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Sep 5, 2017 - The amphipathic nature of bile acids is manifested in their unusual self-assembly ability. For example, lithocholic acid (LCA) in alkali...
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The Formation of Spherulitic J-Aggregates from the Co-Assembly of Lithocholic Acid and Cyanine Dye Samuel Rhodes, Wenlang Liang, Ekaterina Shteinberg, and Jiyu Fang J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01943 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 6, 2017

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The Formation of Spherulitic J-Aggregates from the Co-Assembly of Lithocholic Acid and Cyanine Dye Samuel Rhodes, Wenlang Liang, Ekaterina Shteinberg, and Jiyu Fang* Advanced Materials Processing and Analysis Center and Department of Materials Science and Engineering, University of Central Florida, Orlando, Florida 32816. *Email: [email protected]

Abstract Supramolecular aggregates of organic dyes through noncovalent interactions have attracted great interest because they exhibit collective optical and excitonic properties. In this paper, we report the formation of spherulitic J-aggregates from the co-assembly of lithocholic acid

(LCA)

and

3,3′-diethylthiacarbocyanine

iodide

(DiSC2(3)) in ammonia solution. Each spherulite contains a core, which serves as a nucleus for the growth of radially oriented J-aggregate fibrils. We find that the growth of spherulitic J-aggregates exhibits a sigmoidal kinetic curve with an initial lag time, followed by a period of rapid growth and a finally slow approach to equilibrium.

Keywords: Bile acids, cyanine dyes, the co-assembly, J-aggregates, and spherulites.

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Supramolecular aggregates of organic dyes through noncovalent interactions have attracted great interest in chemistry and materials science because they exhibit collective optical and excitonic properties.1-2 Cyanine dyes with two aromatic nitrogen-containing heterocycles linked by a polymethine chain are an important class of cationic dyes, which are widely used in color photography, nonlinear optics, and photoelectric devices.3 The polarizability and hydrophobicity of cyanine dyes are main driving forces for the formation of supramolecular aggregates. Depending on the spatial stacking of dye molecules, two types of supramolecular aggregates can be formed.4-5 J-aggregates represent an edge-to-edge stacking of dye molecules, showing a redshifted absorption band with respect to the monomer band and a strong florescence emission. On the other hand, H-aggregates represents a face-to-face stacking of dye molecules, showing a blue-shifted absorption band compared with the monomer band and a strong emission quenching. The collective optical and excitonic properties of J- and H-aggregates can be explained by the Frenkel exciton theory.6 Despite the importance of J- and H-aggregates of cyanine dyes in diverse optoelectronic applications, the control of their morphologies remains a challenge. Recently, efforts have been made in the use of the self-assembly of amphiphilic cyanine dyes to form tubular J-aggregates.7-15 The co-assembly of amphiphilic cyanine dyes with surfactants has been used to alter the size and morphology of tubular J-aggregates and consequently their optical properties.16-18 Bile acids are an amphipathic biological surfactant with a rigid steroid skeleton.19 The concave side of the steroid skeleton of bile acids is hydrophilic, while the convex side is hydrophobic. The structure of bile acids is different from that of conventional

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surfactants, which often have a long hydrophobic tail and a hydrophilic polar head group. The amphipathic nature of bile acids is manifest in their unusual self-assembly ability. For example, lithocholic acid (LCA) in alkaline solution is able to self-assemble into well-ordered supramolecular aggregates including micelles, ribbons, and tubes, depending on the condition under which the self-assembly occurs.20-26 The co-assembly of bile acids and other molecules has been proven to be an effective way to form complex supramolecular aggregates.27-31 Herein, we report the formation of spherulitic J-aggregates from the co-assembly of LCA with 3,3′diethylthiacarbocyanine iodide (DiSC2(3)) in ammonia solution. The structure and optical properties of spherulitic J-aggregates are studied at different mixed molar ratios of LCA and DiSC2(3). We show that each spherulite contains a core, which serves as a nucleus for the growth of radially oriented J-aggregate fibrils. The growth kinetics of spherulitic J-aggregates are studied with absorption spectroscopy and optical microscopy.

