Nanoreactors Based on Amphiphilic Uracilophanes: Self-Organization

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J. Phys. Chem. B 2007, 111, 14152-14162

Nanoreactors Based on Amphiphilic Uracilophanes: Self-Organization and Reactivity Study Lucia Ya. Zakharova,*,† Vyacheslav E. Semenov,† Mikhail A. Voronin,‡ Farida G. Valeeva,† Alsu R. Ibragimova,† Rashid Kh. Giniatullin,† Alla V. Chernova,† Sergey V. Kharlamov,† Lyudmila A. Kudryavtseva,† Shamil K. Latypov,† Vladimir S. Reznik,† and Alexander I. Konovalov† A. E. ArbuzoV Institute of Organic and Physical Chemistry of the Russian Academy of Sciences, 8 ul. Akad. ArbuzoV, Kazan 420088, and Kazan State Technological UniVersity, 68 ul. K. Marx, Kazan 420015, Russia ReceiVed: August 16, 2007; In Final Form: October 4, 2007

New amphiphilic pyrimidinic macrocycles (APMs) with two (APM-1) and three (APM-2) decyl tails have been synthesized by quaternization of the bridged N. Complex examination of the APM-based systems with the help of tensiometry, conductometry, dynamic light scattering, and UV and NMR spectroscopy provides evidence for their aggregation. Calculations based on surface tension isotherms and on packing parameter considerations make it possible to assume a lamellar packing of macrocycles when aggregating. Marked differences in the aggregation behavior of APM-1 and APM-2 have been found. The additives of polyethylenimine (PEI) exert little influence on the critical micelle concentration (cmc) of APM-1, while in the APM-2/PEI systems there occurs a pronounced decrease in the cmc and also a ca. 2-fold decrease in the surface area per molecule. The APM-based assemblies are explored as nanoreactors for the hydrolysis of O-alkyl O-p-nitrophenyl (chloromethyl)phosphonates (alkyl ) ethyl, hexyl). The kinetic study reveals a minor rate effect of the APM-1-based systems. In the APM-2-based systems an acceleration of the hydrolysis of both phosphonates occurs as compared to the uncatalyzed process. Within the APM-2 f APM-2/PEI f APM-2/PEI/La(III) series, due to the cooperative contributions of the supramolecular, polymer, and homogeneous catalysis, an increase in the catalytic effect is observed from 30 times to 3 orders of magnitude as compared to that of the basic hydrolysis of the substrates.

The catalysis of reactions in organized media is of increasing interest.1-7 In these systems, aggregates act as nano- or microreactors, compartmentalizing and concentrating or diluting reagents and thereby altering the observed rate of chemical reactions. In our earlier works8-11 the use of multicomponent self-assembling systems was shown to be a powerful strategy for the design of nanoreactors, the role of building blocks being assigned to surfactants and/or other amphiphiles, polymers, and metal ions. The catalytic activity of supramolecular systems is directly due to their ability of self-organization and the types (the size and the architecture) of aggregates and their binding capacity toward the reagents. Therefore, the variation in the composition can be used for the control of the solution behavior and hence of the catalytic effect and also for adapting the catalysts to various chemical processes. Besides, the use of components (surfactants, polymer, cyclophanes) with the catalytic functional groups makes it possible to enhance the summary rate effects with the help of homogeneous catalysis. Recently we have studied supramolecular catalytic systems based on conventional surfactants and polyethylenimines (PEIs) in both the presence and absence of metal ions.8-11 They provide high accelerations of hydrolytic cleavage of phosphorus acid esters due to a complex mechanism of catalysis contributed by micelles and PEIs together with homogeneous catalysis by amino groups of PEI (general basic catalysis) and by metal ions * To whom correspondence should be addressed. Fax: +7 8432 73 2253. E-mail: [email protected]. Phone: +7 8432 73 2293. † A. E. Arbuzov Institute of Organic and Physical Chemistry of the Russian Academy of Sciences. ‡ Kazan State Technological University.

(electrophilic catalysis). A similar polyfunctional mechanism but on a higher level is typical for the enzyme catalysts, which are considered prototypes of supramolecular catalysts. Such an approach based on the step-by-step expansion of the catalytic effect by the addition of components with their own aggregating or catalytic activity seems to be fruitful. The present work extends this approach to a new type of nanoreactors for the hydrolytic cleavage of phosphorus acid esters based on amphiphilic pyrimidinic macrocycles with two hydrophilic groups related to gemini or bolaform amphiphiles similar in structure. Synthesis and applications of geminis and bolas are the perspective directions of modern supramolecular chemistry.12-22 Bolaamphiphiles or bolaform surfactants or bolas are molecules that have a hydrophilic group at both ends of the hydrophobic chain.12-15 Gemini surfactants consist of two hydrophobic chains and two hydrophilic headgroups linked by a spacer which can be both long or short and flexible or rigid.16-22 Due to their structural features bolas and geminis demonstrate a marked ability for self-organization at the interface or in bulk solutions.12-22 The spontaneous curvature of the aggregates formed in aqueous solutions is strongly determined by the geometry of the molecules, the nature of the spacer, hydrophilic-lipophilic balance, etc. Of particular interest are the amphiphilic nucleotide bases and their derivatives, including those of bolaform or gemini character, that play a crucial role in biochemistry, pharmacology, and drug chemistry. It is common knowledge that pyrimidine or purine derivatives are capable of forming different types of associates due to noncovalent interactions, e.g., van der Waals and π-π, hydrogen-bonding interactions with charged or neutral

10.1021/jp076592q CCC: $37.00 © 2007 American Chemical Society Published on Web 12/05/2007

