Biomacromolecules 2004, 5, 1009-1014
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Novel Water-Soluble Photosensitizers from Dextrans Maria Nowakowska,* Szczepan Zapotoczny, Monika Sterzel, and Emilia Kot Faculty of Chemistry, Jagiellonian University, Ingardena 3, 30-060 Krakow, Poland Received December 5, 2003; Revised Manuscript Received March 2, 2004
Novel water-soluble polymeric photosensitizers based on the natural polymer dextran were synthesized and studied. The modified dextran contained photoactive anthracene (An) chromophores. They were soluble in water with the solubility decreasing with an increase in the number of An moieties bound to the polymeric chain. In aqueous solutions, the macromolecules adopted a compact conformation which resulted in the formation of hydrophobic microdomains. The properties of these domains were characterized with molecular probes such as perylene and pyrazolo-quinoline derivative. The polymer absorbed in the UV/vis region and photosensitized reactions mediated by energy and/or electron transfer from electronically excited An to the molecules of organic compounds solubilized in polymeric microdomains or resided in water. Introduction Amphiphilic polymers have attracted considerable attention due to their broad range of applications in modern technologies such as optical information technology, biotechnology, and pharmacology.1,2 Polymeric systems which display specific properties such as photoactivity, especially the ability to participate in an electron transfer processes, became recently of particular importance. So far, most of the studies were carried out with the synthetic polymers.3-5 There is, however, a growing interest in natural polymers for practical applications, especially in biotechnology, medicine, and environmental protection. The most obvious choice for such applications includes polysaccharides, the most abundant polymers in the biosphere. Polysaccharides are massively available. They combine unique structural and molecular features such as high molecular weight, well-defined molecular structure, and strong intramolecular and intermolecular bonding. An unquestionable advantage of natural polymers over the synthetic ones is their ability to undergo degradation by specific microorganisms.6,7 Dextran has been extensively studied in terms of medical applications as a macromolecular carrier for delivery of drugs and proteins and as an imaging agent in magnetic resonance examinations.8,9 Dextran has been also used for separation and purification of biological materials, plasma volume expansion, peripheral flow promotion, and as antithrombolytic agent.10-13 It was also reported on using these polymers for preparation of catalytically active membranes for gas separation and hyperfiltration homo- and electrodialysis.14,15 From the structural point of view, dextrans consist predominately of linear R-1,6-glycosidic linkages with some degree of branching via the 1,3-linkage. The solubility of dextrans in water decreases with increase in branching. They are stable under basic and mild acidic solutions. A large number of their hydroxyl groups provide the numerous attractive chemical modifications. * To whom correspondence should be addressed.
Water soluble synthetic polymers containing various chromophores for photodecomposition of various toxins16-18 have been recently developed in our laboratories. Synthesis and properties of novel, biodegradable naphthyl substituted hydroxyethylcellulose has been recently reported.19 It very efficiently photosensitized photooxidation of cyanides.20 To the best of our knowledge, no polysaccharide has been used so far for those kinds of applications. Polysaccharides labeled with some chromophores such as azulene or fluorescein have been used for improving optical or electrooptical properties of polymers.1,2,21,22 This paper reports on the synthesis and characterization of novel polymeric photosensitizers based on dextran. The main goal for developing these polymers was to obtain a alternative, environmentally friendly photocatalytic system operating with visible light to conduct an efficient degradation of a wide range of pollutants. Experimental Section Apparatus. The UV/vis spectra of the samples were obtained using HP 8452A Diode Array spectrometer (HewlettPackard, Palo Alto, CA). The 1H NMR spectra of the polymers were measured in DMSO-d6 solution using a Bruker AMX 500 spectrometer (Bruker, Rheinstetten, Germany). The IR spectra were recorded in KBr pellets using a Bruker IFS 48 spectrometer (Bruker, Rheinstetten, Germany). The steady-state fluorescence measurements were done using SLM Aminco 8100 spectrofluorometer (SLM, Rochester, NY) in the L-type geometry. The GPC analyses of the polymers were carried out using Waters GPC system (Waters, Milford, MA) equipped with Ultrahydrogel (30 Å, 40 Å, 50 Å) × 8 µm column system. Detection was done using Waters 2410 Refractive Index Detector and Waters 2996 Photodiode Array Detector (Waters, Milford, MA). NaCl solution (0.05M) was used as the mobile phase at flow rate 1 mL/min. Irradiation of polymer-viologen-TEA systems was performed with the use of the mercury lamp equipped with a 365 nm interference filter.
10.1021/bm034506w CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004
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Biomacromolecules, Vol. 5, No. 3, 2004
Nowakowska et al.
