Photoshrinkage in Polysaccharide Gels with Trivalent Metal Ions

2 Nov 2012 - Department of Chemical Materials, Daito Kasei Kogyo Co. Ltd., Asahi-ku ... (MAX-303, Xenon Light Source 300W, Asahi Spectra) was used for...
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Photoshrinkage in Polysaccharide Gels with Trivalent Metal Ions. Maiko K. Okajima,† Quyen Thi le Nguyen,† Seiji Tateyama,† Hideaki Masuyama,† Takumi Tanaka,‡ Tetsu Mitsumata,§ and Tatsuo Kaneko*,† †

School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan Department of Chemical Materials, Daito Kasei Kogyo Co. Ltd., Asahi-ku, Osaka, 535-0005, Japan § Department of Polymer Science and Engineering, Graduate School of Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa, 992-8510, Japan ‡

ABSTRACT: The giant anionic polysaccharide “sacran”, which is composed of 6deoxyhexoses, pentoses, uronic acids as well as hexoses, showed hydrophobization and insolubilization phenomena in response to ultraviolet light irradiation. The sacran solution became turbid, and microparticles were formed by photoirradiation. To visualize the results of this photoreaction, anionic polysaccharide gels cross-linked by metal cations were used. As a result, we observed that sacrangels with trivalent metal ions gradually contracted depending on the photoirradiation energy. In contrast, alginate gels used as a comparison degraded instead of contracting. This photoshrinkage of the sacran gels may be attributed to the hydrophobization of uronic acid based on photodecarboxylation. We propose that sacran-metal ion gels can function as effective, photoresponsive gels.

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strated. This type of research may lead to fields of soft materials and photoresponsive gels using polysaccharides.

INTRODUCTION

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Stimuli-responsive gels have attracted researchers’ attention in the field of smart materials. In particular, photoresponsive materials have advantages in remote controllability by light, aiming toward the development of photocontrolled-release biorelated materials that are preferably composed of biocompatible molecules such as polysaccharides. The photoresponsive polysaccharide gels were conventionally prepared by the incorporation of photoreactive moieties.1−3 In particular, uronic acid is a kind of ultraviolet absorbable sugar that was contained in various polysaccharides such as glycosaminoglycans (GAGs); then, it has been reported that the decarboxylation of uronic acid occurred due to photoirradiation.4 The photoirradiation to GAGs containing uronic acid causes their degradation accompanied with the reduction of the molecular weight and viscosity.5−8 It is difficult to prepare self-supporting hydrogel materials of simple GAGs in vitro because they do not have a sufficiently high molecular weight and the formation of a 3D network is difficult. Therefore, there have been no reports on the photoresponsiveness and reactions of polysaccharides containing uronic acid in a gel state. The cyanobacterial polysaccharide sacran9 (structure: Figure 1a) contains uronic acid (22% per sugar residue) and is a supergiant polyanion with a molecular weight over 1.6 × 107 g/mol.10 In addition, it was found that sacran can easily form gels with trivalent metal ions by just mixing them together.11 Here the optical responsiveness of the metal-complexed gels of polysaccharides with uronic acid was investigated using sacran as well as alginate, which is known for acidic sugarpossessing uronic acids in all sugar residues as comparison. As a result, the novel phenomenon of a volume change in polysaccharide gels mediated by UV irradiation was demon© 2012 American Chemical Society

