Cooperative and Noncooperative Assembly of Oligopyrenotides

Article pubs.acs.org/Macromolecules

Cooperative and Noncooperative Assembly of Oligopyrenotides Resolved by Atomic Force Microscopy Alexander V. Rudnev,†,‡,⊥ Vladimir L. Malinovskii,†,⊥ Alina L. Nussbaumer,† Artem Mishchenko,†,§ Robert Han̈ er,*,† and Thomas Wandlowski*,† †

Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, 3012 Bern, Switzerland A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, Moscow 119991, Russia

‡

S Supporting Information *

ABSTRACT: The supramolecular assembly of amphiphilic oligopyrenotide building blocks (covalently linked heptapyrene, Py7) is studied by atomic force microscopy (AFM) in combination with optical spectroscopy. The assembly process is triggered in a controlled manner by increasing the ionic strength of the aqueous oligomer solution. Cooperative noncovalent interactions between individual oligomeric units lead to the formation of DNA-like supramolecular polymers. We also show that the terminal attachment of a single cytidine nucleotide to the heptapyrenotide (Py7-C) changes the association process from a cooperative (nucleation−elongation) to a noncooperative (isodesmic) regime, suggesting a structure misfit between the cytidine and the pyrene units. We also demonstrate that AFM enables the identification and characterization of minute concentrations of the supramolecular products, which was not accessible by conventional optical spectroscopy.

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INTRODUCTION The understanding of building principles that govern the organization of molecular objects represents the key to the creation of nanoscale functional systems.1−5 In the past two decades, the topic of supramolecular polymers (SPs) rapidly emerged as a separate field of research with promising prospects for the development of new materials.6−11 The spontaneous self-association of monomer units toward the formation of polymeric structures may proceed reversibly or irreversibly, depending on the conditions such as concentrations of monomers, temperature, pH, solvent polarity, ionic strength, etc. Importantly, noncovalent synthesis under reversible conditions allows creating nanostructures without defects due to self-healing and/or self-sorting processes.12−14 Therefore, noncovalent synthesis offers a unique strategy for producing nanomaterials of crystalline purity. The formation of extended ordered assemblies (i.e., nanoobjects of defined shape, size, composition, and chirality) versus disordered aggregates is possible only when cooperative noncovalent interactions take place during the association process.2,6,15 Cooperativity arises from the interplay of two or more interactions within an associate and leads to a system with novel properties not present in the individual molecular blocks.2,6,15 Cooperative association represents a process, where the equilibration of a system is significantly shifted to one of the limiting forms of the material (all-or-nothing behavior), e.g. “fully bound−fully unbound ligand”, “monomer−polymer”, etc.15 Such systems are usually characterized by a steep change of properties after a certain point, which is identified as nucleation step. However, the prediction of © 2012 American Chemical Society

cooperativity and the search for suitable conditions in reversible assembly are challenging tasks. Since efficient screening methods are lacking, approaches in this research area have been largely driven by intuition. Products of the self-assembly process are often characterized by microscopic methods, such as TEM, AFM, or STM, and light scattering techniques.16−18 However, the main method for monitoring supramolecular polymerization processes is optical spectroscopy. Differentiation between a noncooperative (isodesmic19) and a cooperative (nucleation−elongation6) mechanism of association is commonly achieved through analysis by UV−vis, circular dichroism (CD) spectroscopy, and, occasionally, fluorescence spectroscopy.20−24 These methods enable following aggregation and/or dissociation processes down to the lower micromolar range. However, they are usually insufficient to discriminate and characterize minute fractions of ordered assemblies in the presence of bulk material due to the additive character of optical signals. In this work, we studied the assembly of an amphiphilic heptapyrenotide Py7 and its cytidine derivative Py7-C (Figure 1a) in water with atomic force microscopy (AFM) and compare these results with data from optical spectroscopy (UV−vis, CD). The microscopic investigations enabled us to distinguish between cooperative and noncooperative aggregation of Py7 and Py7-C, respectively, as well as to clarify the origin of the loss of supramolecular chirality in their mixtures. The conclusions Received: April 14, 2012 Revised: July 6, 2012 Published: July 20, 2012 5986

