmethacrylamide (APMA). Propranolol hydrochloride and sodium salt of ibuprofen served as cationic and anionic ligands for the oppositely charged polyanionic NIPAAAc and polycationic NIPA-APMA networks, respectively. Thermodynamic parameters of the gel collapse in the presence of the drugs were determined and analyzed in terms of ionic and hydrophobic binding of the drugs preferentially by the swollen or collapsed state of the gels.

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(1)

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RESULTS AND DISCUSSION Thermograms of the NIPA-AAc and NIPA-APMA gels in 5 mM phosphate buffer at pH 7.0 are shown in comparison with the thermogram of the NIPA homopolymer gel in Figure 1. The thermograms of all three gels were 100% reproducible upon repeat heating−cooling cycles in the calorimeter. The NIPA gel shows a well-resolved narrow heat capacity peak corresponding to its collapse transition.18,19 The collapse parameters of the NIPA gel are the transition temperature Tt = 35.8 °C, the enthalpy Δth = 40 J g−1, the heat capacity increment Δtcp = −1.2 J g−1 K−1, and the width ΔtT = 4.8 °C. Both polyelectrolyte gels revealed very broad and small heat capacity peaks shifted notably to high temperatures relative to the homopolymer gel peak. The NIPA-AAc gel has the following collapse parameters: the transition temperature Tt = 60.4 °C, the enthalpy Δth = 4.4 J g−1, the heat capacity increment Δtcp = −0.13 J g−1 K−1, and the width ΔtT = 28.2 °C. The corresponding parameters of the NIPA-APMA gel are Tt = 54.5 °C, Δth = 7.3 J g−1, Δtcp = −0.15 J g−1 K−1, and ΔtT = 18.8

MATERIALS AND METHODS

N-Isopropylacrylamide (NIPA), acrylic acid (AAc), N-aminopropylmethacrylamide (APMA), N,N′-methylenbis(acrylamide), 2,2′-azobis(isobutyronitrile), propranolol hydrochloride (PPN), and sodium ibuprofen (IBN) were from Sigma-Aldrich. The gels of the NIPA-AAc and NIPA-APMA copolymers were prepared by free-radical copolymerization of N-isopropylacrylamide (700 mM) with acrylic acid (60 mM) or N-aminopropylmethacrylamide (60 mM) in aqueous medium using N,N′-methylenbis(acrylamide) (60 mM) as a cross-linking agent. After adding initiator 2,2′-azobis(isobutyronitrile) (10 mM), the comonomer solution was placed into a glass ampule and degassed under vacuum. The polymerization reaction was performed at room temperature for 24 h. After the reaction was completed, the gels were taken from the ampules and washed consecutively with solutions of HCl (100 mM), NaOH (100 mM), and deionized water to remove nonreacted monomers. The gels were swollen in water at 4 °C for 1 week. The 4166

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Figure 2. Excess heat capacity functions of the NIPA-AAc copolymer gel at different concentrations of propranolol hydrochloride: 0 (1), 1.0 (2), 2.0 (3), 5.0 (4), 10 (5), and 50 mM (6). 5 mM potassium phosphate buffer, pH 7.0; polymer concentration 5−7 mg mL−1.

Figure 1. Apparent partial heat capacity of polymer vs temperature for the NIPA homopolymer gel (a), for the polyanionic NIPA-AAc copolymer gel (b), and for the polycationic NIPA-APMA copolymer gel (c). Thin lines show the transition baselines. Heating rate 1 K min−1; polymer concentration 1−5 mg mL−1; 5 mM potassium phosphate buffer, pH 7.0.