EXPERIMENTAL SECTION

Materials. Sacran extracted from Aphanothece sacrum was purchased from GSM (Green Science Materials), Japan. Sodium alginate (500−600 cP) was purchased from Wako, Japan. Preparation of Gel Beads. Sodium alginate and sacran at concentrations of 1 wt % were dropped into an europium chloride solution (0.01 M) using a micropipet (200 μL) to make gel beads with diameters of 5 ± 1 mm, respectively. After the gel beads were incubated in the europium chloride solution for 8 h at room temperature, they were placed in pure water for 24 h, and the pure water was exchanged three times until the amount of Eu3+ was achieved an equilibrium both inside and outside of the gel bead. UV Irradiation to Gel Beads. A high-pressure mercury lamp (MAX-303, Xenon Light Source 300W, Asahi Spectra) was used for irradiation of the gel beads. The gel bead was fixed in a bottom of a glass tube filled with pure water to prevent vaporing of the water from gel, and UV light (wavelength range: 250−450 nm, irradiation intensity: 1.81 mW/cm2) was irradiated onto the beads. The distance between the UV lamp and the gel bead was 6 cm. No specific treatment for screening the air was made. The weight change rate of the gel bead before and after UV irradiation was calculated as follows: weight change rate = (W /Wo) × 100%

(1)

where W is the weight of the gel bead after irradiation and Wo is the initial weight of the gel bead. Particles Size Measurement. The size of small particles appearing from sacran solution by UV-irradiation was estimated as follows. Five to ten particles were found in an image taken through the optical microscope, and ten images were taken per a sample. The Received: September 13, 2012 Revised: October 25, 2012 Published: November 2, 2012 4158

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Figure 1. (a) Chemical structures of sacran and alginate. (b) Photoreactions of europium uronate resides where the decarboxylation mediated by UV irradiation.

Figure 2. (a) Effects of UV-irradiation on sacran solubility in water. White light (arrow) was irradiated to the solution from the top. The inset pictures are microscopic images showing that small aggregates appeared following UV irradiation. (b) Changes in the size of sacran particles as a function of UV-irradiation energy. (c) Changes in the fluorescence intensity of ANS mixed with the polysaccharide solution as a function of UVirradiation energy. particle sizes averaged from n = 50−100 data sets are plotted in Figure 2b, where the bars show maximum and minimum sizes. ANS Measurement. The polysaccharides solution (0.5%, 8 mL) was mixed with EuCl3 solution (0.01M, 2 mL) and strongly stirred for 3 h. UV lamp was irradiated on each solution under the same condition with gel beads. The very small amount (10 μL) of saturated

solution of 8-aniline-1-naphthalene sulfonic acid (ANS) was then added to each UV-irradiated sample. The fluorescence intensity of these solutions was measured using FP-6500 spectrofluorometer (JASCO Company). Fluorescence emission (400−620 nm) was monitored at an excitation wavelength of 360 nm (excitation bandwidth: 3 nm, emission bandwidth: 5 nm). 4159