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temperature. Equilibration was monitored from few minutes up to three weeks. In “sergeant-and-soldiers” experiments, Py7 and Py7-C building blocks were mixed at different ratios in buffer solution (10 mM, 1 M NaCl, pH 7.0) while keeping the total concentration constant at 2.5 μM. The assembly was started with preheating of the reaction mixture up to 60 °C followed by cooling to room temperature and a subsequent equilibration period. Optical Spectroscopy. CD spectra were recorded with a JASCO J-715 spectrophotometer at 20 °C using quartz cuvettes with an optical path of 1 cm (scanning speed: 100 nm/min; data pitch: 0.5 nm; bandwidth: 1.0 nm; response: 1 s). The UV−vis spectra were measured with a Varian Cary 100 Bio-UV−vis spectrophotometer equipped with a Varian Cary block temperature controller in 200 to 700 nm at 20 °C. The optical path was 1 cm. Atomic Force Microscopy. AFM imaging was performed under ambient conditions in air with a Nanosurf FlexAFM (Nanosurf AG, Switzerland). All measurements were carried out in tapping mode employing PPP-NCHR-W cantilevers from Nanosensors (resonance frequency ∼280 kHz, tip radius ∼10 nm). The mica substrates (20 × 20 mm2) were attached to a steel baseplate with Scotch tape and freshly cleaved prior to each new experiment. For surface modification, 5 μL of a 2.5 or 7 μM oligomer solutions were mixed with 10 μL of 1− 5 mM NiCl2 solution in a vial (the final concentrations of Ni2+ ranged between 0.7 and 3.3 mM, depending on the specific experiment). The mixture was subsequently drop-casted onto the mica substrate. After an incubation time of 8 min, the modified mica sample was thoroughly rinsed with Milli-Q water (>18 MΩ cm−1, 3 ppb TOC) and dried in a gentle stream of 5 N Ar (Alpha Gas). This experimental protocol enabled uniform distributions of supramolecular polymers (SPs) over more than 1 cm2 of the mica surface.

Figure 1. (a) Oligomeric pyrene strands: achiral with X = H (Py7) and chiral with X = cytidine (Py7-C). (b) Formation of rodlike, supramolecular polymers by interstrand stacking interactions in aqueous solution triggered by increasing the ionic strength.

derived from AFM results are in agreement with complementary investigations using solution-based optical methods. We highlight that the strong electrostatic substrate−polymer interactions facilitate the identification and characterization of minute concentrations of the supramolecular assemblies at an early stage of their formation, which extends significantly the detection limit as accessible from optical spectroscopy.

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EXPERIMENTAL SECTION

Synthesis of Oligopyrenotides. The pyrene building blocks Py7 and Py7-C (Figure 1a) were synthesized according to a published procedure.25 Nucleoside phosphoramidites from SAFC (Proligo reagents) were used for oligomer synthesis. The oligomers were prepared via an automated oligonucleotide synthesis by a standard synthetic procedure (“trityl-off” mode) on a 394-DNA/RNA synthesizer (Applied Biosystems). Cleavage from the solid support and final deprotection was done by treatment with 30% NH4OH solution at 55 °C overnight. All oligomers were purified by reverse phase HPLC (LiChrospher 100 RP-18, 5 μm, Merck), eluent A = (Et3NH)OAc (0.1 M, pH 7.4); eluent B = MeCN; elution at 30 °C; gradient 5−50% B over 39 min. Preparation of the Oligopyrenotide Polymer Solutions. For Py7 and Py7-C (Figure 1a): 7 μM oligomer in 1 mL of buffered solution (10 mM sodium phosphate, pH 7.0, 1 M NaCl) was heated to 80 °C. The mixture was subsequently allowed to cool to room