°C. As compared to the NIPA gel, the transition temperature of both polyelectrolyte gels increases, the enthalpy decreases, the heat capacity increment decreases by an absolute value, and the width of the transition increases markedly. The obtained results indicate that introduction of charges into the gel subchains effectively inhibits the collapse. The system becomes less cooperative. All these somewhat expected effects reflect a shift in the hydrophobic−hydrophilic balance of the system to the more hydrophilic side.20,33−35 To examine interactions of the polyelectrolyte hydrogels with drugs, we have performed calorimetric measurements at different concentrations of propranolol (PPN) and ibuprofen (IBN). Figures 2 and 3 show thermograms of the NIPA-AAc and NIPA-APMA gels at various concentrations of PPN and IBN, respectively. The behavior of both gels is similar. Upon increasing amount of the ligands the transition peaks shift to lower temperatures, increase in height and become narrower. As the ligand concentration increases further, the transitions undergo opposite changes most visible as a peak broadening. The dependences of the collapse parameters of the NIPAAAc and NIPA-APMA gels on the ligand concentrations are shown in Figures 4 and 5, respectively. One can distinctly see the common features of both systems. Upon increasing the ligand concentration, the transition temperature of the gels decreases gradually, reaches the transition temperature of the neutral NIPA gel, and further decreases. In the case of NIPAAAc gel some increase in the transition temperature is observed at high concentrations of PPN (Figure 4). For the NIPAAPMA gel this effect is not seen (Figure 5), but it most likely

Figure 3. Excess heat capacity functions of the NIPA-APMA copolymer gel at different concentrations of sodium ibuprofen: 0 (1), 0.2 (2), 1.0 (3), 5.0 (4), 10 (5), and 100 mM (6). 5 mM potassium phosphate buffer, pH 7.0; polymer concentration 5−7 mg mL−1.

takes place at higher concentrations of IBN. The transition enthalpy passes through a maximum being however always lower than the enthalpy of the NIPA homopolymer gel. The heat capacity increment decreases to a constant value being always less negative as compared with this parameter for the NIPA gel. The transition width decreases, reaches the width of the NIPA gel, and then tends to increase slightly. The observed changes in the transition temperature, enthalpy, and width of the NIPA-AAc and NIPA-APMA gels are in agreement with the previously reported data on the effect of cationic surfactants on the collapse of the NIPA-AAc gel.20 However, cationic surfactants did not affect the collapse heat capacity increment, while both PPN and IBN led to a significant increase in the absolute value of this parameter for the NIPA-AAc and NIPAAPMA gels (Figures 4c and 5c). 4167

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Figure 4. Dependences of the transition temperature (a), enthalpy (b), heat capacity increment (c) and width (d) for the NIPA-AAc copolymer gel on the propranolol hydrochloride concentration. The dash-dotted lines indicate values of the transition parameters of the NIPA-AAc copolymer gel in the absence of the ligand. The dashed lines indicate values of the transition parameters of the NIPA homopolymer gel. 5 mM potassium phosphate buffer, pH 7.0; polymer concentration 5−7 mg mL−1.

Figure 5. Dependences of the transition temperature (a), enthalpy (b), heat capacity increment (c) and width (d) for the NIPA-APMA copolymer gel on the sodium ibuprofen concentration. The dashdotted lines indicate values of the transition parameters for the NIPAAPMA copolymer gel in the absence of the ligand. The dashed lines indicate values of the transition parameters of the NIPA homopolymer gel. 5 mM potassium phosphate buffer, pH 7.0; polymer concentration 5−7 mg mL−1.

The observed changes in the collapse parameters induced by the ligands reflect changes in the hydrate structure of the gel. The polyelectrolyte gel subchain represents an alternation of the hydrophobic NIPA sequences and charged groups (Figure 6). The hydrate shell of the hydrophobic NIPA sequences is highly ordered in such a way that prevents self-association of the sequences at low temperatures. At temperatures high enough the hydrate shell melts, the NIPA self-association occurs and as a result the gel collapses. The collapse enthalpy, heat capacity increment, and width depend on the hydrate shell size: the larger the size, the higher the enthalpy and absolute value of the heat capacity increment and the lower the width of the transition. Introduction of charges perturbs a continuity of the NIPA hydrate shell. For this reason the polyelectrolyte copolymer gels initially have the degenerated transitions with low enthalpy and small changes in heat capacity upon the collapse. Besides, the osmotic pressure of counterions impedes the collapse, thus shifting it to higher temperatures (Figure 1). As a result of the ligand binding, the osmotic pressure effect of counterions disappears and the hydrate structure of the copolymer becomes more homogeneous and extended (Figure 6a). Thermodynamically this manifests itself in an increase in the transition enthalpy and absolute value of the heat capacity increment as well as a decrease in the transition temperature and width. The situation changes drastically at rather high ligand concentrations when neutralization of the permanent