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NMR Spectroscopy. Solid-state 13C DD-MAS NMR spectra (125.8 MHz) of the sacran samples were recorded on a Bruker Avance III 500 spectrometer using a 4 mm MAS probe head (4 mm VTN probe). The acquisition time and recycle delay were 27 ms and 20 s, respectively. The spectra were externally referenced to the carbonyl signal of glycine at 176.03 ppm. Solution-state 1H NMR spectra (400.2 MHz) were obtained on a Bruker NMR spectrometer model AVANCE III 400 with BBFO plus an ATMA probe in D2O. Measurement of Metal Ion Concentration. After the UV irradiation on the gel beads, the amount of trivalent metal ions released from the gel beads was measured by ICP-AES (inductively coupled plasma-atomic emission spectroscopy). Samples of the solution from outside of the gel bead (3 mL) were filtered with a 0.2 μm micro filter and were then treated with HNO3 to decontaminate any organic substances. The concentrations of the metal ions at each irradiation energy value were then measured by ICP-AES (Shimadzu ICPS-8100). Molecular Weight Measurement. Absolute weight-average molecular weights of the sacran samples were measured by a sizeexclusion chromatography multiangle laser light scattering (SECMALLS) system. The chromatographic system was a SHODEX GPC consisting of a pump and an Alliance autosampler. Three columns, a Shodex OHpak (8.0 mm ID × 300 mm length) SB-807G (Guard), a SB-807 HQ, and an SB-804 HQ were used. The column temperature was kept at 40 °C. The eluent was NaNO3 aqueous solution (0.1 M) at a flow rate of 1 mL/min. The sample solution (injected volume: 100 mL, concentration: 0.01 wt %) was filtered using a filter with a pore size of 5 mm just before the measurement. The light scattering instrument, a DAWN Heleos II multiangle laser light scattering detector (detecter angles: 13.0, 20.7, 29.6, 37.5, 44.8, 53.1, and 61.1°) from Wyatt Technology, operating at 665.2 nm, was placed between the SEC and a precise refractive index (RI) detector (Wyatt Technology Optilab T-rEX; laser wavelength (658.0 nm)). The DAWN detector was calibrated with toluene and normalized using a pullulan standard with a molecular weight of 22 000 g/mol. The dn/dc values for sacran was 0.108 mL/g (25 °C), as determined by the RI machine Optilab T-rEX. Fourier-Transformed Infrared Spectroscopy (FT-IR) Spectra. Gel beads of the polysaccharides with metal ions were washed with pure water three times to remove any extra amount of metal ions on the surface of the gel beads, and beads were then freeze-dried. The FTIR spectra were recorded at 25 °C and were measured using a PerkinElmer Spectrum GX 2000 (Boston, MA) and a Nicolet 400D (CA). The spectra were recorded in transmittance mode over the range of 4000−400 cm−1 using a diamond−attenuated total reflection (ATR) accessory. Zeta Potential Measurement. To evaluate the change of each polysaccharide charge before versus after UV irradiation, the zeta potential was measured at 25 °C using a Malvern Zetasizer 2000 (Malvern Instruments, England), where the zeta-potential value was calculated from the measured mobility according to the Smoluchowski equation. The zeta potential was measured using the filtrate that passed through a 0.45 μm filter. The polysaccharide solutions were prepared at concentration of 0.5%. The average from two replicates was used. The standard of silica colloid with a potential range of −50 ± 5 mV (Malvern Instruments) was used for the calibration.

hot water after irradiation with energy of 117 J/cm2, and insoluble materials were observed. Higher water solubility of hyarulonan may be due to a larger amount of hydrophilic component except for uronic acid. Next, solutions of each polysaccharide were prepared and UV light with energy of 117 J/cm2 was irradiated onto them. Slight Tyndall phenomenon in the original sacran solution in the left image of Figure 2a was due to small bubbles entrapped in the highly viscous sacran solution. The insoluble particles appeared in only the sacran solution to show clear Tyndall phenomenon (right image of Figure 2a). In addition, the optical microscopic observation of the UV-irradiated sample revealed the microparticle formation (inset image of Figure 2a). The size of microparticles increased with an increase in UV-irradiation energy, which was controlled by irradiation time (Figure 2b). We hypothesized that the difference in particle formation behavior from their solution between the alginate and sacran was due to the differences in structural changes and then the molecular weight change of the polysaccharides before versus after UV irradiation was measured by SEC-MALLS. UV light with 1100 J/cm2 was irradiated onto a 0.5% solution of alginate and sacran, and the Mw of alginate decreased dramatically from 5.1 × 105 to 1.6 × 103 g/mol. In contrast, the Mw of sacran barely changed from (4.0 to 3.2) × 107 g/mol. Regarding the distribution of the molecular weights, the Mw/Mn value ranged from 1.5 to 1.6 in the case of alginate, but the Mw/Mn range of sacran varied greatly from 1.5 to 4.2. Thus, sacran was basically refractory to photolysis as compared with alginate, and the partially decomposed intermediates of sacran contributed to the broadening of the Mw distribution. From the above-mentioned results, the scission of the sugar chains of alginate by light energy must have occurred. The high-molecular weight materials of sacran hardly undergo photolytic degradation, presumably owing to the bundle formation of sacran chains.11,12 The bundle formation may be attributed to the intrinsic hydrophobic interaction of sacran chains containing relatively hydrophobic sugars, pentose, and 6-deoxyhexose compared with normal hexose. In general, it is considered that the polysaccharides easily form bundle associates dependently on the concentration. In the present experiment, the 0.5% concentration in sacran solution was 58 times higher than its critical concentration at which chain overlap occurs, c*, which are calculated as 0.0086 wt % using the equation:13