Nanoreactors Based on Amphiphilic Uracilophanes SCHEME 1: Chemical Structure of Uracilophanes and Substrates

substrates, etc. This ability was explored to design supramolecular assemblies that mime the molecular organization in biological systems.23,24 A series of publications is devoted to nano- and mesostructure constructions using multiple complementary hydrogen bonds of amphiphilic nucleotides.25-28 Macrocycles containing derivatives of nucleotide bases are new promising candidates for self-assembly study. However, there is one report on the NMR study of the association behavior of pyrimidinocyclophanes with the uracil unit and the quaternized bridged nitrogen atom.29 To our knowledge, information on the catalytic properties of bolas and geminis is scarce,30-36 while supramolecular catalysts based on gemini pyrimidinic macrocycles have not been previously explored. In this study, new amphiphilic pyrimidinic macrocycles (APM-1 and APM-2) with two hydrophilic groups are synthesized (Scheme 1). These APMs consist of two alkyl-substituted uracil units bridged together by polymethylene chains, in general called cyclophanes37 and in particular uracilophanes containing uracil moieties. They are used for constructing nanoreactors by noncovalent self-assembly. As mentioned above, catalysis in supramolecular systems is due to the formation of nanoaggregates (nanoreactors), which provide specific concentration and microenvironmental conditions responsible for the reaction rate alteration. Therefore, selforganization which is largely focused on problems related to the chemical reactivity is examined prior to the reactivity study. The following points are supposed to be elucidated: (i) the attestation of the aggregation in the systems; (ii) the estimation of the cmc values; (iii) the determination of the size and types of the organized structures. Although the self-organization section was intended as an auxiliary part of the present study, it was given much attention, and a complex of complementary methods was used to elucidate different aspects of the structural behavior of these systems. This was due to the fact that the APMs are related to new types of monomers capable of selfassembling through a rather complicated mechanism. Besides, the parallel study of aggregation and reactivity within the same framework makes it possible to discuss the correlation between the structural behavior and catalytic properties of the supramolecular assemblies, which is of particular interest for us.38 Besides single APM solutions, their mixed systems with PEI and lanthanum salt are studied. This makes it possible to modify the APM solutions to enhance the catalytic effect due to general basic and polymer catalysis contributed by PEI and electrophilic catalysis contributed by La(III) ions. As a model process, the hydrolysis of O-alkyl O-p-nitrophenyl (chloromethyl)phosphonates (alkyl ) ethyl (S1), hexyl (S2)) has been studied (Scheme 1). The transfer of a phosphoryl group is one of the most fundamental chemical and biochemical reactions.39 Phosphorus acid esters are compounds with interesting biological and pharmacological properties and are widely used as pesticides,

J. Phys. Chem. B, Vol. 111, No. 51, 2007 14153 drugs, and nerve gases.40 The hydrolysis of phosphorus acid esters has been studied in detail in aqueous alkali solutions,41 in single surfactant solutions,1-4,38 and in PEI solutions.42-44 The reaction proceeds through breaking of the P-O bond, which is facilitated by the presence of electron-withdrawing groups. At high solution pH basic hydrolysis occurs in the absence of PEI. Cationic micelles catalyze the reaction, while anionic micelles inhibit it.38 The second-order rate constants of the basic hydrolysis of substrates S1 and S2 at 25 °C are equal to 4.0 and 3.0 M-1s-1, respectively. There is much evidence that amines including polyamines accelerate the hydrolysis of phosphorus acid esters via general basic catalysis.42,43 The regularities revealed within the framework of this study for the APM-based systems are compared with those typical for reactivity in conventional cationic surfactants. Experimental Section The Synthesis of Isomeric Uracilophanes 3a, 3b, 4a, and 4b from 1,3-bis(5-bromopentyl-1)-6-methyluracil (1) and 1,3bis[5-(ethylamino)pentyl-1]-6-methyluracil (2a) or 1,3-bis[5(ethylamino)pentyl-1]-5-decyl-6-methyluracil (2b) is shown in Scheme 2. These reactions imply the formation of isomers which differ from one another by the mutual arrangement of the carbonyl and methyl groups at different pyrimidine rings: cisand trans-isomers. Compound 2b has been prepared from the reaction of 1,3-bis(5-bromopentyl-1)-5-decyl-6-methyluracil with ethylamine according to the known protocol.45,46 The synthesis of isomeric uracilophanes 3a and 3b and in particular the individual macrocycle 3a with a trans-arrangement of the carbonyl C(4)dO group at different uracil moieties was described elsewhere.45-47 In the case of preparation of uracilophanes 4a and 4b we did not succeed in isolating individual geometric isomers of macrocycles and obtained a mixture of macrocycles 4a and 4b with trans- and cis-arrangements of the carbonyl and methyl groups. 16,32-Dimethyl-15-decyl-7,23-diethyl-1,7,13,17,23,29hexaazatricyclo[27.3.1.113,17]tetratriaconta-15,31-diene-14,30,33,34-tetraone (4a) and 14,32-Dimethyl-15-decyl-7,23-diethyl1,7,13,17,23,29-hexaazatricyclo[27.3.1.113,17]tetratriaconta14,31-diene-16,30,33,34-tetraone (4b). Potassium carbonate (1.1 g, 8 mmol) was added to a solution of dibromide 1 (2.0 g, 3.6 mmol) and diamine 2b (1.7 g, 3.6 mmol) in CH3CN (150 mL). The reaction mixture was stirred at 50-60 °C for 14 h and then at the boiling temperature of the solvent for 7 h. The precipitate formed was filtered off. The solution was concentrated to 1020 mL and transferred to a column with Al2O3 (activity II). The column was successively washed with ether, and a 15:1 ether-diethylamine mixture. An oily mixture of isomeric uracilophanes 4a and 4b was obtained from the etherdiethylamine fractions in a yield of 0.45 g (15%). Anal. Found: C, 70.10; H, 10.52; N, 11.21. Calcd for C44H78N6O4: C, 69.98; H, 10.41; N, 11.13. HRMS (EI): m/z 754.6080; C44H78N6O6 requires 754.60846. 1H NMR (CDCI3): δ 5.56 (s, 1H, C(5)urH); 3.89 (m, 4H, 2N(3)urCH2); 3.78 (m, 4H, 2N(1)urCH2); 2.51-2.49 (m, 14H, 6NCH2, C(5)urCH2); 2.25 (s, 3H, C(6)urCH3); 2.24 (s, 3H, C(6)urCH3); 1.63-1.29 (m, 40H, 20CH2); 1.01 (m, 6H, 2NCH2CH3); 0.88 (t, 3H, CH3(CH2)9, J ) 5.9 Hz). General Procedure for the Synthesis of Amphiphilic Uracilophanes (Scheme 3). The solution of uracilophane 3a or the mixture of isomeric uracilophanes 4a and 4b (0.20 mmol) and a 4-fold excess of n-decyl bromide in CH3CN (30 mL) was refluxed for 20 h. The solvent was distilled off. The residue was thoroughly triturated in ethyl ether (5 × 30 mL), each time decantated, and finally the solvent was evaporated.