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RESULTS AND DISCUSSION It is well-recognized that product distribution upon association is governed by thermodynamic and kinetic factors. Because of the dynamic nature of supramolecular systems products may be formed under thermodynamic control (assembly with the lowest Gibbs free energy) or under kinetic control. Kinetic control usually leads to a set of energetically similar and/or different structures and, in rare cases, to ordered assemblies, not necessarily of the lowest energy. In consequence, probing and tuning of aggregation under equilibrium conditions requires the

Figure 2. Tapping mode AFM images of (a) (Py7)n polymers and (b) B-DNA molecules (600 bp DNA fragment) deposited on mica using Ni2+ cations as binding agent, 3 × 3 μm2. The polymers were formed from 2.5 μM achiral oligomers Py7 in solution by addition of 1 M NaCl. After more than three weeks equilibration time an aliquot of 5 μL was mixed with 10 μL of 5 mM NiCl2 (final c(Ni2+) = 3.3 mM) and deposited on mica. The DNA samples were also prepared in the presence of 1 M NaCl. Subsequently, 5 μL of the DNA solution (0.18 μg μL−1) was mixed with 10 μL of 5 mM NiCl2 (final c(Ni2+) = 3.3 mM) and then deposited on mica. The insets show magnifications of panels a and b and typical cross sections of the two polymer strands, 0.3 × 0.3 μm2. (c) Schematic representation of the helical structures of the (Py7)n-type polymers: top and side views. Further details are given in the text. 5987

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Figure 3. Tapping mode AFM images (recorded in air) of (Py7)n initially formed in 1 M NaCl solution (equilibration time >3 weeks) and subsequently deposited on mica with Ni2+ as additive (frame size 3 × 3 μm2). The concentration of Py7 monomers was 2.3 μM. The Ni2+ concentration varied from (a) 1.3 mM, (b) 2.3 mM to (c) 3.3 mM. A ring-shaped polymer is highlighted in (b).

determined width is ascribed to the convolution of the AFM tip radius R with the molecular feature under study. Assuming that the minimum distance between tip and sample surfaces is constant, the sharp edges of objects under study broaden, and the apparent lateral size dapp increases by the factor 2(2Rdreal − dreal2)1/2.35 Taking R = 10 nm (cantilever specification) and the measured width dapp = 24−40 nm, we estimated the real width dreal as ∼2.0−2.9 nm (see details in Supporting Information, Figure S1). The apparent height of the (Py7)n and DNA strands are obtained as 0.65 ± 0.20 nm as extracted from experiments with different cantilevers and scanning parameters (Figure 2). The underestimation of the real height of DNA molecules in AFM experiments is well-known. It is attributed to deformations as induced by tip−sample interactions. 36 Assuming a similar deformation in case of the (Py7)n polymers, we calibrated their “real” height with the known DNA data as reference (see Supporting Information for details, Figure S2). The ratio of the apparent heights of the (Py7)n polymer and of DNA, as obtained under identical experimental conditions, is equal to 1.0 ± 0.1 (cf. insets in Figures 2a,b). On the basis of this comparison, we extracted 2.4 ± 0.4 nm as the “real” height or diameter of the rodlike polypyrene polymers. Figure 2a demonstrates that AFM imaging enabled the detection of assemblies formed from as little as ∼10−11 mol of the Py7 block, which is 100−1000 times less than the amount of material required for optical experiments. We assume that the high sensitivity of the AFM approach for monitoring supramolecular polymerization is due to the selection of individual aggregated products on the substrate surface. In particular, Ni2+ cations were used as an additive to induce attractive electrostatic forces between the phosphate containing pyrene aggregates (Py7)n and the negatively charged mica surface. Similar strategies were also applied for the immobilization of DNA.16,30,31 These interactions via multiple Ni2+ bridges result in a strong binding of the longer polymers to the substrate surface, whereas the molecular building blocks and small aggregates are bound weaker. This surface-mediated selectivity enables the detection and analysis of small amounts of long aggregates, even in a pool of monomeric building blocks and other types of weakly associated aggregates (see also below the data from time-dependent experiments). The role of Ni2+ ions in polymer attachment and their influence on the morphology of the SPs was mapped in a systematic series of experiments with (Py7)n in the presence of different concentrations of Ni2+ ions (Figure 3). Effective