c* ≈ a−3N −4/5

(2)

where a is the length of the monosaccharide (0.65 nm) and N is the number of monosaccharide residues in sacran (2.2 × 105 calculated from Mw: 4.0 × 107). The value means that the experimental condition in the solution was very effective on sacran chain association; however, 0.5% in alginate was only 1.8 fold higher than its c* (0.28 wt %),13 which was calculated by eq 2 using a = 0.65 nm and N = 2.8 × 103 (Mw 5.1 × 105 g/ mol). If the similar condition with sacran is given for alginate, then 16% was necessitated, which is too high to make an experiment in solubility and viscosity problems. As a whole, the high-molecular-weight polysaccharides, sacran, have an advantage of easy association at a low concentration. Furthermore, the aggregate particle formation of the sacran chains seemed to contribute to the remaining high-molecular-weight residues.14 To investigate the phenomenon of sacran insolublization, we mixed a hydrophobicity-sensitive dye 8-anilino-1-naphthalenesulfonic acid (ANS)15 into the sacran solution, and the UV irradiation energy dependency of the emission intensity was

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RESULTS AND DISCUSSION UV Irradiation to Solution. The photosensitivity of polysaccharides with uronic acid was investigated. Ultraviolet light (250−450 nm) was irradiated onto dried sodium alginate with carboxylates in all sugar residues, hyaluronan with carboxylates in 50% of the sugar residues, and sacran with one carboxylate (22%) per around five sugar residues (Figure 1a), and the solubility in water of the resulting samples was examined. Only hyaluronan could be completely dissolved in hot water after irradiation with energy of 117 J/cm2. In contrast, alginate and sacran were not sufficiently dissolved in 4160

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potential change in Figure 3a. This phenomenon might be caused by carboxylic acid binding to a metal ion, which generates radical species by ultraviolet light absorption to induce more effective decarboxylation (Figure 1b). In addition, two slight changes in the DD-MAS 13C NMR spectra by UV irradiation were recognized (white arrows). (1) Two signals around 30 and 10 ppm and (2) a broad signal around 140 ppm were detected. In (1), structure variety of C5 and C6 was increased by the speculation that some reactions were provoked by the C5 radical produced by decarboxylation: A) methylation by a hydrogen radical attachment (pentose formation) and (B) a cross-linking reaction by coupling between C5 radicals (Figure 1b), which both induce the hydrophobization of urinate resides. In (2), such signals might appear if the carbon double bond between C4 and C5 was induced by the elimination of RO radical (C) in Figure 1b accompanied by glycoside linkage scission. Although NMR in the solution state was tried, the viscosity of polysaccharides was too high to give clear signals in 1 H NMR. Moreover the pH value was decreased after UV irradiation in both polysaccharides, which is a phenomenon supporting decarboxylation, especially the pH of sacran, which decreased remarkably (alginate: from 6.5 to 5.0, sacran: from 9.0 to 4.8). The decarboxylation was also supported by infrared analyses of the sacran gels discussed later (Figure 5). Photoshrinkage of Gels. We investigated the photoreaction of polysaccharide gels in the water-swollen state to materialize the photoresponse in polysaccharides. Exploiting the quality of alginate and sacran to form gel beads with metal ions, we evaluated the volume change and configuration of the polysaccharides gels following UV irradiation. In this experiment, we selected the trivalent ions that allow gel formation of sacran and alginate.13 Each polysaccharide solution was dropped into individual trivalent ion solutions of 10−2 M to form spherical gel beads (Figure 4a). As a result, volume change of the gels cross-linked by all trivalent ions used here was observed, as shown in representative images of polysaccharide/ Eu3+ gels (Figure 4a), and the shape variation of the gel beads depending on the UV irradiation energy was then investigated. The alginate/Eu3+ gel began to deform at the irradiation energy of 23 J/cm2, and it was observed that the gel began to deteriorate from the irradiation energy of 46 J/cm2. The gel was almost collapsed after 280 J/cm2 and completely disappeared after 530 J/cm2 of UV irradiation (Figure 4a). Alginates were degraded by photoirradiation even in the gel state, which were confirmed by the decrease in dried sample weight. The sacran/ Eu3+ gel gradually contracted without deterioration of the gel; the shape was maintained even after UV irradiation for 539 J/ cm2, and finally the gel contracted into a small blob of material (Figure 4a). This result corresponded to the appearance of the insoluble substance from the solution after UV irradiation. Because the high-molecular-weight materials of sacran hardly undergo photolytic degradation, the sacran/Eu3+ gel did not deteriorate but contracted little by little due to hydrophobization of the sacran chains. To quantify the volume change rate of the each polysaccharide gel, the weight change of the gels depending on the UV irradiation energy was examined. The sacran/Eu3+ gel contracted at a rate of 0.13 mm3/(J/cm2) corresponding to 22 mm3/h, whereas the volume change rate of the alginate/ Eu3+ gel was 0.36 mm3/(J/cm2); the volume change speed of the alginate gel was around three times faster than that of the sacran gel because of the efficient photolysis of alginate gels (Figure 4b). If divalent calcium ion was used, then the alginated