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SCHEME 2: Synthesis of Uracilophanes

SCHEME 3: Synthesized Uracilophanes with Quaternary Ammonium in the Spacers

16,32-Dimethyl-7,23-diethyl-7,23-di-n-decyl-1,7,13,17,23,29hexaazatricyclo[27,3,1,113,17]tetratriaconta-15,31-diene-14,30,33,34-tetraone Dibromide (5, APM-1). Yield: 85%. Mp: >80 °C dec. Anal. Found: C, 61.22; H, 9.46; Br 15.14; N, 7.82. Calcd for C54H100Br2N6O4: C, 61.35; H, 9.53; Br, 15.12; N, 7.95. 1H NMR (D2O): δ 5.76 (s, 2H, 2C(5)urH); 3.88 (m, 8H, 4NurCH2); 3.33 (m, 4H, 2NCH2); 3.23 (m, 12H, 6NCH2); 2.34 (s, 6H, 2C(6)urCH3); 1.76 (m, 12H, 6CH2); 1.67 (m, 8H, 4CH2); 1.45 (m, 8H, 4CH2); 1.38 (s, 8H, 4CH2); 1.28 (m, 26H, 10CH2, 2NCH2CH3); 0.85 (t, 6H, 2CH3(CH2)9, J ) 6.5 Hz). 16,32-Dimethyl-15-decyl-7,23-diethyl-1,7,13,17,23,29hexaazatricyclo[27.3.1.1 13,17]tetratriaconta-15,31-diene-14, 30,33,34-tetraone Dibromide (6a) and 14,32-Dimethyl-15-decyl7,23-diethyl-1,7,13,17,23,29-hexaazatricyclo[27.3.1.113,17]tetratriaconta-14,31-diene-16,30,33,34-tetraone Dibromide (6b, APM-2). Yield: 97%. Mp: >50 °C dec. Anal. Found: C, 64.27; H, 10.21; N, 7.05; Br,13.46. Calcd for C64H120N6O4Br2: C, 64. 19; H, 10.10; N, 7.02; Br, 13.35. 1H NMR (D2O): δ 5.75 (s, 1H, C(5)urH); 3.89 (m, 8H, 2N(3)urCH2, 2N(1)urCH2); 3.423.25 (m, 16H, 8NCH2); 2.33 (m, 8H, C(5)urCH2, 2C(6)urCH3); 1.74-1.26 (m, 78H, 36CH2, 2NCH2 CH3); 0.85 (m, 9H, 3CH3(CH2)9).

Compounds used in the UV and NMR studies as models (Scheme 4), in particular 1,3-bis(4-bromobutyl)-6-methyluracil (7),48 1,3-dibutylthymine (8),49 1,3-bis(3,6-dimethyluracil-1-yl)propane (9),50 and 1,3-bis[4-(3,6-dimethyluracil-1-yl)butyl-1]thymine (10),49 were reported earlier. Substrates S1 and S2 were prepared according to the published procedure.51 La(NO3)3‚6H2O (99% grade) and branched PEI with an average molecular mass of 50000 were from Aldrich. The solution concentrations of PEI are given as the molar concentration on a monomer basis (mol of monomer/L of solution). In the micellization and reactivity studies, a fixed PEI concentration of 0.05 M was generally used. This corresponds to 4.3 × 10-5M when recalculated for the macromolecule as a whole. The reaction was controlled by monitoring the p-nitrophenolate anion absorption at 400 nm. A Specord M-400 spectrophotometer with temperature-controlled cell holders was employed. All runs were performed at a substrate concentration of 5 × 10-5 M. The observed rate constants (kobsd) were determined from the equation ln(A∞ - A) ) -kobsdt + constant, where A and A∞ are the absorbances of the micellar solutions at point t during and after completion of the reaction, respectively. The kobsd values were calculated using the weighted leastsquares computing methods. Each value of kobsd is the mean of at least three independent determinations differing by no more than 4%. The UV spectra were measured on a Specord UV-vis spectrophotometer, the spectra of APM-1 and reference compound 7 were determined at least three times provided that they were reproducible, and average parameters were taken into account. The oscillator strength was calculated on the basis of the integral intensity of the corresponding absorption band. High-resolution mass measurements were performed on a Finnigan MAT-212 mass spectrometer by a mass matching procedure, the reference substance being perfluorokerosene. Dynamic light scattering (DLS) measurements were performed by means of PhotoCor complex dynamic light scattering

SCHEME 4: Reference Compounds for UV Study (7) and NMR Study (8-10)

Nanoreactors Based on Amphiphilic Uracilophanes

J. Phys. Chem. B, Vol. 111, No. 51, 2007 14155

Figure 1. Surface tension isotherms of the single APM-1 system and the binary APM-1/PEI system, 25 °C.

equipment consisting of a goniometer and a digital correlator. A He-Ne laser operating at 633 nm wavelength and emitting vertically polarized light was used as a light source. The experimental details are described elsewhere.10 All of the NMR experiments were performed with a Bruker AVANCE-600 spectrometer with a 5 mm diameter inverse probe head with Z-active shielded gradients working at 600.013 MHz in 1H. Chemical shifts (ppm) are internally referenced to the tetramethylsilane signal in CDCl3 and the signal of the sodium salt of 3-(trimethylsilyl)propionic-2,2,3,3-d4 acid (DSS) in D2O. The 2D DOSY (diffusion-ordered spectroscopy) experiments were performed by a BPP-STE-LED (bipolar pulse pair-stimulated echo-longitudinal eddy current delay) sequence.52 Data were acquired with a 50.0 or 100.0 ms diffusion delay in all experiments, with a bipolar gradient pulse duration from 2.2 to 14.4 ms (depending on the system under investigation), a 1.1 ms spoil gradient pulse (30%), and a 5.0 ms eddy current delay. The bipolar pulse gradient strength was varied incrementally from 0.01 to 0.32 T/m. The experimentally observed diffusion coefficients (D) were then determined from 2D DOSY plots obtained with the Bruker XWinNmr software package. Several measures of D were obtained at more than one place in the spectrum, and all experiments were carried out in duplicate or triplicate. The temperature was set and controlled at 298 K with a 535 L/h airflow rate to avoid any temperature fluctuations owing to sample heating during the magnetic field pulse gradients. Surface tension measurements were performed using the du Nouy ring detachment method. The experimental details are described elsewhere.53 Conductivity measurements were performed using an inoLab Cond level 1 instrument. Results and Discussion Synthesis of Uracilophanes. We have recently reported that the interaction of 1,3-bis(ω-bromoalkyl)-6-methyluracils with 1,3-bis[ω-(ethylamino)alkyl]-6-methyluracils gives a series of isomeric bis(ethylamino)(1,3)uracilophanes with trans- and cisarrangements of the carbonyl groups at the C(4) atoms of different uracil moieties.45-47 In particular, coupling of dibro-