The chemical structure of 3′-diethylthiacarbocyanine iodide (DiSC2(3)) and lithocholic acid (LCA) used here is shown in Figures 1a-1b. We find that LCA is able to induce the J-aggregation of DiSC2(3) in 1% w/w ammonia solution. As can be seen in the absorption spectra shown in Figure 1c, DiSC2(3) in 1% w/w ammonia solution shows a strong band at 557 nm, which agrees with the monomer absorption band (556 nm) reported in the literature.32 The addition of 1 mM LCA in DiSC2(3) solution at the mixed molar ratio of 5:1 and 2:1 results in the significant decrease and the slight shift of the monomer band. Interesting, a new strong band at 590 nm appears, which is red-shifted with respect to the monomer band. The red-shifted band at 590 nm is a result of the J-aggregation of DiSC2(3) through the interaction with LCA. At the mixed molar ratio of 2:1, a weak H-band at 459 nm is also observed, suggesting that a small fraction of DiSC2(3) forms H-aggregates. The weak H-band disappears at the mixed molar ratio of 5:1. 3 ACS Paragon Plus Environment

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When the mixed molar ratio is reduced to 1:1, there are still a large fraction of DiSC2(3) presenting as monomers, which is evident from the strong monomer band (Figure S1). Our results suggest the higher mixed molar ratio of LCA and DiSC2(3) is required to form Jaggregates. Interestingly, the absorption spectrum of DiSC2(3) in 1% w/w ammonia solution remains unchanged after the addition of either deoxycholic acid (DCA) or cholic acid (CA) with the concentration below and above their critical micelle concentrations (Figure S2), suggesting that they are unable to induce the J-aggregation of DiSC2(3). CA, DCA, and LCA in 1% w/w ammonia solution with pH 11.3 are ionized due to the deprotonation of the carboxyl group. These bile acids differ from each other in the number and position of hydroxyl groups on their steroid skeleton. It has been shown that the order of the hydrophobicity is LCA > DCA > CA.33 Thus, we conclude that the hydrophobic interaction of LCA and DiSC2(3) plays a major role in the formation of J-aggregates. Optical microscopy images reveal that the J-aggregates from the co-assembly of LCA and DiSC2(3) have a spherulitic morphology (Figure 1d). Each spherulite contains a core surrounded by radially oriented fibrils with pink color, which is the evidence of the presence of DiSC2(3) in the fibrils. The absorption spectra shown in Figure 1c suggest that DiSC2(3) in the fibrils forms J-aggregates. The spherulitic J-aggregates exhibit a characteristic Maltese-cross extinction pattern of light extinction when being viewed under a polarizing optical microscope (Figure 1e). However, the core of the spherulites appears to be non-birefringent, suggesting that it does not have a long range ordered molecular arrangement. The structure of J-aggregate spherulites is further examined with a scanning electron microscope (SEM). Although SEM images show that the spherulites collapse after being dried on substrates (Figure 1f), individual J-aggregate fibrils can be still resolved. They either stick out or lay down at the surface of the collapsed spherulites. 4 ACS Paragon Plus Environment

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The diameter of the J-aggregate fibrils is measured in the range from 200 nm to 300 nm. Lowresolution optical microscopy and SEM images reveal the presence of many spherulitic aggregates in LCA/DiSC2(3) solution (Figure S3). Occasionally, we can see the core in the SEM image of collapsed spherulitic aggregates (Figure S4). The surface of the core appears to be featureless. In general, spherulites can be divided into two categories based on nucleation mechanisms.34 Category I spherulites are a result of heterogeneous nucleation, in which fibers grow outward radially from a nucleation site and branch intermittently to maintain a space filling character. In contrast, Category II spherulites are a result of homogeneous nucleation, in which needles first form in solution and then subsequently split at their tips. Clearly, the spherulitic J-aggregates from the co-assembly of LCA and DiSC2(3) resemble Category I spherulites. The representative optical microscopy images inset in Figure 2 show the growth of J-aggregate fibrils from the core over time. The formation mechanism of spherulitic J-aggregates is similar to that of spherulitic amyloid fibrils.35-36 In addition, the representative optical microscopy images also show that the core gradually disassociates by changing its color over a long incubation time. The radius of Jaggregate spherulities formed in LCA/DiSC2(3) solution with the mixed molar ratio of 2:1 increases from ~ 5.0 µm to ~ 34 µm as the incubation time increases from 1 hour to 48 hours, beyond that it remains almost unchanged (Figure 2). It is also evident in Figure 2 that the radius of spherulitic J-aggregates formed at the mixed molar ratio of 2:1 is slightly larger than that formed at the mixed molar ratio of 5:1 for the same incubation time. We note that spherulitic Jaggregates formed at the mixed molar ratio of 5:1 has a slightly smaller core, compared to the spherulitic J-aggregates formed at the mixed molar ratio of 2:1. In addition, the number of

