Synthesis and properties of isomeric pyridyl-containing chitosan derivatives

International Journal of Biological Macromolecules 62 (2013) 426–432

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Synthesis and properties of isomeric pyridyl-containing chitosan derivatives S.Yu. Bratskaya a , Yu.A. Azarova a , A.S. Portnyagin a , A.V. Mechaev b , A.V. Voit a , A.V. Pestov b,∗ a b

Institute of Chemistry, Far East Branch of RAS, 159, Prosp. 100-letiya Vladivostoka, Vladivostok 690022, Russia I.Ya. Postovsky Institute of Organic Synthesis, Ural Branch of RAS, 20, S. Kovalevskoy street, Yekaterinburg 620990, Russia

a r t i c l e

i n f o

Article history: Received 26 August 2013 Accepted 18 September 2013 Available online 27 September 2013 Keywords: Chitosan Pyridyl derivative Physical–chemical properties

a b s t r a c t Here we report on the method of synthesis in gel of a new heterocyclic aminopolymer-N-2-(4pyridyl)ethylchitosan (4-PEC) via direct addition of 4-vinylpyridine to chitosan that yields a derivative with the substitution degree (DS) up to 0.8. The comparison of reactivity, thermal, spectroscopic, and sorption properties of a new derivative and its isomer N-2-(2-pyridyl)ethylchitosan (2-PEC) is presented. 2-PEC has higher sorption capacity and forms more stable chelates with [PdCl4 ]2− and [PtCl6 ]2− ions than 4-PEC, but the latter shows higher selectivity to noble metals ions in the presence of Cl− ions. A gradual increase of the sorption capacities and the affinity coefficient for Cu2+ and Ni2+ in the row chitosan < 4-PEC < 2-PEC was related to the increase of electron donor nitrogen atoms content and chelating properties of 2-PEC. A nearly negligible increase of the 4-PEC sorption capacity for Ag+ , as compared to plain chitosan, was suggested to be dependent on the difference in complexation models for 2-PEC and 4-PEC derivatives. The density functional theory (DFT) calculations have shown that the “pendant” model of the complex with Ag(I) is energetically favorable only for 2-PEC derivative, while in cases of chitosan and 4-PEC only “bridge” complexes can be formed that results in lower sorption capacity. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.

1. Introduction The correlation between the structure of molecules and their chemical properties is a fundamental problem in organic chemistry. Finding such correlations for macromolecules is even more complicated due to additional contribution of strong intra- and intermolecular interactions and a significant effect of conformation and structural heterogeneity on polymers functional properties. Changes in the chemical structure of newly synthesized macromolecules in general and isomerization in particular often result in unexpected changes in properties of functionalized polymers [1,2]. However, such correlations are crucial for the targeted design of macromolecules. Different chemical activities of poly2- and poly-4-vinylpyridines, which result not only from different electron-donor properties, but also from the spatial structure, show how isomerization affects properties of macromolecules. Indeed, fluorescence of poly-2-vinylpyridine is six times higher than that of poly-4-vinylpyridine [3]; it is also more efficient for chromatographic separation of aromatic amines [4]. However, poly-4-vinylpyridine shows higher activity in complexation with

∗ Corresponding author. Tel.: +7 343 3623439. E-mail address: [email protected] (A.V. Pestov).

poly(monomethy1 itaconate) [5], Zn(II) [6], and alkaline metal ions [7] and provides better oxidation activity for poly(vinyl pyridine)supported silver dichromates [8]. The structure of functional fragments grafted to chitosan, a natural aminopolymer, allows tuning its properties and fabrication of chitosan-based materials for specific applications [9–11]. It is known that, due to polyfunctionality of native chitosan, its modification, depending on the reactions used, can lead to different regioisomers [12–14]. Functionalization of chitosan with isomeric groups is described in literature for a very limited number of examples. Isomeric derivatives – 1-carboxy- and 2carboxyethylchitosans synthesized by the methods reported in [15,16] differ in protolytic and complexing properties [17,18] and biological activity [15,19]. The isomeric structure of grafted pendant groups is also known for pyridyl-derivatives of chitosan. N-(2-Pyridylmethyl)chitosan, which is capable to form chelates, shows a 3-fold increase of sorption capacity and sorption constant toward Cu(II) ions in comparison with chitosan functionalized with non-chelating 4-pyridylmethyl group [20]. Here we report on the synthesis of a new chitosan derivative – N-2-(4-pyridyl)ethylchitosan (4-PEC) and comparison of its spectral, thermal, and sorption properties with those of the isomeric derivative N-2-(2-pyridyl)ethylchitosan (2-PEC) to elucidate the influence of isomeric pendant group structure on chitosan properties.