synthesis and identification of suitable building blocks (intrinsic properties) and the choice of appropriate external parameters (concentration, medium, temperature, etc.). Time-dependent and structure-sensitive experiments are essential to identify and to verify thermodynamically driven assemblies. We addressed these points by the design of the specific experiments described below. Identification and Characterization of Supramolecular Polymers. An achiral pyrene25 was used as aromatic unit for the preparation of covalently linked oligopyrenotides.26 The synthesis of the amphiphilic heptapyrenotide Py7 and its cytidine derivative Py7-C (Figure 1a) was carried out via standard phosphoramidite chemistry.27 These oligomers were used as building blocks for noncovalent association. The length of the oligomers was chosen on the basis of previous studies, which demonstrated that seven consecutive pyrene units exhibit cooperativity in the formation of interstrand hybrids from oligonucleotide−pyrene conjugates.28,29 The oligomer Py7 is achiral, whereas Py7-C is rendered chiral by the terminally attached cytidine nucleotide. The nucleotide was, thus, introduced as a symmetry-breaking element (Figure 1a). We discovered in a previous spectroscopic study23 that the oligomer Py7 forms supramolecular, rodlike polymer assemblies (Py7)n in aqueous solution. High ionic strength (1 M NaCl) supports the aggregation of the hydrophobic pyrene units and leads to the formation of extended linear polymers (Figure 1b). In the present AFM study, we employed the same experimental conditions for the supramolecular assembly of Py7 and Py7-C in solution. Aliquots of the sample solution were subsequently deposited on freshly cleaved mica. The protocol of the AFM experiments followed a strategy originally developed for DNA.30,31 Figures 2a and 2b show typical AFM images of (Py7)n and 600 bp B-DNA molecules on mica (see Supporting Information for details). The insets illustrate enlarged areas of the corresponding main panels as well as cross-section profiles. These data demonstrate qualitative similarities between the shapes of (Py7)n assemblies and that of DNA molecules. The apparent width dapp of both polymers and DNA molecules was found to be similar and amounts to 32 ± 8 nm (insets in Figures 2a,b). However, the exact diameter of double-stranded B-DNA molecules is much smaller. Published data range between ∼2.0 nm in the solid state32,33 and 2.2−2.6 nm in aqueous solution.34 The differences are attributed to the hydration shell. The rather high value for the experimentally 5988

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Figure 4. Tapping mode AFM images recorded in air for the assembly of (Py7)n on mica with 7 μM Py7 monomer solutions (frame size 3 × 3 μm2): (a) in the absence of 1 M NaCl after 3 weeks equilibration; (b−d) in the presence of 1 M NaCl after (b) 40 min, (c) 1 week, and (d) 3 weeks of equilibration. The concentration of Ni2+ was 2.3 mM. (e) UV−vis spectra of the sample solution before (t = 0) and after addition of 1 M NaCl at different equilibration times. (f) Time-dependent changes of the intensities of the three main UV−vis absorption peaks upon Py7 aggregation in 1 M NaCl solution.

