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Photochemistry in Confined Environments for Single Chain Nanoparticle Design Hendrik Frisch, Jan P. Menzel, Fabian R. Blößer, David Marschner, Kai Mundsinger, and Christopher Barner-Kowollik J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018
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Journal of the American Chemical Society
Photochemistry in Confined Environments for Single Chain NanoParticle Design Hendrik Frisch,† Jan P. Menzel,† Fabian R. Bloesser,† David E. Marschner,†,‡ Kai Mundsinger† and Christopher Barner-Kowollik*†,‡ †
School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology (QUT), 2 George Street, Brisbane, QLD 4000, Australia ‡
Macromolecular Architectures, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76131 Karlsruhe, Germany Supporting Information Placeholder ABSTRACT: Emulating nature’s protein paradigm, single chain nanoparticles (SCNP) are an emerging class of nanomaterials. Synthetic access to SCNPs is limited by ultra-low concentrations, demanding reaction conditions and complex isolation procedures after single chain collapse. Herein, we exploit the visible light photodimerization of styrylpyrene units as chain folding mechanism. Critically, their positioning along the polymer chain creates a confined environment, increasing the photocycloaddition quantum yields dramatically, enabling single chain folding at unrivaled high concentrations without subsequent purification. Importantly, the enhanced photoreactivity allows for single chain folding at = 445 nm LED-irradiation within minutes as well as via ambient light, enabling an unprecedented folding system. The herein demonstrated enhancement of quantum yields by steric confinement serves as a blueprint for all photochemical ligation systems.
Introduction Proteins are – along nucleic acids –a key class of polymeric biomolecules produced by living organisms. Their plethora of functions ranging from catalysis, signal transduction and transport to macroscopic movement allows a glimpse at the potential that lies beyond perfectly tailored macromolecules. These functions are encoded by the sequence of amino acids and the resulting 3D-folding, motivating polymer chemists to seek avenues to control sequence and folding of synthetic polymers. In the last decade, significant progress has been made to move beyond sequence-controlled polymers1 (low dispersity) merely consisting of block copolymer segments and to generate true sequence-defined polymers (dispersity of unity and exact placement of the monomers within the chain) comprising perfect primary structures.2,3 In contrast, the translation of primary chains into secondary and tertiary structures for the generation of well-defined macromolecular architectures on a 3D-level is still in its infancy.4 A promising synthetic approach towards bioinspired chain folding is the compaction of single polymer chains via intramolecular crosslinking of specific binding sites distributed along the polymer backbone.5–14 In order to access such single chain nanoparticles (SCNPs), a large toolbox of chemical reactions for chain compaction – ranging from non-covalent interactions15–17 to coordinative bonds18–21 to covalent crosslinks22,23 – has been established in recent years. Folding processes of synthetic polymers based on non-covalent bonds are, however, closest to the folding of proteins, as both are spontaneous processes governed by their
lowest energy state.24 The energy landscape defining the lowest energy state of a folded system is defined by its chemical and physical environment including solvent and temperature. On the contrary, the formation of covalent bonds classically requires addition of reagents, control of temperature and careful choice of solvent to tune the energy landscape towards the desired bond formation. Consequently, no synthetic macromolecular systems are known, where covalent bond formation proceeds during single chain folding as a spontaneous process as observed in nature. Contrary to thermally driven reaction control, light-induced ligations offer precise temporal and spatial control and can provide highly specific reaction channels gated by wavelength and intensity.25,26 Highly energetic UV light is, however, commonly required for light-induced covalent bond formation, which can cause photodamage and reduce bioorthogonality.27 As a result, photochemical chain collapse has thus far been limited by its triggering wavelengths.28,29 Employing a pyrene functionalized tetrazole, we have recently shifted the activation of a single chain collapse into the critical visible light regime using irradiation between = 410–420 nm, yet still requiring relatively high light intensities.30 While the choice of the employed chemistry allows to control the properties of induced folding points, the exact folding process of macromolecules remains largely unexplored. Since the specific folding of a protein dictates its activity and function, misfolding dramatically affects its function and is a cause for various diseases.24,31 Thus, to reach the potential displayed by natural analogs, the in-depth understanding of the folding process is critical, yet limited by current synthetic techniques. One of the key limitations besetting SCNP formation is the applicable concentration range. Typically, concentrations 90%, the calculated quantum yield of = 0.022 is still significantly lower than for the intramolecular dimerization within P1 – P3 (Fig. 2C). Thus, confinement of photoreactive groups within single polymer chains consequently leads to extremely high local concentrations of 255 mM (P1), 173 mM (P2) 60 mM (P3). As the quantum yield of P2 and P3 are in the same order of magnitude although both polymers display different local concentrations of the photoreactive moiety, the distinct confinement of the photoreactive groups within the macromolecules dictates the photoreactivity to a further extent than solely by increasing the effective concentration. This observation is important for the folding of SCNPs, since the intramolecular reaction has to compete with undesired intermolecular crosslinking. The strongly diffusion dependent intermolecular reaction is hence drastically