Figure 2. Specific conductivity versus APM-1 concentration for the single APM-1 system and the binary APM-1/PEI system, 25 °C.

mide 1a with diamine 2a afforded isomeric uracilophanes 3a (trans-arrangement of C(4)O) and 3b (cis-arrangement of C(4)O) which were separated and described in the solid state and in solution.45,47 Herein in the same manner uracilophanes 4a and 4b are synthesized from compounds 1b and 2b (Scheme 2). In this case, we did not succeed in isolating individual isomers 4a and 4b and obtained only their mixture. Water-soluble uracilophane 5 designated as APM-1 was prepared by quaternization of N atoms in bridges of trans-isomer 3a with n-decyl bromide in CH3CN according to the established procedure.45,46 The reaction of the 4a and 4b mixture with n-decyl bromide resulted in a mixture of trans- and cis-isomeric uracilophanes 6a and 6b designated as APM-2. Synthesized uracilophanes with quaternary ammonium in the spacers are shown in Scheme 3. Self-Organization Study. Surface Tension Measurements. The cmc values of aqueous solutions of APM-1 and APM-2 are determined by tensiometry (Figures 1 and 3, Table 1). For comparison the conductivity data are also given in Figure 2. Using surface tension isotherms, the quantitative parameters characterizing the adsorption of APM-1 and APM-2 at the air/ water interface and their micellization are estimated. The surface excess, Γmax, and the surface area per molecule, A, have been calculated using the Gibbs equation:

Γmax )

1 lim [dπ/d(log C)] 2.3nRT Cfcmc

(1)

Amin ) 1018/(NΓmax)

(2)

where R ) 8.31 J mol K-1 (gas constant), π is the surface pressure obtained from the surface tension of water minus the surface tension of the surfactant solution, and T is the absolute temperature (K), while dπ/d(log C) is obtained from the tangency at the cmc. NA is the Avogadro number (6.02 × 1023 mol-1). The parameter n represents the number of species at the interface, the concentration of which changes with the


Zakharova et al.

TABLE 1: Surface Excess Concentration, Γ, Surface Area per Headgroup, A, Standard Free Energy of Micellization, ∆Gm, and Standard Free Energy of Interfacial Adsorption, ∆Gad, for the APM-Based Systems

a

system

106 × Γmaxa (mol m-2)

105 × cmc (M)

Amina (nm2)

Aminb (nm2)

πcmc (mN m-1)

-∆Gm (kJ mol-1)

-∆Gad (kJ mol-1)

APM-1 APM-1/PEI APM-2 APM-2/PEI

1.45 1.23 1.06 1.83

85 (100c) 65 (61c) 9 0.6

1.14 1.35 1.64 0.91

1.71 2.02 2.45 1.36

25.15 19.24 37.51 17.52

35.0 36.4 46.2 59.6

52.3 52.0 63.8 72.7

For n ) 2 (eq 1). b For n ) 3 (eq 1). c Conductivity data.

surfactant concentration. The constant n takes the value 2 for an ionic surfactant where the surfactant ion and the counterion are univalent and the value 3 for a dimeric surfactant made up of a divalent surfactant ion and two univalent counterions in the absence of a swamping electrolyte. The reported A values for geminis were calculated on the basis of n ) 2,54,55 on the assumption that one of the two charged groups is neutralized by a bound counterion, or n ) 3, on the assumption of a full dissociation of the dimeric surfactants.56,57 In our study two sets of values of A based on n ) 2 and 3 are reported (Table 1). The standard free energy of micellization per mole of monomer unit and the standard free energy of interfacial adsorption at the air/saturated monolayer interface were evaluated by applying eqs 3 and 4.

∆Gm ) RT ln(cmc)

(3)

∆Gad ) ∆Gm - (πcmc/Γmax)

(4)

The results of the calculations are summarized in Table 1. The lower values of cmc for APM-2 as compared to APM-1 are probably due to differences in their hydrophobicity and/or in their packing modes. A decrease in the cmc values for both macrocycles occurs in the presence of PEI (Table 1). It is currently accepted58,59 that the decrement of the cmc’s can be considered as a measure of the affinity between the components in the amphiphile/polymer pairs. From this viewpoint, APM-2 shows a higher affinity for PEI than APM-1, which is supported by the ca. 10-fold decrease in the cmc of APM-2 when the polymer is added. It is noteworthy that in the APM-2/PEI system two break points exist in the surface tension plot, as in the

Figure 3. Surface tension isotherms of the single APM-2 system and the binary APM-2/PEI system, 25 °C.

conventional surfactant/polymer systems.58,59 The two critical concentrations correspond to the onset (critical aggregation concentration, cac) and completion (polymer saturation concentration, psc) of the mixed aggregation in the solutions. The current view is that the aggregates bound to the polymer are formed within the interval concentration limited by the values of cac and psc, while the polymer-free assemblies are formed above the psc.58,59 For the macrocycle APM-1 a more detailed study of the solution behavior is performed with the help of NMR and UV spectroscopy. NMR Study of APM-1. In general, the association properties of the molecular systems are explored by the dependence of the NMR chemical shifts (CSs) on the concentration. However, this approach may fail when there are no strong magnetically anisotropic groups in the molecules. In such cases self-diffusion coefficient (D) measurements can provide the information needed. The data can be obtained by the DOSY method.60,61 The method is utilized in studying association, complexation, and aggregation phenomena in a number of cases.62,63 There is a clear qualitative relationship between D and the size of the system: the slower the diffusion, the greater the size of the system. This forms a basis for drawing a conclusion in regard to the size of the aggregate and in so doing in regard to its association properties. However, there is no quantitatively well-defined general relationship between the diffusion coefficient and molecular volume. There are attempts to derive empirical relationships in the range of some classes of compounds which are similar in the sense of shape and are not prone to association.64,65 The key correlation can be expressed in the form D ) BVR, where B is a constant, V molecular volume, and R the shape factor. For a spherical particle R is accepted to be ca. -1/3; for polymer-like (i.e., rodlike) molecules R is ∼-1/2.65 For a definite type of molecule the B and R values can be determined from the double-logarithmic plot of D vs molecular volume.64 To begin with, 1H CSs of APM-1 were measured in the range of concentration from 0.05 to 5.60 mM in a solution of D2O (DSS, 100 mM, served as a CS reference). There was no indication of the association of APM-1 (the CS change was less than 0.02 ppm). Therefore, to see whether any association of APM-1 takes place, diffusivity measurements were also carried out at different concentrations (Table 2). To take into account solution viscosity changes with concentration, the self-diffusion coefficients in D2O + DSS solution (D1) were corrected66-68 (D1,corr) using the DDSS values, which are usually assumed to be unaffected by self-association. However, we found that correction by DDSS significantly changes the D1 values, and in addition, the slopes of D1 and D1,corr vs C differ as well (Figure 4). These facts prompted us to check the assumption of whether DSS does not associate. Therefore, additional experiments in D2O solution were repeated with dioxane (100 mM) as an internal viscosity reference.68 We found that in the 0.1-0.8 mM concentration range of APM-1 the Ddioxane changes were less than 2% while the DDSS changes