network charge is reached (Figure 6b). Under these conditions hydrophobic binding of the ligand comes into play. It causes inversion of the network charge and fragmentation of the hydrate structure of the network (bottom image). An overcharged complex is formed, and as a consequence the transition temperature tends to increase and the transition enthalpy and absolute value of the heat capacity increment decrease, while the transition broadens. To estimate the binding energetics of the investigated gel− drug systems we have used a thermodynamic approach described earlier.20 A general measure of the ligand binding modulated by the gel collapse is the excess free energy of collapse, ΔtGE. Figure 7 shows dependences of ΔtGE(L) for the NIPA-AAc and NIPA-APMA gels. In general, the excess free energy is negative. This suggests that both ligands favor the gel collapse as a result of their preferential interaction with the polymer network in the collapsed state. Such behavior was shown to be typical of thermoresponsive hydrogels holding functional sites to particular ligands.14,20,21 Preferential binding by the collapsed network is promoted by condensation of sites upon collapse thus providing cooperativity of the binding. At the same time, the excess free energy passes through a minimum in the vicinity of the ligand concentration of 10 mM and then approaches positive values at the ligand concentration of about 100 mM. These tendencies reflect contribution of the ligand binding by the network in swollen state which is small at 4168

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Figure 7. Excess transition free energy vs ligand concentration: (a) NIPA-AAc hydrogel + propranolol system, Tt° = 333.0 K; (b) NIPAAPMA hydrogel + ibuprofen system, T°t = 327.5 K; (○) experimental; () results of fitting by eqs 2−6. The fitting parameters are given in Table 1.

ligand with subchain in the extended state can be expressed in a following way because of the weakness and nonlocal character of this interaction: HΦ ΔGext = RTBHΦL

where BHΦ is the second viral coefficient of the NIPA−ligand interaction in swollen state of the gel. In order to calculate the free energy of cooperative binding of ligand by the collapsed gel, we will refer to the general definition of the free energy of binding:15

Figure 6. Schematic presentation of changes in hydrate structure of a polyelectrolyte copolymer network and corresponding changes in the collapse parameters as a result of binding of an oppositely charged amphiphilic ligand: (a) formation of a stoichiometric polyelectrolyte− ligand complex at moderate ligand concentrations; (b) inversion of the network charge and formation of an overcharged polyelectrolyte− ligand complex at high ligand concentrations. Tt, Δth, |Δtcp|, and ΔtT represent the temperature, enthalpy, absolute value of the heat capacity increment, and width of the collapse transition, respectively. Values Δth > 0 and Δtcp < 0 are apparent measures of suppression of the network hydrophobic hydration as a result of the gel collapse.

* ΔbGcnd = −RTncnd

r=

L

r(L) d ln L

(5)

Kcnd exp(wr )L 1 + Kcnd exp(wr )L

(6)

where Kcnd is the intrinsic binding constant; w = u/RT, and u is the free energy of dissociation of a secondary contact between neighboring bound ligands. The cooperativity of binding is measured by value of the parameter w. The larger the parameter w, the higher the cooperativity. The concentration of free ligand in eqs 3−6 can be equated without much error to the total ligand concentration in the system. Then, it can be assumed that the accessibility of sites (ionogenic groups) to ligands does not substantially change upon the gel collapse, i.e., n*ext ≃ n*cnd = n*. Starting from the feed monomer composition of the gels, one can estimate that the number of sites per subchain in both cases is n* ∼ 1. Equations 2−6 were fitted to the experimental dependences of the excess free energy of collapse on the ligand concentration using Kext, BHΦ ext , Kcnd, and w as adjustable parameters. The fitting results are presented in Figure 7 and Table 1. A good approximation was achieved for both gel−ligand systems (r2 ∼