adhesion of polymers was not detected at c(Ni2+) ≤ 0.7 mM. Surface immobilization was successful at c(Ni2+) ≥ 1.3 mM. Increasing the Ni2+ concentration up to 2.3 mM results in a substantial increase in coverage of the adsorbed material (from 28 to 66 rods/μm2 with increasing the Ni2+ concentration from 1.3 to 2.3 mM, Figures 3a,b). Higher Ni2+ concentrations (≥3.3 mM) lead to the overlapping of polymer strands on the substrate, which hampers the quantitative analysis. Therefore, the Ni2+ concentration of 3.3 mM is considered as an upper limit. Increasing the Ni2+ concentration up to 3.3 mM does not change the rodlike shape and the uniform width of the individual (Py7)n polymers (Figure 3). The morphology of the individual polymers is also not changed upon mixing of the (Py7)n containing sample solutions with Ni2+ directly on the mica substrate (Supporting Information, Figure S3). The only difference, as compared to the typical strategy applied (mixing of Ni2+ and sample solution in a vial followed by deposition on mica), comprises in the less uniform distribution of SPs on the substrate surface. The quantitative analysis of the AFM images demonstrates that the number of polymers with a contour length from 25 to 300 nm decreases exponentially. Figure S4 illustrates a set of data as obtained for a (Py7)n sample on mica with 2.3 mM Ni2+ from 20 separate AFM images acquired at different surface spots. The exponential decay of the length distribution suggests supramolecular polymerization according to a nucleation− elongation mechanism.6 Occasionally, we also observed (Py7)n polymers of circular contour. End-to-end intrasupramolecular assembly leads to ring-shaped polymers37,38 such as marked in Figure 3b (see also Figure S7).

Kinetics Study of Supramolecular Polymerization. Slow equilibration of aggregates represents a major drawback in the design and the study of assemblies based on hydrophobic interactions. Equilibration times, often longer than days and even weeks, have been reported for polymer materials involving hydrophobic units.13,19 The slow dissociation kinetics of aggregates in polar medium is recognized as a key factor.39 We addressed the question of equilibration in a time-dependent study. Py7 aggregation was initiated by addition of NaCl in the monomer-containing solution (Figure 4). No linear elongated polymer chains are formed in the absence of sodium chloride. Figure 4a reveals only relatively small, noncooperative aggregates of undefined size and shape. Cooperative polymerization of Py7 is induced by the addition of NaCl to the solution (Figure 4b). Increasing the equilibration time leads to an increase in the number of (Py7)n polymers (Figures 4b−d). The concentration of SPs on the substrate as obtained by averaging data from 5 to 10 AFM images at different spots amounts to 4, 39, and 66 SPs per 1 μm2 for equilibration times of 40 min, 1 week, and 3 weeks, respectively (see Supporting Information for details, Figure S5 and Table S1). UV−vis spectra of Py7 building blocks show the typical 1Ba (S4 ← S0), 1Bb (S3 ← S0), and 1La (S2 ← S0) transitions in pyrene40 around 225−260 nm (region I), 260−300 nm (region II), and 310−400 nm (region III), respectively (Figure 4e). The addition of 1 M NaCl to the sample solution leads to a substantial decrease of the UV−vis signals during the first few minutes (compare t = 0, i.e., before NaCl addition, and t = 10 min in Figure 4e). This observation is attributed to the initiation of the aggregation process. A further decrease in the absorbance of the three characteristic peaks corresponds to an increasing fraction of aggregated pyrene blocks (Figure 4f), 5989

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Figure 5. Tapping mode AFM images of mica samples modified with polymers formed in oligopyrene solutions, which contained different molar fractions of Py7-C (frame size 3 × 3 μm2): (a) 0% (pure Py7), (b) 20%, (c) 40%, (d) 60%, and (e) 100% (pure Py7-C). The total concentration of pyrene oligomers in all samples was 2.5 μM. The ionic strength was given by 1 M NaCl. The sample solutions were equilibrated for more than 3 weeks. Panel (f) shows the coverage of the aggregates as a function of molar fraction of Py7-C in mixed oligopyrene solutions (see Table S2).