less favored under the concentrations employed for the folding of P1 – P3 and only the intramolecular single chain collapse occurs (Fig. 3A). Within the confined environment of the macromolecular chain, the quantum yield of the [2+2] photocycloaddition is consequently drastically enhanced, thus establishing an unprecedented selectivity of the intrachain collapse over the intermolecular crosslinking (Fig. 3A). The achieved selectivity clearly allows to overcome the generally required ultra-dilute reaction conditions for the folding of single polymer chains. To investigate the maximum concentration at which the single chain collapse is the dominant reaction, the irradiation of P2 was conducted at increasing concentrations. To further demonstrate the practical feasibility of the styrylpyrene mediated single chain collapse, these experiments were carried out using a commercially available LED (3 W) emitting at max = 445 nm and without deoxygenation of the solutions, which is usually required for photocycloadditions to proceed efficiently.38,44 The SEC traces of P2 after irradiation including the sample at 15 mg mL-1 show a shift towards higher elution volumes, which is expected for the intramolecular single chain collapse (Fig. 3B). However, with increasing concentration the small shoulder at lower elution volumes, initially resulting from the synthesis, begins to increase, indicating intermolec-
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Journal of the American Chemical Society ular crosslinking. The occurrence of a more pronounced intermolecular crosslinking at higher concentrations can be explained by the overall increased concentration of styrylpyrene units in the reaction solution reaching at 15 mg mL-1 (10.5 mM styrylpyrene), the concentration regime at which the quantum yield for the dimerization in free solution becomes significant – although it is still lower than within P2 ( = 0.022 vs. 0.034). Moreover, in the case of P3 containing around 60% less photoreactive moieties than P2, it was even possible to induce single chain collapse at 25 mg mL-1 (S8). To the best of our knowledge, the exploitation of quantum yield effects in confined environments reported herein allows visible light activated styrylpyrene dimerization at the highest to date reported constant concentration for SCNP folding. Importantly, only by operating in the concentration regime between 1 and 5 mg mL-1 accessible through the here presented concept, folding processes of SCNPs can be studied comprehensively in real-time in-situ, since NMR and scattering techniques typically operate within this concentration regime. This marks important progress in the field of SCNPs, since even light-induced reactions in which photons are the only added reagent still require isolation after SCNP collapse to increase the concentration prior to analytical assessment.28– 30,45,46 In contrast, all analytical measurements including the DOSY NMR experiments presented in the current study were carried out without isolation after irradiation in the same solvent, i.e. THF. Thus, it was possible to investigate the SCNPs in its native state in solution avoiding effects from the subsequent change of solvents, precipitation, heating or effects resulting from concentration increases. While the quantum yield of the photoreaction was efficiently increased through the macromolecular confinement of the photoreactive moieties, its effect on the triggering wavelengths holds the potential to additionally increase the reactivity window into the critical visible light regime. To investigate the wavelength dependence of the styrylpyrene dimerization within single polymer chains, P2 was irradiated with a constant number of photons (2.06 mol photons within 60 s) at different monochromatic wavelengths using a tunable laser setup (Fig. 4). To compare the photodimerization within P2 with a dimerization in free solution, the wavelength dependent dimerization per photon was compared to our previous investigation of hydroxy-styrylpyrene (10 mM, 242 µmol photons).38 Even though the different concentrations of styrylpyrene moieties and number of photons prevent a quantitative comparison of both systems, it provides an excellent qualitative guideline for conducting the photoreactions. We note that the behavior of the obtained action plot does neither align with the UV/vis-absorption spectrum of the photoreactive moieties as also observed in earlier studies of us44,47 nor with the action plot in free hydroxy-styrylpyrene. While P2 and the dimerization in free solution both show that the longest possible wavelength to trigger the dimerization lies between 470 and 450 nm, they differ drastically at shorter wavelengths. In free solution, the dimerization has a distinct maximum reactivity at = 435 nm, strongly decreasing towards the shorter wavelength regime yielding almost no conversion below = 370 nm. However, the dimerization within the polymer proceeds with a significantly higher dimerization yield per photon, in agreement with the increased quantum yield close to = 430 nm. Moreover, the reactivity maximum
is shifted to = 410 nm and the dimerization is still highly efficient at lower wavelengths. Since in the shorter wavelength regime also the cycloreversion of the cyclobutane ring in the dimer is triggered, both reactions compete in the UV-regime. In free solution, the cleaved dimer can diffuse apart, while the cycloreversion within the formed SCNP is kinetically hindered to the point where both reaction rates are within the same order of magnitude yielding a wavelength dependent photostationary state.48 This is the first time that tuning of photoreactivity towards a broader activation window – solely through the confinement of the reactive units within a polymer coil – is observed across the electromagnetic spectrum. Consequently, the dimerization of styrylpyrene in a polymer coil is able to harvest a larger portion of the visible light realm with a higher efficacy than in free solution. Importantly, as the underlying [2 + 2] photocycloaddition is exclusively induced by light, the photoreactive polymers are stable over more than half a year if kept under exclusion of light at ambient temperature and atmosphere.