TABLE 2: Values of D for APM-1 and Reference Compounds 8-10 in D2O + DSS (D1) and D2O + Dioxane (D2) Solutions, for DSS (DDSS), and for Dioxane (Ddioxane) and Corresponding Standard Errors (10-10 m2/s) compound (Vse,a Å3)

C (mM)

APM-1 (1070)

0.05 0.07 0.10 0.15 0.20 0.40 0.30 0.50 0.60 0.80 1.00 5.60 1.00 1.00 1.00

8 (225) 9 (260) 10 (465)

D1

DDSS

2.39 ( 0.05 2.44 ( 0.09 2.67 ( 0.03 2.60 ( 0.03 2.86 ( 0.01

6.11 ( 0.04 6.36 ( 0.04 5.99 ( 0.02 5.87 ( 0.02 6.24 ( 0.01

2.64 ( 0.01

5.81 ( 0.01

2.59 ( 0.02 2.52 ( 0.01 2.47 ( 0.03 2.11 ( 0.04 5.12 ( 0.03 4.80 ( 0.02 4.50 ( 0.03

5.82 ( 0.02 5.73 ( 0.01 5.63 ( 0.02 5.22 ( 0.02 6.00 ( 0.02 5.90 ( 0.01 6.00 ( 0.01

D2

Ddioxane

2.65 ( 0.04 2.64 ( 0.03 2.69 ( 0.03

9.73 ( 0.01 9.69 ( 0.01 9.62 ( 0.03

2.66 ( 0.02

9.65 ( 0.03

2.59 ( 0.01

9.55 ( 0.01

D1,corrb 2.68 ( 0.05 2.63 ( 0.09 3.05 ( 0.03 3.03 ( 0.03 3.14 ( 0.01 3.12 ( 0.01 3.05 ( 0.02 3.02 ( 0.01 3.00 ( 0.03 2.78 ( 0.04 5.85 ( 0.03 5.58 ( 0.02 5.14 ( 0.03

D2,corrb

2.65 ( 0.04 2.64 ( 0.01 2.71 ( 0.03 2.64 ( 0.02 2.64 ( 0.01

a Connolly solvent-excluded volume (Å3; the values were estimated in the ChemOffice packet). b D corrected for solution viscosity changes (see the text for details)

Figure 4. Dependence of the D of APM-1 vs its concentration in D2O + DSS (D1) and D2O + dioxane (D2) solutions.

were ca. 5% (Table 2 and Figure 4). Hence, it appears that DSS is not so inert. Experiments on APM-1 (0.07 mM, D2O) with two different concentrations of DSS (ca. 100 and 1000 mM) proved this hypothesis. Namely, both D1 and DDSS values decrease by ca. 15% (from 2.44 × 10-10 to 2.10 × 10-10 m2/s and from 6.36 × 10-10 to 5.40 × 10-10 m2/s, respectively) when the DSS concentration increases. Thus, it appears that the solution viscosity depends on the DSS concentration, and hence, one should use DSS as an internal viscosity reference with great care. As one can see on the graphs (in Figure 4 principal dependences are summarized; data are given only in the 0.05-1 mM range) noncorrected D1 values in D2O + DSS are in good agreement with those in D2O + dioxane solution (D2). Meanwhile in the D2O + dioxane solution the correction by Ddioxane changes the self-diffusion coefficients only slightly, while in the D2O + DSS solution the correction by DDSS values produces a large shift. Therefore, hereinafter we have analyzed only noncorrected data. As one can see (Figure 4) the D1 vs C dependence is rather strange: at low concentration D1 is small, and it increases strongly with an increase of concentration, reaching a maximum value at ca. 0.2 mM. After that D1 decreases monotonously as the concentration increases to 5.6 mM. While the behavior of D1 vs C within the concentration range from 0.20 to 5.60 mM can be simply explained as a consequence of an increase of the

population of associated species or/and enlargement of its size due to association (in frame, e.g., isodesmic model), the first part of the graph (from 0 to 0.2 mM) cannot be rationalized on the basis of only this approach. Probably, at low concentrations the protonation effects due to trace acids of the solvent may cause self-association of APM1. To verify this hypothesis, experiments with the addition of acid and base were carried out. In the acidic sample (0.3 mM APM-1 in D2O + DSS (70 mM) + HCl) the diffusion coefficient decreased (D ) 3.5 × 10-12 vs 2.65 × 10-10 m2/s), which corresponded to the enormous increase of the molecular volume (by ca. 106). In the second case, the addition of NaOH leads to an increase of the D of APM-1 (from 2.39 × 10-10 to 3.20 × 10-10 m2/s), which corresponds to the destruction of associates. To estimate the size of the associates of APM-1, the D values for a set of molecular fragments of APM-1, compounds 8-10 (Scheme 4), with known solvent-excluded volumes (SEVs) were measured. The following correlation is roughly estimated for compounds 8-10: D ) (1.43 × 10-9)Vse-0.17 m2/s, where Vse is SEV (Å3). Thus, for APM-1 (D ) 2.97 × 10-10 m2/s at 1 mM) the SEV of the associate can be estimated as ca. 104 Å3. Taking into account the SEV for the monomer (1070 Å3), the aggregation number is ca. 10. UV Study of APM-1. Ultraviolet spectra of APM-1 in water solution have been interpreted in terms of hyperchromic and hypochromic effects (increase or decrease of light absorbance, respectively, compared to that of monomeric compounds). The latter phenomenon, i.e., hypochromism, has been widely used as evidence of stacked structures of various π-systems, including nucleic acid bases in solution.69 According to the theories of Tinoco70 and Rhodes,71 hypochromism (parallel stacking of the chromophores) or hyperchromism (linear array of the chromophores) is observed depending on the relative orientation of the transition moments. Values of hypochromism, H, were calculated from the oscillator strength, f (eq 5), of the studied uracilophane and 1,3-bis(4-bromobutyl)-6-methyluracil (7), which was used as a reference compound simulating building units of the macrocycle (Scheme 4):