gates of Py7 and Py7-C. It was found that the copolymerization process in a mixture of Py7 and a small fraction of Py7-C resulted in a pronounced CD response (Figure S10). However, the CD signal ceased if the molar fraction of Py7-C amounted to 60% or more. We suggested in that work23 two possible scenarios: (i) a change in the aggregation process from a cooperative to a noncooperative mechanism with increasing content of the mismatching unit Py7-C or (ii) the occurrence of multiple reversals45 in the SPs. Both mechanisms are reasonable, based on the original spectroscopic data. However, the higher resolution and the direct access to the polymer morphology in the present AFM experiments demonstrated clearly that mechanism (i) is predominant. Spectroscopic data were also collected to explore cooperative and noncooperative association. Figure 6 displays UV−vis spectra of the polymer product as obtained for three equilibrated (more than 3 weeks) mixtures of Py7 and Py7-C monomers together with data of the pure building blocks in 1 M NaCl solution. Increasing the Py7-C concentration leads to a broadening of the features in the characteristic absorption regions I to III (see Supporting Information for more details). The vibronic structure in region III is deconvoluted in three individual peaks labeled P1, P2, and P3. Figures 6b and 6c represent the two limiting cases: 100% Py7 and 100% Py7-C. The apparent vibronic structure of the optically allowed transition S2 ← S0 in region III is clearly resolved for Py7, but much less pronounced for Py7-C. This finding indicates the lack of highly ordered aggregates with an uniform pyrene− pyrene arrangement (with restricted vibrational motions and increased electronic nearest-neighbors pyrene−pyrene interactions) in the Py7-C sample, which is distinctly different for

reaching equilibrium after ca. 2 weeks. This trend is in agreement with the present AFM results (Figures 4b−d). However, hypochromism itself does not allow distinguishing between cooperative (formation of ordered extended polymers) and noncooperative (disordered aggregates) association. The latter challenge was resolved in the more sensitive AFM experiments. We note that the AFM approach allows detecting the appearance of rodlike polymers (Py7)n already 40 min after the addition of NaCl (Figure 4b). The combination of AFM and optical (UV−vis, CD) data provides clear evidence that supramolecular polymerization can be initiated in a controlled manner by applying an appropriate stimulus, in analogy to classical chemical polymerization.41,42 Cooperative and Noncooperative Association. In this section, we explore the copolymerization of Py7 and Py7-C at a constant total concentration of pyrene units. The AFM data shown in Figure 5 illustrate that the density of linear polymer strands decreases with increasing content of the cytidinemodified heptapyrenotide (Py7-C) (see Supporting Information for details, Table S2 and Figure S6). No rodlike polymers were detected for samples prepared from a reaction solution with more than 60% of Py7-C (Figures 5e,f). This supports that the nucleation step, a prerequisite to cooperative association,6,43 is suppressed. Thus, the attachment of a terminal cytidine nucleotide onto the pyrene oligomer changes the association mechanism from cooperative (pure Py7, Figure 5a) to noncooperative (pure Py7-C, Figure 5e), which can be regarded as a sort of structure misf it between cytidine and the ordered multipyrene assembly. This conclusion is in agreement with results of our previous CD spectroscopic study,23 where the “sergeant-and-soldiers” effect43−45 was explored with coaggre5990

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Figure 6. (a) UV−vis spectra of Py7 and Py7-C equilibrated in 1 M NaCl solutions. The mole fractions of Py7-C are indicated. The total concentration of the pyrene oligomers was kept at 2.5 μM. The three adsorption regions are labeled I to III. (b, c) Deconvolution of region III into three Voigt-type peaks for (b) 100% Py7 and (c) 100% Py7-C. (d) Dependence of the intensity ratios A362/A347 of the UV−vis signals in region III on the Py7-C fraction.

chirality.48 The parity between M- and P-helices is broken by the presence of small amounts of the cytidine-containing building block Py7-C, which preferentially stabilizes P-helical polymers. While this work was in progress, we identified other nucleotide-containing oligopyrenotides, which stabilize the Mhelical SPs.49 The structure of the SPs is schematically illustrated in Figure 2c. The helical chirality is the result of a twisted arrangement of stacked pyrene units along the propagating helical axis.48 The negatively charged phosphate backbones support a moderate stiffness of the rodlike aggregates and prevent interhelical aggregation. With an inner core of stacked aromatic units and a phosphate backbone on the outside, the Py7 polymers can be regarded as structural relatives of nucleic acids.32 As in DNA, the structural organization of polymers studied is based on noncovalent cooperative interactions.