0141-8130/$ – see front matter Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ijbiomac.2013.09.017

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427

DCL = {(C/N)cross-linked polymer − (C/N)non-cross-linked where C/N – is the molar ratio of carbon and nitrogen in 2-PEC or 4-PEC before and after cross-linking, 3 – is a number of carbon atoms in cross-linker residue.

2. Experimental

formula:

polymer }/3,

2.1. Materials and methods Chitosan was purchased from JSC “Sonat” (Moscow, Russia). The degree of acetylation (DA) was determined by 1 H NMR spectroscopy to be 0.18; the average molecular weight of 2.5 × 105 Da was measured using viscometry according to [21]. 4-Vinylpyridine (Sigma–Aldrich) was distilled before the synthesis and stabilized with hydroquinone (0.05%). All other chemicals were of an analytical grade and used without further purification. The C, H, N-elemental composition was determined by a Perkin Elmer Elemental Analyzer. FTIR spectra were recorded on a Perkin Elmer Spectrum One FTIR spectrometer using a Diffuse Reflectance Sampling Accessory (DRA). 1 H NMR spectra were recorded on a Bruker AVANCE-500 spectrometer at 70 ◦ C to increase the solubility of samples and attain better resolution of the signals. Samples were dissolved in D2 O/DCl (concentration 10 mg/ml), sodium 3-(trimethylsilyl)-1-propanesulfonate was used as an internal standard. Suppression of the solvent signal during the spectrum recording was realized by the “presaturation” technique. The degree of substitution (DS) was calculated using 1 H NMR spectra according to the following formulas: total DS (DS) =

m + 2d , m+d+a

monosubstitution DS (DSm ) =

disubstitution DS (DSd ) =

m , m+d+a

2d , m+d+a

where m is the mole fraction of monopyridylethylated glucosamine units, d is the mole fraction of dipyridylethylated glucosamine units, a is the mole fraction of glucosamine units. The calculations of m, d, and a values were performed using integral intensities of the corresponding signals in 1 H NMR spectrum. Thermograms were registered using a Mettler Toledo TGA analyzer. 2.2. Preparation of chitosan derivatives N-2-(2-Pyridyl)ethylchitosan (2-PEC) with various degrees of substitution (DS) was synthesized by the method of chitosan transformation “synthesis in gel” escribed in [22]. N-2-(4Pyridyl)ethylchitosan (4-PEC) was synthesized using a similar scheme: the mixture of 0.33 g (0.002 mol NH2 ) of chitosan and 0.42 ml (0.004 mol) of 4-vinylpyridine and 1.56 ml (0.002 mol) 4.6% HCl was held about 10 min until formation of the gel-like mass, thereafter the gel was heated up to 70 ◦ C, held for 48 h, and cooled. 7.18 ml of 0.85% HCl (0.002 mol) was added to the mixture, upon homogenization of the solution the product was precipitated with 50 ml of acetone. Then the product was extracted by hot isopropanol during 24 h and dried at 50 ◦ C to the constant weight. 1 H NMR spectra of 4-PEC, DS = 0.62 (D O/DCl), ı ppm: 2.07 (CH ), 2 3 3.24–3.95 (CH2 CH2 Py CH2 OH, H-2, 3, 4, 5, 6), 4.56 (H-1 GlcNHAc), 4.93 (H-1 GlcNH2 ), 5.04 (H-1 GlcNHCH2 CH2 Py), 5.12 (H-1 GlcN(CH2 CH2 Py)2 ), 7.95–8.70 (H Py). To obtain insoluble sorbents, 2.25 g of 4-PEC was mixed with 250 ml of water containing 21 g NaOH, 7 ml of epichlorohydrine was added to the mixture under constant stirring. Upon heating the mixture at 50 ◦ C during 2 h, precipitate was filtered, washed with water until negative reaction to Cl− ions, and dried at 50 ◦ C to the constant weight. The degree of cross-linking (DCL) of the sorbents obtained was determined using the elemental analysis data according to the