∫

f ) (4.32 × 10-9) ((λ)/λ2) dλ

(5)

H (%) ) {1 - [f5/2f7]} × 100

(6)

where f5 is the oscillator strength of macrocycle 5 (APM-1) and f7 is the oscillator strength of the reference compound 7. Table


Zakharova et al.

TABLE 3: Ultraviolet Absorption Spectra and Percentage Hypochromism of APM-1 in H2O Solution at Different Concentrationsa,b C (mM)

λmax (nm)

(M-1cm-1)

f

H (%)

0.1 0.22 0.32 0.65 1.1

268 268 267 269 268

22000 23410 23125 22464 17668

0.379 0.371 0.378 0.374 0.296

8.0 10.0 8.0 9.2 28.2

a Key: λ, wavelength; , molar extinction coefficient; f, oscillator strength; H, hypochromism value. b For reference compound 7 the following parameters were determined: λmax ) 267 nm, ) 10 157 M-1cm-1, f ) 0.206.

3 represents UV data and calculated parameters for APM-1 in H2O solution at different concentrations. There is a linear relationship between the concentration and absorbance (the Beer-Lambert law) in the region 0.01-1.0 mM. This means that the molar extinction coefficient, , the oscillator strength, f, the hypochromism, H, of APM-1 are almost the same in the region. Positive hypochromism values of 8-10% may indicate either intramolecular stacking between uracil moieties69,72 or the formation of well-defined intermolecular aggregates of APM-1, even at such low concentrations. The latter assumption agrees with the NMR data. When the concentration of APM-1 in H2O solution is 1 mM, and the integral intensity of absorbance dramatically decrease, and as a result hypochromism increases. It is obvious that at 1 mM molecules of APM-1 form structures different from those at low concentrations. This critical point is similar to cmc values identified by tensiometry and conductivity studies. Addition of PEI to the APM-1 solution does not involve any changes in the uracilophane absorption spectra. The absorption of uracilophane with or without PEI (0.05 M) in the concentration region 0.01-1.0 mM is almost the same. DLS Study. The DLS data show that above the cmc large aggregates of APM-1 and APM-2 are formed with a mean radius of ca. 65 nm (5 mM APM-1) and 84-94 nm (1.5-7.5 mM APM-2), respectively. In the absence of PEI the solutions are practically monodispersed. The large size of the aggregates makes it possible to exclude the formation of the micellelike assemblies and to assume a lamellar packing of the macrocycles (Scheme 5). Due to a rather long hydrophobic spacer, its contact with water appears to be unfavorable. Therefore, despite the presence of the hard pyrimidinic fragment within the spacer, there occurs a high probability of its considerable bend, when molecules are ordered at the interface or in the bulk solution. Thus, two types of packing modes can a priori be expected, namely, self-assembly of the extended molecules and of the folded ∩-shaped monomers (Scheme 5). On the addition of PEI, the solutions become polydispersed. Above the cmc, aggregates with a hydrodynamic radius (RH) corresponding to the single APM assemblies exist together with the smaller particles (RH ≈ 25 nm), which can be attributed to the macromolecules of PEI.10 Probably, in the APM/PEI systems there exist aggregates of APMs like those in their single solution, while PEI plays the role of a polymer matrix (Scheme 5). To support the above assumptions, the geometry and packing parameters for APMs are considered. Packing Parameter Considerations. The self-assembly behavior can approximately be rationalized and predicted in terms of the surfactant packing parameter, given by73

P ) V0/(la)

Here V0 is the volume of the hydrophobic spacer, a is the polar head surface area at the interface of the hydrophobic core/water, and l is the chain length of the hydrophobic fragment. According to this model, a packing parameter of less than 1/3 will yield spherical micelles, while for a packing parameter between 0.5 and 1 there occurs a layered ordering. Later this approach was extended to bolas.74 Figure 5 shows the geometry of macrocycle 3a afforded by the X-ray diffraction study, which makes it possible to calculate the P factor with acceptable accuracy. Taking into account the cyclic structure of the APM molecules, we can consider the hydrophobic spacer made up of two symmetrical truncated cones with a common base. In this case, V0 ) (2h/3)[S1 + (S1S2)1/2 + S2], where S1 and S2 are the truncated cone bases. For bola 3a (nonhydrophobized analogue of APMs) V0 is calculated to equal 638 Å3. The volumes of two and three decyl radicals should be added to this value for APM-1 and APM-2, respectively. The contribution of the alkyl radicals can be estimated as the volume of a cylinder with a radius of 2.4 Å.75 The l value can be taken from Figure 5. The V0 and l values for decyl radicals can also be calculated using the relationships76