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spherulitic J-aggregates decreases with the increase of the mixed molar ratio. When the mixed molar ratio increases to 10:1, there is a very few J-aggregate spherulites observed. Spherulitic J-aggregates can be disrupted by sonicating spherulite solution in an ultrasonic bath (Figures 3a-3b). The J-aggregate fibrils broken from the spherulites were examined with atomic force microscopy (AFM) and transmission electron microscopy (TEM). Both individual J-aggregate fibrils and branched fibrils are observed in the AFM image shown in Figure 3c. The J-aggregate fibrils have a smooth surface with the tip at their ends. After the sonication, the intensity of the J-band at 590 nm slightly decreases and a weak H-band at 453 nm appears (Figure S5), suggesting that a small fraction of DiSC2(3) transits into H-aggregates in the fibrils. We would like to point out that 1 mM LCA is able to self-assemble into nanotubes with the diameter of ~ 70 nm in 1% w/w ammonia solution only after 1 week incubation (Figure 3d), in which helical ribbons serve as a precursor of LCA nanotubes. The closing of the gap of helical ribbons leads to the formation of nanotubes. However, there is no hollow structure observed in the TEM image of J-aggregate fibrils (Figure 3e). The formation mechanism of J-aggregate fibrils is different than that of LCA nanotubes. Figure 4a shows that the time-dependent adsorption spectra of LCA/DiSC2(3) solution with the mixed molar ratio of 2:1. At the early stage after the addition of LCA in DiSC2(3) solution, a broad absorption band with a maximum at 459 nm is observed, suggesting the formation of Haggregates. The intensity of the less well-defined H-band decreases over time. At the same time, a well-defined J-band at 590 nm appears and its intensity increases over time. There is an isosbestic point in the time-dependent absorption spectra, i.e., the absorbance at 530 nm is not changed during the growth of J-aggregates and the decay of H-aggregates. The isosbestic point represents the equilibrium between J-aggregates and H-aggregates. Figure 4b is the plot of the 6 ACS Paragon Plus Environment

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intensity of the J-band as a function of incubation time, showing a sigmoidal shape with an initial lag time, followed by a period of rapid growth and a finally slow approach to the equilibrium position. The sigmoidal growth kinetics of J-aggregate fibrils resembles to that of amyloid β peptide fibrils.37 Bile acids are known to form either primary or secondary micelles in aqueous solution as their concentrations approach to critical micelle concentration (CMC)38. Primary micelles contain a small aggregation number (2-10) formed through the hydrophobic interaction of bile acids, while secondary micelles are formed via the hydrogen bonding of primary micelles. It has been reported recently that hydrophobic molecules can be incorporated in the hydrophobic cavity of bile acid micelles.39-40 In our experiments, the concentration of LCA is 1 mM, which is close to its CMC.41 Thus, it is reasonable for us to assume that DiSC2(3) molecules are incorporated in the hydrophobic cavity of LCA micelles through hydrophobic interaction. To verify this hypothesis, we carried out the co-assembly of LCA and DiSC2(3) in 1% w/w ammonia solution with the mixed molar ratio of 5:1, where LCA concentration was 0.5 mM, which is lower than its CMC. In this case, a broad absorption band with a maximum at 459 nm is observed (Figure S6). The intensity of the less well-defined H-band at 459 nm remains unchanged even after 24 hour incubation. The intense J-band at 590 nm, which appears at the LCA concentration of 1 mM, is not observed at the LCA concentration of 0.5 mM. The length of LCA molecules from its hydroxyl to its carboxyl group (∼1.5 nm) is slightly longer than the length of DiSC2(3) molecules (~ 1.4 nm). Thus, the hydrophobic cavity of LCA micelles should be able to accommodate DiSC2(3) molecules. The formation of the intense Jband at 590 nm suggests that the DiSC2(3) molecules incorporated in the hydrophobic cavity of

