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Bioinspired Supramolecular Nanosheets of Zinc Chlorophyll www.nature.com/scientificreports OPEN Bioinspired supramolecular nanosheets of zinc chlorophyll assemblies Received: 9 April 2019 Sunao Shoji 1,3, Tetsuya Ogawa2, Shogo Matsubara1 & Hitoshi Tamiaki 1 Accepted: 5 September 2019 Two-dimensional sheet-like supramolecules have attracted much attention from the viewpoints of their Published: xx xx xxxx potential application as functional (nano)materials due to unique physical and chemical properties. One of the supramolecular sheet-like nanostructures in nature is visible in the self-assemblies of bacteriochlorophyll-c–f pigments inside chlorosomes, which are major components in the antenna systems of photosynthetic green bacteria. Herein, we report artifcial chlorosomal supramolecular nanosheets prepared by the self-assembly of a synthetic zinc 31-methoxy-chlorophyll derivative having amide and urea groups in the substituent at the 17-position. The semi-synthetic zinc chlorophyll derivative kinetically formed dimeric species and transformed into more thermodynamically stable chlorosomal J-aggregates in the solid state. The kinetically and thermodynamically formed self- assemblies had particle-like and sheet-like supramolecular nanostructures, respectively. The resulting nanosheets of biomimetic chlorosomal J-aggregates had fat surfaces and well-ordered supramolecular structures. The artifcial sheet-like nanomaterial mimicking chlorosomal bacteriochlorophyll-c–f J-aggregates was frst constructed by the model molecule, and is potentially useful for various applications including artifcial light-harvesting antennas and photosyntheses. Two-dimensional sheet-like nanostructures have been made by organic and/or inorganic materials, for exam- ple, graphene, supramolecules, polymers, covalent organic frameworks, metal organic frameworks, and hexag- onal boron nitride, and interested in wide felds such as electronics, optoelectronics, catalysts, energy storage, energy generation, sensors, separation, and biomedicines due to their unique physical and chemical proper- ties1–3. Supramolecular assembling systems are progressive strategy for preparing well-ordered nanostructures4–6. In nature, one of the supramolecular sheet-like nanostructures have been observed in major light-harvesting antenna systems of photosynthetic green bacteria, called chlorosomes7–9. Chlorosomes are egg-like, ellip- soidal, extramembranous antenna apparatuses (Fig. 1a), and one of the most efcient photofunctional nan- odevices in nature, which have attracted attention from the viewpoints of their functions with photophysical properties, supramolecular structures, and nanostructures10–13. A single chlorosome contains a large amount of bacteriochlorophyll-c, d, e, and f pigments (Fig. 1b), magnesium complexes of 31-hydroxy-131-oxo-chlorin, which are surrounded by lipid monolayers and form J-aggregates in hydrophobic environments without any protein assistance14,15. Teir self-assembly is mainly organized by coordination bonding (31-O∙∙∙Mg), hydrogen bonding (31-O–H∙∙∙O=C-13), and π–π stacking of chlorin skeletons16,17. Teir supramolecular (nano)structures are still in discussion, but suggested to be rods, tubes, and/or lamellar sheets. Model compounds of such chlorosomal bacteriochlorophyll-c–f pigments have been synthesized from natu- rally occurring chlorophyll-a18,19. Teir rod-, tube-, and/or lamellar-shaped supramolecular nanostructures have been prepared using model molecules in solution and solid states as well as on substrates (Fig. 1c)20–24. Recently, we have reported that a zinc 31-hydroxy-131-oxo-chlorin having amide and urea groups in the 17-substituent self-assembled to form rod-like supramolecular nanostructures with a spacing of 1.4 nm25. In this study, we syn- thesized zinc 31-methoxy-chlorophyll derivatives lacking the 31-hydroxy group, which was requisite for chloroso- mal J-aggregation through specifc hydrogen bonding (31-O–H∙∙∙O=C-13). Te zinc 31-methoxy-131-oxo-chlorin having amide and urea groups in the 17-substituent gradually formed chlorosomal J-aggregates in the solid state, and the supramolecular nanostructures of the resulting solids were mainly nanosheets that mimic the natural 1Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga, 525-8577, Japan. 2Institute for Chemical Research, Kyoto University, Uji, Kyoto, 611-0011, Japan. 