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And Water Soluble Polymer Ligands with Potential Use in Controlled Drug Delivery Nada Verdel,1,* Andrej Kr`An,2 Mojca Ben~Ina2 and Majda @Igon2

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And Water Soluble Polymer Ligands with Potential Use in Controlled Drug Delivery Nada Verdel,1,* Andrej Kr`An,2 Mojca Ben~Ina2 and Majda @Igon2 Acta Chim. Slov. 2013, 60, 651–659 651 Scientific paper Determination of the Interactions Between Zn2+ and Water Soluble Polymer Ligands with Potential Use in Controlled Drug Delivery Nada Verdel,1,* Andrej Kr`an,2 Mojca Ben~ina2 and Majda @igon2 1 Faculty of Chemistry and Chemical Technology, University of Ljubljana, Askerceva 5, 1000 Ljubljana, Slovenia 2 National Institute of Chemistry, Hajdrihova 19, 1000 Ljubljana, Slovenia * Corresponding author: E-mail: [email protected] Received: 28-01-2013 Abstract Biodegradable copolymers of aspartic and lactic acids were synthesized for potential use in controlled drug release. The proportion of aspartic acid moieties in the copolymers was 0.9 and 0.1, the molecules were partially branched and had absolute molar masses over 100,000 g/mol. The drug could be attached to the copolymer via metal (particularly zinc) ions, so a method to estimate the interactions between zinc ions and the water-soluble polymers by fluorescence spec- troscopy was developed. The stability constants of binding of zinc and the concentrations of zinc bound to polymer we- re determined. The results confirm that zinc ions at pH 6 preferentially bind to side groups of aspartic acid units of the copolymers. Keywords: Polyamino acid, controlled drug delivery, biodegradable carrier, fluorescence spectroscopy, zinc, polymer coordination compound. biological conditions. However, it is insoluble in organic 1. Introduction solvents and does not have thermoplastic properties, which makes it difficult to obtain PA films or mouldings; This study focused on biodegradable polymers that it can only be processed as an aqueous solution or in a can potentially be used for pharmaceutical purposes in hygroscopic powder form.4 controlled drug release. Polymers as drug carriers are On the other hand, polylactide or poly(lactic acid), used in two types of delivery systems, colloidal carriers PLA, is one of the most intensively studied biodegradable and polymer-drug conjugates. In colloidal formulations, thermoplastic polymers and contains lactic acid monomer the polymer encapsulates drug within micro- or nanopar- units. PLA is hydrophobic and soluble in organic solvents. ticles.1 In polymer-drug conjugates, the drug is covalently It is currently used in a number of biomedical applica- bound to the polymer.2 For such purposes, the drug could tions, such as sutures or fibres,5 stents, implants for bone be attached to the polymer via dative covalent bonds or fixation6 and drug delivery devices.7 It is also being eva- coordinate bonds with metal ions like zinc, and for the luated as a material for tissue engineering.8 One of the polymer components lactic and aspartic acid moieties shortcomings of this polymer is that there is no pendant could be combined. functional group on the backbone. Poly(aspartic acid), PA, is a typical hydrophilic bio- One of the possibilities to combine the advantages degradable polymer with aspartic acid monomer units. of both PLA and PA derivatives is to make amphiphilic The most common method for the synthesis of PA invol- copolymers.4 PA can bind metal ions and is water-solub- ves the polycondensation of aspartic acid into polysucci- le,9 whereas PLA has good mechanical properties and dis- nimide, followed by its hydrolysis to sodium polyasparta- solves in organic solvents. By changing the ratio of aspar- te. The hydrolyzed form, sodium polyaspartate, has tic and lactic acid units we can therefore obtain amphiphi- hydrophilic carboxylate side groups that can interact with lic copolymers to which a drug could be attached via zinc metal ions.3 It degrades quickly under physiological and ion bridges. For many years zinc has been used to treat Verdel et al.: Determination of the Interactions Between Zn2+ ... 652 Acta Chim. Slov. 2013, 60, 651–659 epithelial disorders, ranging from wound healing to diarr- acid indicator, “cell impermeant”, purchased from Molecu- hea and ulcerative colon disease.10 It is found in all body lar Probes, was dissolved in twice deionized Milli-Q water tissues, with ∼85% of the whole body zinc in muscle and and kept protected from light at –20 °C. 8 g of N-(carba- 11 2+ bone, and another 11% found in the skin and liver. Typi- moylmetyl)iminodiacetic acid buffer (ADA) (Kd(Zn ) = cally, humans appear to have the capacity to regulate who- 127 nmol/L at an equimolar ratio of Zn2+ and ADA, 20 °C, le body zinc content over a ten-fold change in intake.12 pH 7 and ionic strength 0.1 mol/L), purchased from Sigma, Zinc ions bind to copolymers of aspartic and lactic was dissolved in 50 mL 1 N NaOH. acids to form polymer coordination compounds. In poly- mer coordination compounds metal ions interact with li- 2. 