l (nm) ) 0.154 + 0.126nC V0 (nm3) ) (27.4 + 26.9nC) × 10-3 where nc equals 10 for a decyl radical. Both methods give similar results. The summary values of V0 and l are equal to 1108 Å3 and 42 Å for APM-1 and 1343 Å3 and 54.8 Å for APM-2. The area per headgroup, a, of amphiphiles is to a great extent determined by their charge character and can be influenced by the solution conditions. The a value can be estimated from the molecular geometry (Figure 5), which yields a = 25 Å2. This value takes into account only the steric component, ignoring the electrostatic repulsion of the headgroups, thus determining the upper boundary of the permitted packing parameter. It is in good agreement with a found for trialkylammonium headgroups in ref 77 on the basis of only steric effects. In this case the packing parameters are equal to 1.03 and 0.98 for APM-1 and APM-2, respectively. Thus, the planar layered packing is strongly dependent on the geometry of both molecules, while the real ordering mode and the aggregate morphology can be corrected by the solution conditions, contributions of electrostatic interactions, etc. In accordance with the literature,15 for the bolas and geminis when packing, both the expanded and folded conformations can be considered (Scheme 5). The calculations based on the surface tension isotherms show that with no PEI added the cross-section area per headgroup equals 114 Å2 for APM-1, which is in a good agreement with the literature value for geminis and is about twice as much as conventional cationic surfactants.78,79 The APM-2 molecule occupies a much larger surface area as compared to APM-1 (Table 1). This situation can be interpreted in two ways. First, this is probably due to the differences in the packing modes: extended for APM-1 and folded (∩-shaped) for APM-2, for which the “tail-to-tail” packing is the most possible one when aggregating (Scheme 5). Therefore, the transition from the extended to the folded mode of ordering is favorable due to a considerable contribution of the hydrophobic effect to the free energy of aggregation. This can underlie the lower cmc of APM-2 as compared to APM-1. Meanwhile, the hypochromic effect indicating the intramolecular stacking interactions (Table 3) testifies to the ∩-shaped packing for APM-1 as well. Therefore, the second explanation is that both macrocycles demonstrate a layered packing mode with folded



SCHEME 5: Schematic Illustration of the Packing Modes of Gemini APM-1 in a Single Solution, Extended (a) and Folded (b) Monolayer and Folded Bilayer (c), and in a APM-1/PEI Mixed System (d, e)

molecules involved. In this case the lower A value for APM-1 requires an efficient screening of the headgroups by counterions. Besides, aggregates can differ in their curvature, which is probably close to zero for APM-1 and is somewhat positive for APM-2. This is consistent with the lower A value for APM-1 as compared to APM-2 (Table 1). It might be assumed that vesicles are probably formed in the APM-2 system. The addition of PEI markedly influences the packing mode. In the APM-1/PEI system some loosening of the molecules at the surface layer probably occurs, which is confirmed by an increase in the A value to 135 A2 (Table 1). In the APM-2based systems a 2-fold decrease in the surface area per molecule is found. This can with high probability be attributed to the change in the packing mode, i.e., the transition from ∩-shaped to the extended form of the macrocycle. Catalytic Activity. In the kinetic section, the hydrolysis of phosphonates S1 and S2 of different hydrophobicities is examined in single APM solutions and their mixtures with PEI. The influence of substrate hydrophobicity on the catalytic effect can be regarded as a test for the ratio of the contribution of the

micellar and polymer catalysis to the summary effect. When the solubilization mechanism of substrate binding is predominant (micellar catalysis), a higher reactivity is observed for the more hydrophobic substrate.38 On the contrary, when the sorption pattern of substrate binding is prevalent (polymer catalysis), the less hydrophobic phosphonate shows a higher reactivity.9,10 In this case the electronic and steric effects rather than hydrophobicity are responsible for the reactivity, similar to molecular solutions. The latter tendency is assumed to occur in the case when the inclusion mechanism contributes to substrate binding as well. As shown in our earlier work, in single cationic micelles of conventional surfactants a ca. 20-fold acceleration of the hydrolysis of phosphonates S1 and S2 occurs,38 while in the cationic surfactant/PEI systems acceleration of the hydrolysis by up to 3 orders of magnitude is observed.8 In the individual and mixed systems based on cationic surfactants higher rate constants for the more hydrophobic phosphonate S2 were found. The catalytic effect of the single PEI solution is attested in refs 42-44. The general basic catalysis of the hydrolysis of


Zakharova et al.

Figure 5. Geometry of macrocycle 3a afforded by the X-ray diffraction study (the sizes are given in angstroms).

SCHEME 6: Schematic Representation of the Reaction under Study in Polymer and Micellar Solutions

Figure 6. Observed rate constant of hydrolysis of S1 and S2 as a function of the APM-1 concentration in the single APM-1 system (0.01 M NaOH) (a) and the APM-1/PEI system (0.05 M PEI, pH 10.5) (b), 25 °C.

TABLE 4: Results of Quantitative Treatment of the Kinetic Data in Terms of Eq 7

phosphorus acid esters occurs in the presence of amines including polyamines. The first-order law is observed for the dependence of the rate constant on the concentration of lowmolecular-mass amines. For the polyamines, the order of the rate constant decreases with the amine concentration, close to zero (the S-shaped kinetic curve showing a plateau at high concentrations). This is typical for the reactions with preliminary equilibrium processes, such as the formation of the catalytic complex substrate (S)-PEI (Scheme 6a). Similar principles underlie the kinetics of nucleophilic substitution in organized media (Scheme 6b). This kind of kinetics obeys the formalism similar to the Michaelis-Menten equation for enzyme catalysis (7),1 where

kobsd )

kw + kcatKSC 1 + KSC

(7)

KS is the binding constant of the substrate (S), the indices “w” and “agg” refer to water and the organized pseudophase (or polymer), respectively, and kw and kcat (s-1) are the pseudofirst-order rate constants in water and catalytic complexes, respectively. In the unbuffered PEI-based systems a solution pH of g10 is generated due to acid-base interactions between amino groups and water. However, the observed rate constant of hydrolysis is practically not contributed by the basic hydrolysis, and catalysis occurs through the general basic mechanism. Basic hydrolysis of the substrates is studied in the single APM systems. Figure 6 shows the kinetic data for the hydrolysis of phosphonates S1 and S2 in the APM-1-based systems. An inconsiderable influence of APM-1 on the reaction rate is observed in both the single and mixed systems. In the single APM-1 solution a ca. 1.3-fold retardation of the hydrolysis of both substrates occurs as compared to the uncatalyzed processes.

system

substrate

kcat (s-1)

KS (M-1)

kcat/kw

APM-1 APM-1 APM-1/PEI APM-1/PEI APM-2 APM-2 APM-2/PEI APM-2/PEI

S1 S2 S1 S2 S1 S2 S1 S2

0.031 0.026 0.0004 0.0012 0.0007 0.0045a 0.0062 0.0172

8200 2900 9300 1360 4390 7320 250 4840

0.7 0.8 0.6 2.1 1.4 7.4 10 33

a

Value from a plateau.