(2)

where ΔbGcnd and ΔbGext are the free energies of the ligand binding by the subchain in the condensed and extended states, while ΔGHΦ ext is the free energy of hydrophobic interaction of the ligand with the subchain in the extended state. Let us assume that the ligand binding by the swollen gel proceeds to the identical independent sites which are represented by ionogenic groups of the copolymer network. In this case the free energy of binding per a subchain in the extended state is expressed as15 * ln(1 + KextL) ΔbGext = −RTnext

∫0

where n*cnd is the number of sites in the condensed subchain and r(L) is the binding ratio of ligand. Alvarez-Lorenzo et al.36 have shown that in case of cooperative ligand binding by a collapsed polymer gel the function r(L) can be represented as follows:

low concentrations and dominates at higher concentrations of the ligand. It is reasonable to consider three contributions into the excess free energy of collapse:20 HΦ Δt GE = ΔbGcnd − (ΔbGext + ΔGext )

(4)

(3)

where n*ext is the number of sites in the extended subchain, Kext is the intrinsic binding constant, and L is the concentration of free ligand. The free energy of hydrophobic interaction of 4169

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Table 1. Binding Parameters of Propranolol and Ibuprofen to the NIPA-AAc and NIPA-APMA Hydrogels at Temperature Tt° gel

ligand

Tt° a

Kextb

c BHΦ ext

Kcndd

we

Kcnd/Kext

NIPA-AAc NIPA-APMA

propranolol ibuprofen

333.0 327.5

589 ± 115 208 ± 29

−4.8 ± 0.3 −7.2 ± 0.2

470 ± 50 233 ± 19

1.8 ± 0.3 1.0 ± 0.1

0.8 ± 0.2 1.1 ± 0.2

a Transition temperature in the absence of ligand, K. bIntrinsic binding constant for swollen state of the gel, M−1. cSecond viral coefficient of the NIPA−ligand interaction for swollen state of the gel, M−1. dIntrinsic binding constant for collapsed state of the gel, M−1. eFree energy of dissociation of secondary contacts between neighboring bound ligands for collapsed state of the gel in RT units.

0.99). This confirms a validity of the proposed mechanism of interaction of PPN and IBN with the NIPA-AAc and NIPAAPMA gels in swollen and collapsed states. The intrinsic binding constants of the drugs by the swollen and collapsed network are close for both systems. It implies that the key mechanism of binding does not principally change as a result of the collapse. The ligands and subchain sites carry opposite charges, so it is expected that electrostatics provides a main contribution into the ligand−subchain interactions. Apparently, the accessibility of the charged sites for small ligand molecules is not substantially affected by the collapse. However, the collapse may cause a notable spatial rearrangement of the sites. The sites are randomly distributed throughout the volume of the swollen gel so that a mutual secondary interaction of bound ligands is largely excluded. On the other hand, the sites (the network charged groups) in the collapsed state form domains in which a short-range secondary interaction can appear between hydrocarbon tails of the closely located bound ligands. These very cooperative effects provide a total preference of the ligand binding by the polymer network in the collapsed state. The secondary interaction (self-association) of the bound ligands is accompanied by the suppression of the hydrophobic hydration of their hydrocarbon tails but also involves noncooperative van der Waals interactions, stacking, or hydrogen bonding between them. The standard free energy of transfer of a ligand molecule from water to the nonpolar O environment, ΔW G, can serve as the measure of the hydrophobic hydration contribution. According to this value, ibuprofen is a more hydrophobic compound than propranolol (Table 2). However, propranolol surpasses ibuprofen by the

polymer network in the swollen state. Such an interaction appears to be nonlocal like solubilization of nonpolar compounds by surfactants. In fact, the free energy ΔGHΦ ext ∝ BHΦ ext of the more hydrophobic substance ibuprofen reaches the more negative values (Table 1).