Py7. The intensity ratio of the pronounced peak and the valley in the vibronic bands is used to characterize the pyrene aggregates.46 Figure 6d shows the ratio of absorbance intensities at 362 nm (valley) and 347 nm (maximum of signal P2) as a function of the molar fraction of Py7-C. A small fraction of Py7-C (up to 20%) does not change noticeably the extent of cooperative polymerization of Py7. However, the further increase of the Py7-C fraction leads to the suppression of association, which is reflected by the lack of the vibronic structure (the valley−peak ratio approaching 1). Cooperative aggregation of Py7 is also shown in the AFM experiments (Figures 2−5). The comparison of these experimental data reveals an excellent correlation between the results obtained by AFM and those acquired from UV−vis and CD spectroscopy (Figures 4 and 6 and Figures S6 and S10). Origin of Chirality of the Supramolecular Polymers. The control of the handedness and origin of chirality in quasione-dimensional aggregates represents another key aspect in supramolecular polymerization.18 In a previous communication,23 we proposed a helical nature of the polypyrene aggregates on the basis of CD spectroscopy data. Excitoncoupled CD signals47 were observed for Py7 solutions with small fractions of Py7-C indicating the preferential stability of Phelical aggregates, whereas no optical activity was detected for pure Py7 solutions.23 The identity in shape (AFM images in Figures 5) of the products obtained by the polymerization of pure Py7 (optically nonactive product, Figure S10) and by the copolymerization of Py7 with Py7-C (optically active product, Figure S10) reveals that pyrene polymers are intrinsically helical. The polymerization of pure Py7 leads to a racemic mixture of both left- and right-handed products with helical

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CONCLUSIONS The association of heptapyrenotide Py7 and its cytidine derivative Py7-C in aqueous solutions of high ionic strength was studied by AFM and optical spectroscopy. AFM imaging enabled the identification and characterization of minute amounts of supramolecular products and provided fast and reliable information on morphology of these assemblies. We demonstrated that cooperative assembly of artificial amphiphilic oligomeric blocks Py7 leads to the formation of quasi-one-dimensional SPs with high similarity (shape and helical nature) to DNA. Within the chosen range of experimental conditions (concentration of building blocks, relative composition of oligomers, equilibration time) the 5991

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polymers are highly uniform in width. The homogeneity in width confirms that the morphology of the aggregates is encoded in intrinsic properties of the heptapyrenotide building block Py7, composed of hydrophobic aromatic pyrene units linked via hydrophilic phosphate. The appearance of a structure misfit in supramolecular assembly was demonstrated by experiments on copolymerization of Py7 and its cytidine derivative Py7-C. AFM measurements revealed that the cytidine residue changed the mechanism of association from cooperative (pure Py7) to noncooperative (pure Py7-C). The combination of optical spectroscopy and AFM measurements revealed that oligopyrenotide SPs exhibit the general characteristics of supramolecular materials: they are dynamic with a composition, fluctuating through release and reintegration of individual molecular building blocks. The ability to form supramolecular helical polymers and their high structural similarity to nucleic acids render oligopyrenotides as promising candidates for the development of novel, self-pairing systems. Furthermore, polymers of this type may also find applications as optical components in advanced electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

Quantitative analysis of AFM results and supplementary AFM, UV−vis, and CD data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel +41 31 631 4382, fax +41 31 631 8057, e-mail robert. [email protected] (R.H.); Tel +41 31 631 5384, fax +41 31 631 3994, e-mail [email protected] (T.W.). Present Address §

School of Physics & Astronomy, University of Manchester, Manchester M13 9PL, UK. Author Contributions ⊥

These authors have contributed equally and are joint first authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the University of Bern, the Swiss National Science Foundation under Grants 200021-1-124643 (T.W.) and 200020-132581 (R.H.).

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REFERENCES

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dx.doi.org/10.1021/ma3007619 | Macromolecules 2012, 45, 5986−5992