Figure 4. Action plot of the single chain collapse of P2 (1 mg mL-1 in THF) indicating styrylpyrene dimerization yield per incident photon upon irradiation with 2.06 μmol photons at different wavelengths (solid red dots) along with its initial UV/vis spectrum (black line). To compare the reactivity with styrylpyrene in free solution, the dimerization per photon as a function of the wavelength was calculated from a previous investigation (242 µmol photons, 10 mM hydroxy-styrylpyrene, d3-acetonitrile, hollow red dots).38 Note that the absolute concentration of styrylpyrene moieties was more than one order of magnitude higher in the free solution experiment (10 mM vs. 0.7 mM in P2). As the macromolecular confinement of the styrylpyrene units has shown a drastically increased photoreactivity towards longer wavelengths as well as quantum yields, the single chain collapse of P1 and P2 was investigated to explore the mildest possible compaction conditions. Thus, P1 and P2 were dissolved in THF at a concentration of 0.7 mM styrylpyrene in a glass vial, which was subsequently sealed to prevent the evaporation of the solvent. The reaction mixture was placed on a bench in the laboratory, which is not exposed to direct sunlight (624 Lux). After exposure to ambient light for 24 h without any direct sunlight, its SEC trace displayed the expected shift in elution volume resulting from a dimerization degree of 79% as obtained from the UV/vis spectrum (Fig. 5A, B, for P2 see Fig. S9, S10). Importantly, when a solution containing
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hydroxy-styrylpyrene at the same concentration of photoreactive groups (0.7 mM) was placed on the bench for 24 h, no formation of any cycloaddition products could be observed by 1H-NMR (Fig. S11). Thus, the confined environment allows for covalent bond formation to occur spontaneously under ambient conditions, while in its absence the reaction is fully suppressed. While classically the driving force of non-covalent interactions inducing self-assembly is depending on solvent and ambient temperature, the folding of the current styrylpyrene functionalized polymers is driven by the ambient light in the laboratory. When regarding the photonic field inside a laboratory as an integral part of the chemical and physical environment that determines the thermodynamic minimum on the energy landscape, the presented behavior of P1 and P2 can be described as self-folding driven by covalent interactions. The conditions required for the spontaneous folding of P1 and P2 are consequently as mild as for any SCNP formation based on non-covalent interactions – but critically resulting in covalent folding points.
Broadening of the triggering wavelengths into regimes, which are dominated by the cycloreversion in free solution. (ii) Increasing its quantum yield at = 430 nm from zero to (P3), (P2) and (P1). These findings provide a blueprint for highly efficient photoligations within macromolecular architectures, which was applied here to overcome four major challenges of folding single polymer chains: Concentration range, triggering wavelength, post folding isolation and reaction conditions, enabling a unique spontaneous covalent self-folding. The difference in quantum yield of the dimerization within the single polymer chains compared to styrylpyrene in free solution was exploited to carry out single chain folding at unrivaled high constant concentrations. Critically, the enhanced photoreactivity allowed for single polymer chain folding at max = 445 nm LED-irradiation within 15 min or under the mildest conditions using ambient light within 24 h. The presented system thus allows for an additive free single chain folding without additional reagents, catalysts, deoxygenation, high temperatures and subsequent isolation, yet directly in the concentration regime required for its analysis. Experiments to exploit the access to real-time insitu analysis of single chain folding processes is currently underway in our laboratories.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional experiments, experimental details, calculations and simulations, synthetic procedures (PDF).
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS C.B.-K. acknowledges funding from the Australian Research Council (ARC) in the form of a Laureate Fellowship enabling his photochemical research program as well as key support from the Queensland University of Technology (QUT). H. F. acknowledges the Leopoldina for a post-doctoral fellowship. Additional support by the German Research Council (DFG) funding D.M.’s PhD studies is gratefully acknowledged.
Figure 5. (A) Schematic representation of the covalent selffolding of P1 under ambient condition. (B) SEC trace of P1 after standing for 24 h on the bench of the laboratory or in the dark. (C) UV/vis spectra of the same samples of P1 and photograph of the reaction setup enabeling the covalent selffolding. Conclusions The macromolecular confinement of photoreactive styrylpyrene moieties within single polymer chains was investigated as a function of the wavelength and number of photons, allowing to affect the [2+2] photocycloaddition on two levels: (i)
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