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LCA micelles form J-aggregates. The number of coherently coupled molecules (Ncoh) in Jaggregates can be estimated from their absorption spectra according to the equation:18 N coh =

3(∆vmon ) 2 −1 2(∆vJ ) 2

Where ∆νmon and ∆νJ are the full widths at the half maxima of monomer and J-aggregate bands, respectively. Based on the adsorption spectra shown in Figure 1c, we estimate that there are four or five DiSC2(3) molecules incorporated in the hydrophobic cavity of LCA micelles to form Jaggregates (Figure 4c). It has been shown that the carboxylate ion of a LCA engages itself in hydrogen bond with the hydroxyl group of another LCA.22 The linear aggregation of LCA vesicles through hydrogen binding has been reported in the literature.23 Thus, we propose that the growth of J-aggregate fibrils is likely a result of the linear aggregation of J-aggregate incorporated LCA micelles through the hydrogen binding (Figure 4b). In conclusion, we report the spherulitic J-aggregates formed by the co-assembly of LCA and DiSC2(3) in ammonia solution. Mmicroscopy studies show that the spherulities contain a core, which serves as a nucleus for the growth of J-aggregate fibrils. Spectroscopy studies reveal that the growth of spherulitic J-aggregates follows a sigmoidal kinetic curve with an initial lag time, followed by a period of rapid growth and a finally slow approach to the equilibrium. On the basis of spectroscopy measurements, we estimate that there are four or five DiSC2(3) molecules in LCA micelles to form J-aggregates through hydrophobic interaction. The linear aggregation of J-aggregate incorporated LCA micelles through the hydrogen binding leads to the growth of Jaggregate fibrils.

Experimental Methods

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Bile acids used on our experiments are cholic acid (CA), deoxycholic acid (DCA) and lithocholic acid (LCA) purchased from Sigma-Aldrich. 3,3′-diethylthiacarbocyanine iodide (DiSC2(3)) was from Molecular Probes. All chemicals were used without further purification. Ammonia solution was from Sigma-Aldrich. Water used in the experiments was purified with Easypure II system (18 MΩ cm, pH 5.7). Holey Formvar filmed grids were from SPI.

The co-assembly of LCA and DiSC2(3) at different molar ratios was carried out in 1% w/w ammonia solution, in which the concentration of LCA was always kept at 1 mM. The mixture was sonicated in an ultrasonic bath (Branson 1510, Branson Ultrasonics Co.) at ~ 60°C for 5 min and then allowed to cool to room temperature. The structure and morphology of supramolecular aggregates from the co-assembly were characterized with an optical microscope (Olympus BX), a scanning electron microscope (SEM, Hitachi S3500N), a transmission electron microscope (TEM, FEW Technai F30), and an atomic force microscope (AFM, Dimension 3100 Veeco Instruments). The optical property of supramolecular aggregates was characterized with a Cary 300 UV-Vis spectrophotometer. For SEM and TEM measurements, aggregate solution was dried on a holey Formvar filmed grid at room temperature for 24 hours and then imaged at an accelerating voltage of 20 kV and 100 kV, respectively. AFM images were taken with a silicon nitride cantilever (Nanosensors) with a spring constant of 30 N/m and a resonant frequency of 260 kHz in tapping mode. For optical microscope observations, a drop of aggregate solution was placed on a glass substrate, followed by placing a cover glass slide on the top of the drop.