then measured. Because ANS efficiently emits in hydrophobic nanodomains such as micelle, the measurement of ANS fluorescence gave us the information of change in the hydrophobic domain amount by photoirradiation. As a result, strong light emission was detected from the sacran solution after UV irradiation as compared with the alginate solution, and the emission intensity increased with increasing UV irradiation energy (Figure 2c), indicating that sacran achieved a higher hydrophobic state after UV irradiation. By comparing Figure 2b with Figure 2c, the particle size started to increase around 370 J/cm2, which was slightly higher than ANS emission intensity increase occurring around 200 J/cm2. This inconsistency may be due to the observation scale difference between microscopic observation on the microscale and ANS analyses on the molecular scale. It is considered that the microparticles visible in microscopy should be secondary (or higher) aggregates of the hydrophobic assembles. If the decarboxylation occurred as pointed out in the Introduction4 to induce insolubilization, then the ζ-potential of polysaccharides should be changed by UV irradiation. The results for sacran and alginate are shown in Figure 3a. The value

Figure 3. (a) Zeta potential values of polysaccharide solutions (0.5%) before and after UV irradiation. (b) Solid-state DD-MAS 13C NMR spectra of original and UV-irradiated sacran. Block arrow: reduced signal. White arrow: new signals.

of the negative charge in both polysaccharides was decreased depending on the UV irradiation energy, which suggested the decarboxylation. To support the decarboxylation, we measured 13 C DD-MAS NMR of the sacran samples, which was prepared by UV irradiation, followed by lyophilization. The NMR spectra are shown in Figure 3b, revealing the decrease in the integral intensity of carboxylate signal around 175 ppm. Whereas the integral intensities of C1 and main backbone of sugars were kept after UV irradiation, the signals were broadened, indicating an increase in a variety of sugar structures. The signal area ratio of carboxylate in sacran to C1 was decreased from 26 to 19% before versus after UV irradiation. From the above results, the partial decarboxylation of uronic acid in sacran chains was confirmed, supporting the ζ4161

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Figure 4. (a) Photoshrinking behavior of polysaccharide gels with europium ions. (b) Weight change ratio of alginate/Eu3+ gel and sacran/Eu3+ gel beads depending on UV irradiation (250−450 nm) energy. Inset figure: shrinkage rate of sacran gel beads with various trivalent ions. The shrinkage rate was calculated from the initial inclination of the main plot figure.