3Present address: Division of Applied Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido, 060-8628, Japan. Correspondence and requests for materials should be addressed to S.S. (email: [email protected]) or H.T. (email: [email protected]) SCIENTIFIC REPORTS | (2019) 9:14006 | https://doi.org/10.1038/s41598-019-50026-1 1 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 1. Natural and artifcial chlorosomal systems. (a) Schematic of photosynthetic apparatuses in photosynthetic green bacteria. (b) Molecular structures of bacteriochlorophyll-c–f molecules. (c) Synthetic chlorophyll derivatives reported as models of bacteriochlorophyll-d. Figure 2. Supramolecular assembly of zinc 31-methoxy-chlorophyll derivatives. (a) Molecular structures of Zn- 1 and Zn-2. (b) UV-Vis-NIR absorption and (c) CD spectra of Zn-2 (10 μM) in THF (black), and THF/hexane (1:99, vol/vol) (blue). (d) UV-Vis-NIR absorption and (e) CD spectra of Zn-2 solids prepared from THF/hexane (5:95, vol/vol) (100 μM) just afer preparation (blue) and afer standing in solution for 1 week (red). bacteriochlorophyll-c–f supramolecules in chlorosomes. Tis is the frst report of biomimetic supramolecular nanosheets of chlorosomal J-aggregates being prepared from a synthetic zinc chlorophyll derivative. Results and Discussion Zinc chlorophyll derivatives Zn-1 and Zn-2 (Fig. 2a) were synthesized from methyl 3-devinyl-3- methoxymethyl-pyropheophorbide-a, which was prepared from naturally occurring chlorophyll-a extracted from commercially available Spirulina powders according to reported procedures (see synthetic details and spec- tral data in Supporting Information). Te Zn-1 and Zn-2 molecules have ester and amide/urea groups, respec- tively, in the substituents at the 17-position, which are designed as without and with hydrogen bonding moieties. Te UV-Vis absorption spectra of Zn-1 and Zn-2 in tetrahydrofuran (THF) at a concentration of 10 μM showed their monomeric states with Qy/Soret maxima at 649/426 nm (Fig. 2b, black line). Te self-assembly of the zinc SCIENTIFIC REPORTS | (2019) 9:14006 | https://doi.org/10.1038/s41598-019-50026-1 2 www.nature.com/scientificreports/ www.nature.com/scientificreports Figure 3. Supramolecular nanosheets of Zn-2 self-assemblies formed upon standing in THF/hexane (5:95, vol/ vol) for one week. Tapping-mode AFM analysis of Zn-2 J-aggregates on an HOPG substrate, (a) AFM height image and (b) cross-section analysis. Cryo-TEM analysis of Zn-2 on a carbon-coated copper grid, (c) and (d) cryo-TEM images and (e) electron beam difraction. 31-methoxy-chlorins in a hydrophobic hexane-based solution was examined. A solution of the zinc chlorin in THF was added to 99-times the volume of hexane (total concentration: 10 μM). Te UV-Vis-NIR absorption spectrum of Zn-2 exhibited red-shifed Qy/Soret maxima at 667/439 nm (Fig. 2b, blue line), which are ascribable to dimeric species, while Zn-1 gave only a monomeric state similar to its THF solution. Additionally, excitonically coupled circular dichroism (CD) signals were observed in the Qy region for the hexane-based solution of Zn-2 (Fig. 2c, blue), indicating that the molecular dipole moments of the chlorin π-systems were aligned along the y-axis. Tese results indicate that Zn-2 primarily form the dimeric species by intermolecular hydrogen bonding of its amide and urea groups in the 17-substituent, which readily induce stacking of the chlorin π-systems as well as coordination of the 31-O and central zinc in a dimer. At a 10-fold higher concentration (100 μM) in THF/hexane (5:95, vol/vol), Zn-2 was precipitated immedi- ately, while Zn-1 was dissolved in the solution as the monomeric state (Fig. S1). Te resulting Zn-2 solids just afer preparation were adsorbed on a quartz substrate. Te UV-Vis-NIR absorption spectrum exhibited Qy/Soret maxima at 667/437 nm (Fig. 2d, blue line), which was similar to that diluted in the hexane-based solution (Fig. 2b, blue line). Afer standing the precipitates in the above mixed solvent for 1 week in the dark at room temperature, the color of the Zn-2 solids was changed to dark green from blue-green, while Zn-1 was still dissolved in the mixed solvent to give no color change (Fig. S1). Te resulting dark green solids were adsorbed onto a quartz substrate. Te UV-Vis-NIR absorption spectrum showed further red-shifed Qy/Soret maxima at 728/439 nm (Fig. 2d, red line), and the excitonically coupled CD signals in the Qy region were enhanced (Fig. 2e, red line). Tese spectroscopic data indicate that the Zn-2 solids were gradually transformed from dimeric or small oligo- meric species into chlorosomal J-aggregates, in which the molecular dipole moments of the Zn-2 molecules were arranged along the y-axis in a well-ordered manner. Tus, Zn-2 molecules form dimeric species kinetically and transform into more thermodynamically stable J-aggregates in the solid states. Te resulting Zn-2 chlorosomal J-aggregates on a quartz substrate exhibited a 752-nm fuorescence emission peak at the excitation of the Soret band (Fig. S2). Te Zn-2 solids produced in THF/hexane (5:95, vol/vol) afer standing for one week were dispersed by ultra- sonication and the resulting suspension was drop-cast onto a substrate to analyze their supramolecular nanostruc- tures using tapping-mode atomic force microscopy (AFM). Te AFM height image of the Zn-2 self-assemblies on a highly oriented pyrolytic graphite (HOPG) substrate showed sheet-like supramolecular nanostructures with heights of 15–25 nm (Figs. 3a and S3a). Te cross-sectional analysis showed that the surface of the nanosheet was fat over a submicrometer range (Figs. 3b and S3b–e). Te nanosheet had a step of 6 nm (Fig. 3b), suggesting a cleavage of a single layer of the nanosheet. Te Zn-2 solids produced in THF/hexane (5:95, vol/vol) just afer preparation were also examined using tapping-mode AFM.
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