2. Syntheses gand groups situated on the polymer chains. The kind and strength of the metal ion to ligand interaction can be des- Poly(succinimide-co-lactic acid), PSL, copolymers cribed by stability constants. Stability constants of poly- were synthesized in a 100-mL glass flask that was deoxy- mer coordination compounds can be calculated with the genated by continuous degassing and back-filling with ar- help of many theoretical models. For our system, the most gon, except during the third step of synthesis when a va- suitable model is described by Flory’s concept of infini- cuum pump was applied (Table 1). The reaction mixture tely large polymer chains.13,14 consisted of aspartic acid and lactic acid or lactide without In this paper we report a method of determining the addition of catalysts or solvents. ALa copolymers were interactions between polymer ligands and Zn2+ in aqueous synthesized by reaction of aspartic and lactic acids, and solutions by fluorescence spectroscopy. To determine the ALt copolymers with aspartic acid and lactide. The reac- concentration of free zinc, a fluorescent indicator that tions proceeded similarly to the procedure with aspartic enables minimal interference with the equilibrium of zinc, acid and lactide reported by Shinoda et al.4 under the reac- was used. As zinc carriers copolymers of aspartic and lac- tion conditions summarized in Table 1. Upon heating the tic acids were prepared and the properties of the copoly- reaction mixtures changed into viscous liquids, which we- mers for potential use as carriers in chemically controlled re very difficult to stir. At the end of the syntheses the drug release were characterized. reaction flask was taken out of the oil bath and cooled to 45 °C. The products were purified by dissolution in the minimal quantity of DMF and the filtrate was poured into 2. Experimental distilled water to precipitate the polymer. The yellowish white polymer powder was dried for 12 hours at 80 °C in 2. 1. Materials an oven. FTIR (KBr): ν = 1750 (C=O, ester), 1790, 1720 L-aspartic acid (> 98%), L-lactic acid (> 85%), (C=O, succinimide), 1660–1630 (amide I), 1550–1530 L-lactide – the cyclic dimer of lactic acid (98%), N,N-di- (amide II), 1290–1260 (amide III), 640 (amide V), methylformamide (p. a., 99%) and lithium bromide, LiBr, 3700–3000 (N-H and O-H), 2990–2945 (C-H and CH2, (> 99%), purchased from Aldrich, N,N-dimethylacetamide, stretching vibrations), 1130 (C-O, ester) and 1400 cm–1 DMAc, (99%), purchased from Merck and zinc chloride (p. (CH2, deformational vibrations). a., 98+%), purchased from Fluka, were used as received. In the next step, amphiphilic poly(sodium α,β-as- Benzoylated dialysis tubing with pore size of ≤ 1,200 g/mol partate-co-lactic acid) (PAL) copolymers were prepared was purchased from Sigma Aldrich. Sodium poly((α,β)- by hydrolyses of cyclic succinimide rings to more hydrop- D,L-aspartate) (40 w.%) was purchased from Aldrich, with hilic aspartic acid units bearing carboxylate groups. The FTIR: ν = 2600–3600 (N-H and O-H), 1598 (C=O, car- hydrolysis were performed with 1 M NaOH added drop- boxylate), 1648 (amide I), 1240 (amide III), 670 (amide V), wise. The products were purified by three days of dialysis –1 2867 and 2930 (C-H and CH2) and 3086 cm (–HC=, ma- with exchanging distilled water twice per day where com- 1 δ α ≤ leamic acid) and H NMR (in D2O): = 4.7 ( -CH), 4.5 pounds with 1,200 g/mol were separated from com- β β α β ( -CH), 2.76 ( -CH2) and 2.65, 2.55 ppm ( , -CH2). The pounds having molecular weight over 2,000 g/mol. FTIR degree of ionization according to Saudek15 was 1. The fluo- (KBr): ν = 1600 (COO–, carboxylate), 3400 (N-H and –1 rescent dye Fluozin-1, N-(2-methoxyphenyl)iminodiacetic NH2), 1240 (amide III) and 670 cm (amide V). Table 1: Reaction conditions for the synthesis of copolymers of aspartic (A) and lactic acid (La) or lactide (Lt) sample N A La Lt A/Lafeed temperature (°C) / time (h) I. step II. step III. step IV. step melting cooling vacuum (∼ 4 mbar) further reaction AM 4 16 17 / 0.95 180 / 2.5 180–40 / 0.5 40 / 0.3 180 / 2.5 AL 3 42 / 245 0.17 180 / 2.5 180–160 / 0.5 160 / 18 / N = number of parallels, A/Mfeed = feed molar ratio Verdel et al.: Determination of the Interactions Between Zn2+ ... Acta Chim. Slov. 2013, 60, 651–659 653 2. 3. Characterization Methods Above mentioned methods mostly demand a puri- fied polymer coordination compound with no free metal Fourier transform infrared spectroscopy, FTIR, was ions in the solution. However, the equilibrium of free and carried out with a Perkin-Elmer 1725X FTIR spectrome- bound Zn2+ in aqueous solutions of the polymer ligands ter. Typical FTIR spectra were obtained using KBr pellets prevents complete removal of free Zn2+. The extent of the or FTIR spectra in aqueous solutions using AgCl plates, interactions between polymer ligands (L) and Zn2+ was resolution 4-cm–1, range 400–4500 cm–1, 20 °C and with therefore determined by fluorescent spectroscopy where 10 scans for dry or 300 scans for aqueous samples.
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