Table 2. Free Energies of the Water-to-Octanol Transfer C (ΔO WG), Crystallization (ΔL G), and Micellization (ΔMG) for Propranolol and Ibuprofen at 293 K

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ligand

ΔOWG (kJ mol−1)

propranolol ibuprofen

−14 −18

a

ΔCL Gb

(kJ mol−1) −12 −4

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CONCLUSIONS Thermodynamic study on binding energetics of two amphiphilic drugs by two polyelectrolyte gels gave insight into the mechanism of their conformation-controlled affinity. Deconvolution of the binding free energy profile has shown that the extended and condensed conformations of the gel subchains are able to distinguish between charged ligands having a subtle difference in hydrophobicity and ability for association. Further development of the binding energetics evaluation of “smart” polymers, in particular based on the HSDSC method, may be of essential utility in the drug prescreening as well as in tuning drug structure to achieve the desirable affinity and controlled release.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (V.Y.G.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by the Russian Foundation for Basic Research (project 11-03-00320), by the BGTC RAS Program “Design and studies of macromolecules and macromolecular systems of new generations”, and by the Spanish Ministerio de Economia y Competividad (SAF 2011-22771). REFERENCES

(1) Hoffman, A. S. Stimuli-responsive polymers: Biomedical applications and challenges for clinical translation. Adv. Drug Delivery Rev. 2013, 65, 10−16. (2) Schild, H. G. Poly(N-Isopropylacrylamide) - Experiment, theory and application. Prog. Polym. Sci. 1992, 17, 163−249. (3) Shibayama, V.; Tanaka, T. Volume phase transition and related phenomena of polymer gels. Adv. Polym. Sci. 1993, 109, 1−62. (4) Coughlan, D. C.; Corrigan, O. I. Release kinetics of benzoic acid and its sodium salt from a series of poly(N-isopropylacrylamide) matrices with various percentage crosslinking. J. Pharm. Sci. 2008, 97, 318−330. (5) Hoffman, A. S. Bioconjugates of intelligent polymers and recognition proteins for use in diagnostics and affinity separations. Clin. Chem. 2000, 46, 1478−1486. (6) Qian, J.; Wu, F. P. Thermosensitive PNIPAM semi-hollow spheres for controlled drug release. J. Mater. Chem. B 2013, 1, 3464− 3469. (7) Petrusic, S.; Jovancic, P.; Lewandowski, M.; Giraud, S.; Grujic, S.; Ostojic, S.; Bugarski, B.; Koncar, V. Properties and drug release profile of poly(N-isopropylacrylamide) microgels functionalized with maleic anhydride and alginate. J. Mater. Sci. 2013, 48, 7935−7948.

ΔMG (kJ mol−1) c

−4.8 −4.2

a

Calculated from the log P data of Lipinski et al.37 bCalculated from the calorimetric data of Elsabee et al.38 and Dwivedi et al.39 c Calculated from the CMC data of Mosquera et al.40 and Ridell et al.41

binding cooperativity (Table 1). Comparison of the chemical structures of PPN and IBN (Scheme 1) allows one to suggest that secondary noncovalent interactions of hydrocarbon tails should be more pronounced in the case of propranolol. This conclusion correlates with the values of the standard free energies of crystallization and micellization of two drugs (Table 2). In this view our data on the higher cooperativity of binding of propranolol by the collapsed gel as compared with that of ibuprofen have found their explanation. The role of apparent ligand hydrophobicity manifests itself in the parameters characterizing interaction of the ligands with 4170

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dx.doi.org/10.1021/la5005984 | Langmuir 2014, 30, 4165−4171