2.3. Sorption and desorption experiments Sorption properties of 4-PEC or 2-PEC toward ions of noble metals were investigated from solutions of H2 [PdCl4 ] and H2 [PtCl6 ] in 0.1 N HCl, 0.4 N HCl, and 1 N HCl. The solid:liquid ratio in sorption experiments was 1:1000, the contact time was 18 h, and the shaking rate was 200 rpm. The sorption of transition metal ions (Cu(II), Ag(I), and Ni(II)) was studied under the same conditions from solutions of metal nitrates in 1 M NH4 NO3 , pH 5.3. The sorption capacities were calculated using the difference in initial and equilibrium concentrations of the metal ions determined by the atom absorption spectroscopy (AAS) using a Solaar M6 (Thermo, USA) device. Desorption of Pt(IV) and Pd(II) from 4-PEC and 2-PEC phase was studied for derivatives with DS 0.8 using HCl and HCl/thiourea mixtures at solid:liquid ratio 1:100. Preconcentration of the metal ions was carried out from H2 [PdCl4 ] and H2 [PtCl6 ] solutions in 0.1 N HCl containing 200 mg/L of noble metals. After preconcentration, sorbents were rinsed with water, dried, and the quantity of noble metal in the sorbent phase was determined by AAS after decomposition of the sorbent in aqua regia. The contents of metals in the eluent after desorption were determined by AAS. 2.4. DFT calculations of chitosan, 2-PEC, and 4-PEC complexation with Ag+ ions The quantum chemical modeling of all investigated complexes was carried out by means of the GAMESS program package [23]. All calculations were based on the density functional theory (DFT) using the hybrid functional B3LYP [24]. The extended grid compared to the default GAMESS grid by radii (192) and angle (590) was used to increase the accuracy of the calculations. The valence only SBKJC basis set augmented with two polarization d functions and corresponding ECP potential was used for all heavy atoms and unscaled 31 g basis was used for hydrogen atoms [25]. The solvent influence was taken into account in terms of the polarizable continuum model (PCM) [26]. All the studied complexes were optimized by total energy. Since we were interested in the behavior of complexes in aqueous solutions with the pH value close to neutral, the relative formation energies (E) were calculated as a difference between the formation energy of Ag+ – chitosan (2-PEC/4-PEC) complex and silver aquacomplex: E = Ec − Eaq , where Ec is the formation energy of the studied complex, Eaq is the formation energy of [Ag(H2 O)2 ]+ complex, respectively. 3. Results and discussion 3.1. Preparation of 4-PEC We have previously reported on synthesis of a new pyridyl-containing chelating derivative of chitosan – N2-(2-pyridyl)ethylchitosan (2-PEC) using the method of polymer-analogous transformation “synthesis in gel” for chitosan interaction with 2-vinylpyridine [22,27]. The same approach applied for reaction of chitosan with 4-vinylpyridine under the same conditions has yielded an isomeric heterocyclic chitosan derivative – N-2-(4-pyridyl)ethylchitosan (4-PEC). Variation of the synthesis conditions (Table 1) enabled us to obtain derivatives with substitution degree (DS) up to 0.84.

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Table 1 Comparative structural characteristics of products of chitosan reaction with 2-vinylpyridine and 4-vinylpyridine at 70 ◦ C. No. of exp.

1 2 3 4 5 6 7

Chitosan concentration, %

Molar ratio NH2 :C7 H7 N: HCl

1:2:0.5 1:2:1 1:2:1 1:2:1 1:3:1 1:3:1 1:4:1

8 8 14 14 11 14 14

Reaction time, h

24 24 24 48 24 48 24

2-Vinylpyridine

4-Vinylpyridine

DS

DS

Content of amino groups, % mol

0.36 0.64 0.85 0.89 0.72 0.83 1.01

NH2

NHR

NR2

64 36 23 20 28 17 10

36 64 69 71 72 83 79

0 0 8 9 0 0 11

0.54 0.84 0.46 0.62 0.44 0.70 0.83

O HO

NHCOCH 3 DA

HO

HOCH2

O

a

HO

HN

CH 2 R CH 2 R:

N

N

2-PEC

4-PEC

Scheme 1. The general chemical structure of PEC derivatives.

CH2 CH 2

HO

O

O HOCH 2

NR2

54 42 46 32 44 34 45

0 21 0 15 0 18 19

The results of elemental analysis, FT-IR and 1 H NMR spectra allow attributing to PEC derivatives the following structure (Scheme 1): No significant difference was revealed in 2-PEC and 4-PEC IRspectra. Main IR bands, e.g. those at 1654, 1594, and 1477 cm−1 , are only slightly shifted that can be accounted for the difference in sample preparation and spectra recording rather than for the structural difference of derivatives. According to 1 H NMR spectra only aminogroups of chitosan are involved into addition reactions with vinylpyridines. No difference in chemical shifts in 1 H NMR spectra of 2-PEC and 4-PEC was detected in the ı range from 2.0 to 4.0 ppm and from 7.9 to 8.7 ppm. As was earlier observed for 2-PEC [22,27], the chemical shift of anomeric proton H-1 of glucosamine unit (ı 4.5–5.15 ppm) is displaced to the weak field in dependence on aminogroup substitution of isomeric derivative 4-PEC. The chemical shifts of H-1 proton in N-acetylated and deacetylated glucosamine ring are 4.56 and 4.93 ppm, respectively, which is in good

NH 2 O

NHR

44 37 54 53 56 48 36

3.2. