Article pubs.acs.org/Langmuir
Fusogenic Metallosupramolecular Brush Vesicles Lipeng He, Shuai Bi, Hui Wang, Baochun Ma, Weisheng Liu, and Weifeng Bu* Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, and College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou City, Gansu Province, China S Supporting Information *
ABSTRACT: The electrostatic combination of a cationic metallosupramolecular polyelectrolyte (Fe-MSP) with sulfonate-terminated polymers leads to the formation of metallosupramolecular brushes (MSBs). The resulting MSBs can self-assemble into vesicular structures in chloroform/methanol (v/v = 1:1) mixture solvents. The rigid-rod Fe-MSP chain has to bend for the formation of the vesicles, which accompanies the presence of a lateral tension and thus induces a spontaneous vesicle fusion with an hour-scale fusion time. For this much longer fusion process, the arrow-like protrusion, stalk-like intermediate, and hemifusion diaphragm are clearly observed by transmission electron microscopy. The complete fusion into larger vesicles significantly releases the lateral tension.
■
INTRODUCTION Membrane fusion is a key process in living organisms, during which the transport of molecules occurs between and within cells.1 The fusion mechanism is usually believed to occur as follows: The supramolecular interactions between two membranes bring them into close proximity. Such close proximity induces the presence of local disruption in the bilayer structure and thus the formation of a stalk-like structure, where the outer membranes of the approaching lipid bilayers have merged, but not the inner leaflets. The stalk intermediate is further expanded to form a hemifusion diaphragm in which the two compartments are separated by only their inner leaflets. Finally, the inner leaflets are ruptured to generate a small pore, which then enlarges. As a result, the two initial lipid compartments become one, which facilitates the transport of molecules between membranes. Although this physical process has been proposed, the direct observations of the membrane fusion and fusion intermediates remain highly challenging. The main reasons include (1) the real complexity of the biorelated membranes and (2) the high speed of the fusion process. To fully understand the mechanism of the fusion, various vesicular models are developed to mimic the biomembranes and their fusion processes. The fusion processes are usually driven by the defect-sonication induction,2a metal ion coordination,2b host−guest recognition,2c,d hydrogen bonding,2e,f and chemical reaction.2g,h Among them, several examples show the real-time microscopic observations of the fusion process within a second scale.2a−d These works have offered important mechanistic insights into the membrane fusion of the highly complex biological system, which is also important in the fields of drug delivery and targeted therapy due to the transport of molecules between and within cells. Within these works in mind, we therefore wonder if we could obtain a vesicle fusion process with a much longer fusion time than that of liposomes, biomembranes, and branched polymersomes.2 In such a long © 2012 American Chemical Society
fusion process, the fusion intermediates may be directly observed by microscopic measurements. Molecular brushes refer to densely grafted copolymers with multiple side polymers grafted to a linear main polymer by covalent bonds.3 They are usually synthesized by controlled/ living ionic and radical polymerization techniques. The replacement of the covalent bonds with noncovalent interactions leads to the formation of supramolecular polymer brushes,4 which represents an efficient approach to developing dynamic polymer materials with controlled hierarchical nanostructures and functional properties. The side polymers can be grafted onto the backbones by host−guest recognition, forming sliding supramolecular polymer brushes with really tunable functionalities.4a,b Alternatively, polymers bearing shape-persistent macrocycles can self-organize into supramolecular brushes driven by solvophobic interactions.4c On the other hand, polyelectrolyte−surfactant complexes are formed by mixing charged polyelectrolyte backbones and oppositely charged surfactants, which combines the mechanical properties of polymers with highly ordered mesophases of surfactants.5 The electrostatic combination of charged terminated polymers together with oppositely multicharged species results in the formation of supramolecular star polymers.6 The polyoxometalate-based supramolecular star polymers (PSPs) with a Keplerate cluster core and a polystyrene shell can selfassemble into vesicles in organic solvents.6c These works prompt us to develop supramolecular polymer brushes from polyelectrolytes with opposite-charge terminated polymers. Herein, we fabricate metallosupramolecular brushes (MSBs) by the electrostatic connections of a cationic metallosupramolecular polyelectrolyte (Fe-MSP) with sulfonate-terminated Received: July 25, 2012 Revised: September 2, 2012 Published: September 13, 2012 14164
dx.doi.org/10.1021/la303008c | Langmuir 2012, 28, 14164−14171
Langmuir
Article
Scheme 1. Structural Formulas of MSB-1, MSB-2, and MSB-3
supported the complete ionic exchange of Fe-MSP with ST-S100. MSB1 was then isolated as a blue powder after removing the solvent under a reduced pressure. MSB-2 and MSB-3 were prepared by a similar method, but with ST-S162 and ST-S301, respectively. For MSB-1, Anal. Calcd for [(C8H8)100(CH2)3SO3]2(C36H24N6Fe)(CHCl3)5, 22 267.46: C, 88.84; H, 7.43; N, 0.38. Found: C, 88.57; H, 6.97; N, 0.22. UV−vis (CHCl3): λmax = 592 nm. IR (KBr, cm−1): 3088, 3058, 3024, 2920, 2846, 1941, 1872, 1802, 1744, 1600, 1581, 1492, 1450, 1370, 1314, 1218, 1180, 1067, 841, 754, 698. For MSB-2, Anal. Calcd for [(C8H8)162(CH2)3SO3]2(C36H24N6Fe)(CHCl3)9, 35 659.46: C, 89.02; H, 7.45; N, 0.24. Found: C, 88.78; H, 6.99; N, 0.18. UV−vis (CHCl3): λmax = 592 nm. IR (KBr, cm−1): 3088, 3059, 3025, 2920, 2846, 1942, 1870, 1803, 1744, 1600, 1580, 1492, 1450, 1369, 1314, 1219, 1180, 1067, 841, 755, 697. For MSB-3, Anal. Calcd for [(C 8H 8) 301 (CH2 )3 SO3] 2 (C36H 24 N6 Fe)(CHCl3) 24, 66 043.58: C, 88.30; H, 7.40; N, 0.13. Found: C, 88.24; H, 7.31; N, 0.19. UV−vis (CHCl3): λmax = 592 nm. IR (KBr, cm−1): 3089, 3058, 3024, 2920, 2848, 1940, 1874, 1798, 1749, 1600, 1580, 1492, 1450, 1369, 1314, 1218, 1180, 1068, 841, 754, 697. For the preparation of vesicular structures, MSBs cannot be dissolved directly in the mixture of chloroform and methanol containing 50% methanol. To obtain the vesicular aggregates, MSBs were dissolved first in chloroform, and then methanol was added. The final concentrations of MSBs in the solvent mixtures were controlled at 0.5 mg/mL.
polymers (Scheme 1). The resultant MSBs can self-assemble into vesicular aggregates, which further shows a spontaneous fusion in organic solvents with an hour-scale fusion time. For such a long fusion time, the arrow-like protrusion, stalk-like intermediate, and hemifusion diaphragm are clearly observed by transmission electron microscopy (TEM).
■
EXPERIMENTAL SECTION
Materials and Instruments. All air-sensitive reactions were carried out under an argon atmosphere. Iron(II) acetate (99.995%) was purchased from Sigma-Aldrich and used as received. The sulfonic acid-terminated poly(styrene)s (ST-Sn, n = 100, PDI = 1.12; n = 162, PDI = 1.07; n = 301, PDI = 1.05) were purchased from Polymer Source Inc. and used without further purification. Fe-MSP was prepared by a coordinative self-assembly of 1,4-bis(2,2′:6′,2′′terpyridine-4′-yl)benzene with iron(II) acetate according to previous procedures.7 1 H NMR spectra were recorded on a Bruker 400 MHz spectrometer. IR spectra (KBr) were measured with a Nicolet NEXUS 670 spectrometer. Scanning electron microscopy (SEM) measurements were performed on a field emission scanning electron microscope (JEOL JSM-6701F). TEM images were performed with a FEI Tecnai F30 operating at 300 kV. Dynamic light scattering (DLS) measurements were performed on a Brookhaven BI-200SM spectrometer. Elemental analyses were performed with an Elementar VarioELcube. UV−vis absorption spectra were recorded by using a SHIMADZU UV-2550 spectrophotometer. Scanning force microscopy (SFM) measurements were performed on a commercial Multimode AFM (Nanoscope IIIa, Veeco Instrument, Santa Barbara, CA) operated in tapping mode. All measurements were tested at room temperature if not addressed specifically. Surface pressure−area isotherms were recorded at a compression rate of 10 cm2/min by using a NIMA Langmuir trough. The samples were spread on the water surface from their chloroform solutions (0.5 mg/mL). The solutions (160 μL for MSB-1 and 200 μL for MSB-2 and MSB-3) were carefully added to the water surface in 2−5 μL increments using a microsyringe. Prior to each trial, the water surface was cleaned such that the measured surface pressure remained