gel volume increased by photoirradiation. The weaker electrostatic interaction of alginates with Ca2+ than that with Eu3+ may induce the volume increase according to photolysis. Compared with the sacran/Eu3+ system, sacran/Al3+ gels showed a slower photoshrinking behavior. Then, the photoshrinking speed was dependent on metal ion species. The speed was calculated in the sacran gels with various trivalent ions and summarized in the inset of Figure 4b. Among all sacran gels studied here, the gel beads cross-linked with Eu 3+ and Ce3+ contracted prominently. It can be considered that the carboxylate radical is generated according to the formation of metallic free-radical when the metal electrons excited by UV-irradiation drop back to the ground state. Then, the amounts of the metallic freeradical generated in Eu3+ and Ce3+ should be larger than those in other metal ions. The reason for the difference in the photoshrinking rate among these metal ions is not clear, but it might be related to ligand field effects or valence fluctuation phenomenon16 characteristic to the compounds in the 4f electron system. We made an IR study of the sacran/Eu3+ gel beads remained after UV irradiation. As a consequence, the CO stretching vibration at 1620 cm−1 was reduced by UV irradiation, and a new peak at 1579 cm−1 appeared (Figure 5). It was confirmed that the former peak decreased due to the effect of the decarboxylation, whereas the latter peak was derived from carbonate salts. The results also supported the decarboxylation of the sugar by UV irradiation. If decarboxylation did occur in the polysaccharides, then Eu3+ must be released from them. We evaluated the amount of Eu3+ released from the polysaccharide gels by ICP-AEC, in which the gels were immersed in deionized water and then UV-irradiated.

Figure 5. Infrared spectra of sacran (a), sacran gels formed by Eu ion binding (b), and UV-irradiated sacran gels in the presence of Eu ion (c).

The concentration of Eu3+ released from the sacran gels was lower than that of the alginate gels, as shown in Figure 6. This result indicated that the controlled release of metal ions by photoregulation using sacran gels was possible without gel collapse. As a result, we have found the photoshrinkage of gels that were immersed in deionized water (Figure 6) and aqueous solution of EuCl3 with a concentration of 10−2 M (Figure 4b). Under both conditions, photoshrinkage was clearly observed. In aqueous solution of EuCl3 (10−2 M), the ion concentrations were well-balanced inside and outside the bead because the released amount of Eu3+ was only 8 ppm at maximum. Then, it 4162