As is well-known,1,3,4,38 the catalytic effect of ionic surfactants is mainly contributed by the factor of concentrating the reagents. For the basic hydrolysis the effect is controlled by the headgroup charge. Cationic micelles accelerate the reaction of anionic nucleophiles due to their electrostatic attraction to the positively charged micellar surface. The revealed inhibition effect, even that small, is rather unexpected. This can be explained by the following reasons: (i) the low solubilization capacity of the aggregates toward the substrate; (ii) the low surface potential; (iii) the unfavorable localization of the substrates in the micelle interior inaccessible for the highly hydrophilic hydroxide ions. In the APM-1/PEI system a slight retardation of the hydrolysis of phosphonate S1 occurs, while the hydrolysis of S2 is slightly accelerated. In both cases the effects do not exceed 1.5-1.8 times as compared to the single PEI solutions. The results of the quantitative treatment of the kinetic data in terms of eq 7 are summarized in Table 4. As can be seen, the inconsiderable rate effect of the APM-1-based systems does not result from the low solubilization capacity of the aggregates toward the substrates. The binding constants of both phosphonates are high, which provides evidence for the complete binding of the substrates by the APM-1-based assemblies. In the case of basic hydrolysis (Figure 6 a) the most probable explanation


J. Phys. Chem. B, Vol. 111, No. 51, 2007 14161 TABLE 5: Observed Rate Constants of the Hydrolysis of Phosphonates S1 and S2 in the APM-2/PEI/La(III) System (0.05 M PEI, 0.008 M La(NO3)3, pH 8.5) 102kobsd (s-1)

Figure 7. Observed rate constant of hydrolysis of S1 and S2 as a function of the APM-2 concentration in the single APM-2 system (pH 10.5) (a) and the APM-2/PEI system (0.05 M PEI, pH 10.5) (b), 25 °C.

for the inhibition effect is the low charge density of the aggregates, which is consistent with the smaller cross-section values A for APM-1 (Table 1). In the APM-1-based systems higher binding constants are found for the less hydrophobic phosphonate (Table 4). As mentioned, an opposite effect has been observed in the micellar solutions of conventional cationic surfactants. This probably indicates that the solubilization mechanism is not the principal contributor to the reagent binding mode. In fact, the multicentered sorption of the substrates by macromolecules in the APM/PEI assemblies is possible, which is unaffected by the substrate hydrophobicity.9-11 However, the same correlation between the substrate binding constants is observed in the single APM-1 solutions in the absence of PEI. This probably indicates that the inclusion mechanism of substrate binding occurs in the APM-based systems. Since APMs are related to the cyclophanes, at some conformations they can form a cavity in solutions. Providing geometry and energy complementarity toward the substrate, the guest-host inclusion mechanism might contribute to the substrate binding mode. Such a pattern of binding is apparently not influenced by the substrate hydrophobicity either. Figure 7 shows the kinetic data for the APM-2-based systems. The aggregates of APM-2 demonstrate a higher influence on the reaction of hydrolysis of the phosphonates as compared to those of APM-1. In both the single solution and the APM-2/ PEI system accelerations of the reactions occur in the case of both phosphonates. In the APM-2-based nanoreactors regularities typical for conventional cationic surfactants are observed. In particular, higher rate constants and a higher catalytic effect are found for the more hydrophobic phosphonate S2. The maximum catalytic effect (a ca. 30-fold acceleration) is observed for the APM-2/PEI-catalyzed hydrolysis of phosphonate S2 (Table 4). Since the APM-2 assemblies have shown a more effective catalysis, their further modification with lanthanum ions was performed (Table 5). A 5-fold acceleration of the

102kobsd (s-1)

104 × CAPM-2

S1

S2

104 × CAPM-2

S1

S2

0 0.75 0.56 1.7

1.8 3.6 3.7 3.2

2.1 2.0 1.8 1.4

1.5 3.0 4.5

2.8 2.6 2.5

1.2 0.99 0.73

hydrolysis of phosphonate S2 occurs in the APM-2/PEI/La(III) system near the cmc as compared to the binary APM-2/PEI aggregates. Taking into account a 30-fold acceleration of the reaction as compared to that of the single PEI solution, a 150fold catalytic effect is observed. The summary catalytic effect of the APM-2/PEI/La(III) system should be compared with the neutral aqueous hydrolysis of the phosphonates. However, it is a very slow process, the rate of which is hard to measure with acceptable accuracy. Therefore, the real rate accelerations are high and close to the fermentation effects. For the quantitative characteristics to be available, the rate constants in the systems studied are compared to that of the basic hydrolysis of the substrates at the pH spontaneously generated in solution. Due to salt hydrolysis in the presence of lanthanum salt, a marked decrease in solution pH occurs up to pH 8.1-8.6. In this case the summary rate effect of the APM-2/PEI/La(III) system reaches 3 orders of magnitude. Besides, milder conditions are observed as compared to those of strong alkali solutions. Conclusions Thus, the self-organization of new amphiphilic pyrimidinic macrocycles of dimeric structure has been studied in aqueous solutions. The complex examination of the APM-based systems with the help of tensiometry, conductometry, dynamic light scattering, and UV and NMR spectroscopy provides evidence for their aggregation. Calculations based on the surface tension isotherms and on the packing parameter considerations make it possible to assume a lamellar packing of macrocycles when aggregating. Marked differences in the aggregation behavior of APM-1 and APM-2 have been found. In the tensiometry study, a lower cmc and a higher cross-section area per molecule are found for APM-2 as compared to APM-1. The additives of PEI do not exert much influence on the cmc of APM-1. A pronounced decrease in the cmc and also a ca. 2-fold decrease in the surface area per molecule occur in the APM-2/PEI systems. This can indicate the transition from the folded to the extended packing mode. The APM assemblies are explored as nanoreactors for the hydrolysis of phosphonic acid esters. The kinetic study reveals a minor rate effect of the APM-1-based systems. An acceleration of the hydrolysis of both phosphonates occurs in the APM-2-based systems as compared to the uncatalyzed process. The dramatic differences in the catalytic activity of the two macrocycles are assumed to underlie the different morphologies of the aggregates. Within the APM-2 f APM-2/PEI f APM-2/PEI/La(III) series, due to the cooperative contributions of the supramolecular, polymer, and homogeneous catalysis an increase in the catalytic effect is observed from 30 times to 3 orders of magnitude as compared to the basic hydrolysis. Acknowledgment. We thank the Russian Foundation for Basic Researches (Grants 07-03-00392-a and 05-03-32558-a) and the Federal Collective Spectral Analysis Center for Physical and Chemical Investigations of Structure, Properties and Composition of Matter and Materials (CKP SAC) and Federal

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