Letter pubs.acs.org/macroletters
Photochemically Induced Folding of Single Chain Polymer Nanoparticles in Water Carolin Heiler,†,‡ Janin T. Offenloch,†,‡ Eva Blasco,*,†,‡ and Christopher Barner-Kowollik*,†,‡,§ †
Preparative Macromolecular Chemistry, Institut für Technische Chemie und Polymerchemie, Karlsruhe Institute of Technology (KIT), Engesserstr. 18, 76131 Karlsruhe, Germany ‡ Institut für Biologische Grenzflächen, Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany § School of Chemistry, Physics and Mechanical Engineering, Queensland University of Technology, 2 George Street, Brisbane, QLD 4000, Australia S Supporting Information *
ABSTRACT: We pioneer the synthesis of fluorescent single chain nanoparticles (SCNPs) via UV-light induced folding based on tetrazole chemistry directly in pure water. Water-soluble photoreactive precursor polymers based on poly(acrylic acid) (PAA) bearing tetrazole, alkene and tetraethylene glycol monomethyl ether moieties, (PAAn(Tet/p-Mal/TEG)), or simply tetrazoles moieties, PAAn(Tet), were generated via RAFT polymerization. While tetrazole, ene, and acrylic acid containing polymers fold via dual nitrile imine-mediated tetrazole-ene cycloaddition (NITEC) as well as nitrile iminecarboxylic acid ligation (NICAL), tetrazole and acrylic acid only functional prepolymers fold exclusively via NICAL. A detailed study of the underpinning photochemistry of NITEC and NICAL is also included. The resulting water-soluble SCNPs were carefully characterized via analytical techniques such as NMR, UV−vis, and fluorescence spectroscopy, as well as SEC and DLS.
I
(NITEC). Tetrazoles can be activated via light and generate a highly reactive nitrile imine intermediate, which undergoes a 1,3-cycloaddition reaction with a range of activated and nonactivated double bonds.26 The NITEC reaction exhibits several advantages, such as the absence of metal catalysts and the generation of a fluorescence pyrazoline adduct, making it highly interesting for biological applications, for example, as imaging agents. Interestingly, the orthogonality of the NITEC reaction has been recently investigated and put into question.27 Li et al. have demonstrated that the highly reactive nitrile imine formed can undergo rapid nucleophilic reactions with a variety of nucleophiles present in a biological system, including carboxylic acids, along with the expected cycloaddition with alkenes. In the current study, we exploit the photochemistry of tetrazoles directly in water as a versatile tool for the formation of fluorescent water-soluble SCNPs. Poly(acrylic acid) (PAA) is well-known for its water super absorbent properties and biocompatibility, as well as nontoxicity.28 Thus, tetrazolefunctional polymers based on PAA were selected as suitable precursors for the formation of water-soluble SCNPs. Initially,
nspiration from Nature has been a key driving force in macromolecular chemistry. For example, proteins are complex naturally occurring biopolymers comprising a highly specific linear amino-acid sequence that defines their folding into a functional, three-dimensional structure. Polymer chemists aim at mimicking the precise folding of proteins via so-called single chain polymer nanoparticles (SCNPs) by a controlled intramolecular collapse of tailor-made linear precursor polymers.1−4 This intramolecular collapse relies on either dynamic,5,6 dynamic-covalent,7,8 or covalent9 interactions. Water-soluble SCNPs are especially interesting due to their potential applications in variable areas of biomedicine such as imaging,10 catalysis,11−13 or drug delivery.14−18 In most of the reported examples employing water-soluble SCNPs, the collapse is mainly based on noncovalent bonds such as intramolecular hydrogen bonds or coordinative reactions in water.12,19,20 Light-induced reactions are especially interesting, since photonic fields enable spatial and temporal control. UVinduced reactions based on nitrile imine,21 photoinduced Diels−Alder,9,22 and nitrene-mediated photoreactions,24 as well as photodimerization of anthracene23 and coumarin,25 have been successfully employed for the formation of SCNPs, albeit only in organic solvents. Specific attention has been paid to the nitrile imine-mediated tetrazole-ene cycloaddition © XXXX American Chemical Society
Received: November 11, 2016 Accepted: December 21, 2016
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DOI: 10.1021/acsmacrolett.6b00858 ACS Macro Lett. 2017, 6, 56−61
Letter
ACS Macro Letters
Scheme 1. Synthetic Pathways for the Preparation of the Photoreactive Polymers and Their Subsequent Folding in Watera
a
The fluorescence shown corresponds to an excitation wavelength of 366 nm.