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(2) Nam, H. S.; An, J.; Chung, D. J.; Kim, J- H.; Chung, C- P. Controlled release behavior of bioactive molecules from photo-reactive hyaluronic acid-alginate scaffolds. Macromol. Res 2006, 14, 530−538. (3) Jeon, O.; Bouhadir, K. H.; Mansour, J. M.; Alsberg, E. Photocrosslinked alginate hydrogels with tunable biodegradation rates and mechanical properties. Biomaterials 2009, 30, 2724−2734. (4) Blakov, A.; Brezov, V.; Soldanova, Z.; Stasko, A.; Soldan, M.; Ceppan, M. Photocatalytic degradation of heparin over titanium dioxide. J. Mater. Sci. 1995, 30, 729−733. (5) Mogilevskii, M. S.; Laufer, A. L. Effect of ultra violet irradiation on hyaluronic acid. Dokl. Akad. Nauk SSSR 1951, 76, 239−42. (6) Hvidberg, E.; Kvorning, S. A.; Schmidt, A.; Schou, J. Reduction of the viscosity of a potassium hyaluronate solution by ultra-violet irradiation. Nature 1958, 181, 1338. (7) Endre, A.; Balazs, T. C.; Laurent, A. H.; Laszlo, V. Irradiation of mucopolysaccharides with ultraviolet light and electrons. Radiat. Res. 1959, 11, 149−164. (8) Lapčík, L.; Schurz, J. Photochemical degradation of hyaluronic acid by singlet oxygen. Colloid Polym. Sci. 1991, 269, 633−635. (9) Okajima, K. M.; Ono, M.; Kabata, K.; Kaneko, T. Extraction of novel sulfated polysaccharide from aphanothece sacrum (sur.) okada, and its spectroscopic characterization. Pure Appl. Chem. 2007, 79, 2039−204. (10) Okajima, K. M.; Bamba, T.; Kaneso, Y.; Hirata, K.; Kajiyama, S.; Fukusaki, E.; Kaneko, T. Supergiant ampholytic sugar chains with imbalanced charge ratio form saline ultra-absorbent hydrogels. Macromolecules 2008, 41, 4061−4064. (11) Okajima, M. K.; Miyazato, S.; Kaneko, T. The cyanobacterial megamolecule sacran efficiently forms lc gels with very heavy metal ions. Langmuir 2009, 25, 8526−8531. (12) Okajima, K. M.; Kaneko, D.; Mitsumata, T.; Kaneko, T.; Watanabe, J. Cyanobacteria that produce megamolecules with efficient self-orientations. Macromolecules 2009, 42, 3057−3062. (13) De Gennes, P. G. Physics of Liquid Crystals; Oxford Univ Press: Oxford, U.K., 1995. (14) Okajima, K. M.; Nakamura, M.; Mitsumata, T.; Kaneko, T. Cyanobacterial polysaccharide gels with efficient rare-earth-metal sorption. Biomacromolecules 2010, 11, 1773−1778. (15) Slavik, I. Anilinonaphthalene sulfonate as a probe of membrane composition and function. Biochim. Biophys. Acta 1982, 694, 1−25. (16) Lawrence, J. M.; Riseborough, P. S.; Parks, R. D. Valence fluctuation phenomena. Rep. Prog. Phys. 1981, 44, 1−84. (17) Saito, T.; Kimura, S.; Nishiyama, Y.; Isogai, A. Cellulose nanofibers prepared by TEMPO-mediated oxidation of native cellulose. Biomacromolecules 2007, 8, 2485−2491. (18) Finlay, I. G.; Mason, M. D.; Shelley, M. Radioisotopes for the palliation of metastatic bone cancer: a systematic review. Lancet Oncol. 2006, 6, 392−400. (19) Wengler, G.; Wengler, G.; Koschinski, A. A short treatment of cells with the lanthanide ions La3+, Ce3+, Pr3+ or Nd3+ changes the cellular chemistry into a state in which RNA replication of flaviviruses is specifically blocked without interference with host-cell multiplication. J. Gen. Virol. 2007, 88, 3018−3026.

Figure 6. Changes in the amount of Eu3+ released from the polysaccharide gel beads as a function of irradiation energy. The amount of Eu3+ ion released from each gel bead was measured by ICP−AES.

is concluded that Eu3+ leaching out of gels did not contribute to the photoshrinkage. According to the Material Safety Data Sheet (MSDS), the acute toxicity value of EuCl3 is LD50 3500 mg/kg (mouse, oral administration) which is good value comparing to LD50 of NaCl (3000 mg/kg). Then, Eu3+ is a fluorescent ion usable for medicinal assay and the sacran/Eu3+ hydrogels may be useful for remotely UV-controlled assay. Furthermore, uronic acid can be easily converted from widely available hexose by oxidation;17 then, it is possible that such a photoreaction of uronic acid could be extended to other polysaccharides.

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CONCLUSIONS The anionic polysaccharide “sacran”, which is composed of 6deoxyhexoses, pentoses, uronic acids as well as hexoses, showed hydrophobization and insolubilization phenomena in response to ultraviolet light irradiation. The photoreaction mechanism of the polysaccharides was then investigated using the corresponding gels formed by interaction with metal ions. As a result, we observed that sacran gels with trivalent metal ions gradually contracted depending on the energy of irradiated light. In contrast, alginate gels degraded instead of contracting. This photoshrinkage of the sacran gels may be attributed to the hydrophobization of uronic acid based on photodecarboxylation. These phenomena could be seen in other trivalent ions whose controlled release might be effective on cancer targeting18 (if radioactive ions were used) and virus infection control.19 Furthermore, any polysaccharides containing hexose can be materialized using the present photoreaction-mediated phenomenon because uronic acid can be easily converted from hexose by oxidation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

(1) Javvaji, V.; Baradwaj, A. G.; Payne, G. F.; Raghavan, S. R. Lightactivated ionic gelation of common biopolymers. Langmuir 2011, 27, 12591−12596. 4163

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