Table 1. Feed Ratio and Compositions of DXA Polymers
polymer
feed ratio DX:9CMAa
reaction time [h]
degree of substitution [mol-%]a 1H NMRb
UV/visc
DXA(4.5) DXA(2.5) DXA(0.2)
1:1.1 1:0.8 1:0.5
5.5 4.5 3.5
4.5 2.5 e
d 2.8 0.2
a
Calculated with respect to glucose unit of DX. b Calculated from the NMR spectra of the polymers (based on the ratio of broad aromatic signal integration in the range of 7-9 ppm to the integration of DX signals). c Calculated from the absorption spectra of the polymers (based on the extinction coefficient of the highest peak in the vibrational structure of An 356 ) 9700 M-1 cm-1.25 d Limited solubility of the polymer. e Too low degree of substitution. 1H
The solutions were deoxygenated by bubbling with nitrogen for 30 min if necessary. Materials. Dextran, Mw ) 10 000 g/mol, 98% (DX), 9-chlormethylanthracene, 98% (9CMA), 9-methylanthracene, 99% (9MA), perylene, 98% (Pe), methyl viologen dichloride, 98% (MV2+) (all from Aldrich, Milwaukee, WI) and all the solvents: cyclohexane, dimethylformamide (DMF), methanol, tetrahydrofuran (THF), and toluene (all from POCH Gliwice, Poland, HPLC grade or spectrophotometric grade) were used as received. Triethanoloamine, 98%, Aldrich (TEA) was distilled under vacuum. 4,4′-Bipyridinium-1,1′bis(trimethylenesulfonate) (SPV) and [6-(N,N-dimethylamino)]1-(4′-cyanophenyl)-3-phenylpyrazolo[3,4-b]-quinoline (PQ) were prepared according to the procedures described in the literature.23,24 Deionized water was used to prepare polymer solutions.. Synthesis of Anthracene Substituted Dextran (DXA). Three DXA polymers characterized by different content of anthracene were obtained. The polymers were synthesized in a two-phase etherification reaction between DX and 9CMA. The general procedure was as follows: DX (1 g) was dissolved in 5 M NaOH aqueous solution (20 mL). Then solution of 9CMA in toluene (20 mL) was added dropwise within 30 min into DX solution vigorously stirred and heated under nitrogen to 60 °C. To obtain various degrees of substitution the ratio of dextran: 9CMA in reaction mixture and the time of reaction were accordingly adjusted (see Table 1). After the reaction was completed, the mixture was several days dialyzed against DMF and deionized water. Finally, the aqueous solutions of DXA polymers were extracted with cyclohexane until no 9CMA was detected in the organic phase. The polymers were freeze-dried. Results and Discussion Properties of the DXA Polymers. The structure of DXA polymers is shown in Scheme 1. The modified polymers, differing in the chromophore content (see Table 1), were characterized by spectroscopic methods (1H NMR, FTIR, absorption and emission in UV/ vis spectral region) and by gel permeation chromatography (GPC). FTIR spectra of the substituted polymers showed bands in the range of 840 and 914 cm-1, which could be assigned to stretching vibration of the C-H group in aromatics and bands in the range of 1260-1390 cm-1 representing stretch-
Figure 1. Normalized electronic absorption spectra of 9MA in methanol (9) and DXA (2.5) in aqueous solution (b). Scheme 1. Structure of DXA Polymers
ing vibration of C-H group in aromatic ethers. 1H NMR spectra exhibited a broad signal in the range of 7-9 ppm, assigned to the aromatic protons of An. Absorption spectra in the UV/vis spectral region for the polymers (shown in Figure 1) displayed bands with maxima at 356, 374, and 394 nm, characteristic for vibronic structure of An chromophores. They are, however, red-shifted by ca. 8 nm in the case of DXA (2.5) and 2 nm in case of DXA (0.2) in comparison to the spectrum of 9MA in methanol. These differences can be explained considering differences in solvent polarity and specificity of microenvironment experienced by the chromophores attached to the polymer. The GPC chromatogram for starting material, DX, displayed one intensive peak at retention time of 10.3 min (see Figure 2). As expected, the GPC trace for the substituted DX differed considerably from that for the starting material. The main peak characteristic of the modified material was shifted to considerably lower retention time, tR ) 8.75 min. The lower retention time of the modified dextran resulted from its higher molecular weight and some broadening of the peak might reflect the conformational changes of the polymer. Unfavorable interactions between water and hydrophobic chromophores attached to the polymer chain force more compact conformation of the macromolecules. Macromolecules of the same weight but slightly different in hydrophobic substitution can significantly differ in conformation and, hence, in apparent polydispersity. The short retention time shoulder might originate from some contribution of aggregates or small fraction of more substituted DXA.
Water-Soluble Photosensitizers from Dextrans
Figure 2. GPC traces of dextran (refractive detection) (9) and DXA(2.5) (absorption detection at 375 nm) (b).
Figure 3. Stationary emission spectra (λex ) 345 nm) of 9MA (b), DXA (0.2) (1) and DXA (2.5) (9).
The detailed analysis of GPC traces for DXA would require studies on nonsize-exclusion effects which might influence the separation mechanism. This is, however, beyond the scope of that paper. The experimental data presented above confirmed that An chromophores were bounded to the polymeric dextran chain. Polymers with 0.2 and 2.5 mol % of the chromophores were readily water soluble (up to 5 g/dm3), whereas that with 4.5 mol % of An showed limited solubility in water (