tetrazole (Tet), furan-protected maleimide (p-Mal), and triethylene glycol monomethyl ether (TEG) containing a hydroxyl group. The TEG derivative was incorporated into the polymers in order to increase their biocompatibility. 1H NMR spectroscopy confirmed the successful functionalization of both polymers. Figure 1A depicts the NMR spectrum of PAA180(Tet/p-Mal/TEG) as an example. The exact composition was calculated by comparing the resonance integrals for the characteristic protons of the functional groups (Table 1, refer to Section 2.3 in the SI). It was found that for PAA180, 9% of the carboxylic acid groups were functionalized with Tet, 12% with p-Mal, and 26% with TEG units. For the shorter polymer, that is, PAA90(Tet/p-Mal/ TEG), 8% Tet, 14% p-Mal, and 23% TEG was calculated. Both
linear polymers based on PAA were synthesized via RAFT polymerization of acrylic acid using benzyl 2-(dodecylthiocarbono-thioylthio) propanoate as chain transfer agent and 1,4dioxane as solvent. In particular, three PAAs with variable chain length, that is, 180, 90, and 69 monomer units, termed PAA180, PAA90, and PAA69 were prepared. The molecular weight of the polymers was determined by 1H NMR spectroscopy (for further details refer to Section 2.2 in the Supporting Information) by comparing the resonances corresponding to the backbone and the phenyl containing end group. The obtained polymers were subsequently functionalized via Steglich esterification with the corresponding functional alcohol derivatives (Scheme 1). On the one hand, PAA180 and PAA90 were esterified with a 1:2.3:5.3 and 1:2.3:3.3 mixture of 57
DOI: 10.1021/acsmacrolett.6b00858 ACS Macro Lett. 2017, 6, 56−61
Letter
ACS Macro Letters
Figure 1. (A) 1H NMR spectrum of the PAA180(Tet/p-Mal/TEG) with the corresponding labels. (B) Zoom into the PAA180(Tet/p-Mal/TEG) and SCNP1 1H NMR spectrum showing key resonances at 4.93 and 4.80 ppm, indicating the formed pyrazoline product in the photoirradiated sample.
polymers contained unreacted carboxylic groups. As noted above, Li and co-workers recently showed that carboxylic acids can efficiently react with the photogenerated nitrile imines (i.e., nitrile imine-carboxylic acid ligation, NICAL).27 We herein explore NICAL as a photochemical method to generate SCNPs directly in water. Thus, one additional precursor polymer bearing only tetrazole and carboxylic acid as pendant groups was synthesized. For this purpose, PAA69 was partially esterified with the hydroxyl-containing derivative Tet to afford the target polymer PAA69(Tet). Its functionalization was successfully evidenced via NMR and the degree of functionalization was found to be 60% (refer to SI for further details, Figure S14). In a next step, the three prepared photoreactive polymers were analyzed via SEC. The precursor polymers, PAA180(Tet/p-Mal/ TEG) and PAA90(Tet/p-Mal/TEG) showed a narrow molecular weight distribution (Table 1). Unfortunately, due the insolubility of PAA69(Tet) in organic solvents, it was not
Table 1. SEC and DLS Data of the Different Polymer Precursors and the Resulting SCNPs polymer PAA180 (Tet/p-Mal/TEG) SCNP1 PAA90 (Tet/p-Mal/TEG) SCNP1a PAA69(Tet) SCNP2
Teta (%)
p-Mala (%)
Mpb (kDa)
ĐMb
Dhc (nm)
9
12
20
1.5
4.3
8
14
14 12
1.5 1.2
2.3 2.4
10
1.7