Acknowledgements This work was supported by US National Science Foundation (CBET-1264355). 9 ACS Paragon Plus Environment

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31. Liang, W.; He, S.; Fang, J. Y. Self-Assembly of J-Aggregate Nanotubes and Their Applications for Sensing Dopamine. Langmuir 2014, 30, 805–811. 32. Wang, M.; Holmes-Davis, R.; Rafinski, Z.; Jedrzejewska, B.; Choi, K. Y.; Zwick, M.; Bupp, C.; Izmailov, A.; Paczkowski, J.; Warner, B.; Koshinsky, H. Accelerated Photobleaching of a Cyanine Dye in the Presence of a Ternary Target DNA, PNA Probe, Dye Catalytic Complex: A Molecular Diagnostic. Anal. Chem. 2009, 81, 2043–2052. 33. Griffiths, W. J.; Sjovall, J. Bile Acids: analysis in Biological Fluids and Tissues. J. Lipid Res. 2010, 51, 23–41. 34. Shtukenberg, A. G.; Punin, Y. O.; Gunn, E.; Kahr, B. Spherulites. Chem. Rev. 2012, 112, 1805–1838. 35. Krobs, M. R. H.; MacPhee, C. E.; Miller, A. F.; Dunlop, I. E.; Dobson, C. M.; Donald, A. M. The formation of Spherulites by Amyloid Fibrils of Bovine Insulin. Proc. Natl. Acad, Sci. U.S.A., 2004, 101, 14420–14424. 36. Krebs, M. R. H.; Bromley, E. H. C.; Rogers, S. S.; Donald. A. M. The Mechanism of Amyloid Spherulite Formation by Bovine Insulin. Biophys J. 2005, 88, 2013–2021. 37. Eden, K.; Morris, R.; Gillam, J.; MacPhee, C. E.; Allen, R. J. Competition between Primary Nucleation and Autocatalysis in Amyloid Fibril Self-Assembly. Biophy. J. 2015, 108, 632– 643. 38. O'Connor. J.; Wallace, R. G. Physico-Chemical Behavior of Bile Salts. Adv. Colloid Interface Sci. 1985, 22, 1–111. 39. Li, G.; McGown, L. B. A New Approach to Polydispersity Studies of Sodium Taurocholate and Sodium Taurodeoxycholate Aggregates using Dynamic Fluorescence Anisotropy. J. Phys. Chem. 1993, 97, 6745–6752.

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40. Ju, C.; Bohne, C. Dynamics of Probe Complexation to Bile Salt Aggregates. J. Phys. Chem. 1996, 100, 3847–3854. 41. Hofmann, A. F.; Rods, A. Physicochemical Properties of Bile Acids and Their Relationship to Biological Properties: An Overview of the Problem. J. Lipid Res. 1984, 25, 1477–1489.

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Figure Captions

Figure 1: Chemical structure of 3,3′-diethylthiacarbocyanine iodide (DiSC2(3)) (a) and lithocholic acid (LCA) (b). (c) Adsorption spectra of LCA/DiSC2(3) solution with the mixed ratio of 2:1 and 5:1 after 24 hour incubation at room temperature, together with the adsorption spectrum of 0.5 mM DiSC2(3) in 1% w/w ammonia solution. (d) Optical microscopy, (e) polarizing optical microscopy, and (f) SEM images of spherulitic J-aggregates formed in LCA/DiSC2(3) solution with the mixed ratio of 2:1 after 24 hour incubation. The concentration of LCA is 1 mM.

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Figure 2: Growth kinetics of J-aggregate spherulites formed in LCA/DiSC2(3) solution with the mixed molar ratio of 2:1 and 5:1. The representative optical microscopy images of J-aggregate spherulites formed at the mixed molar ratio of 2:1 are inset in Figure 2. The concentration of LCA is 1 mM.

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Figure 3: Optical microscopy images of broken spherulitic J-aggregates by 1 min (a) and 5 min (b) sonication. (c) AFM image of J-aggregate fibrils from broken spherulitic J-aggregates by 10 min sonication. TEM images of Jaggregate fibrils (d) and LCA nanotubes (e). The spherulitic J-aggregates were formed in LCA/DiSC2(3) solution with the mixed ratio of 2:1. The concentration of LCA is 1 mM.

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Figure 4: (a) Time-dependent adsorption spectra of LCA/DiSC2(3) solution with the mixed ratio of 2:1. (b) Intensity of J-band at 590 nm as a function of incubation time. The concentration of LCA is 1 mM. Schematic representation of a LCA micelle with the J-aggregates of DiSC2(3) molecules in its hydrophobic cavity (c) and autocatalytic aggregation of J-aggregate incorporated LCA micelles into Jaggregate fibrils (d).

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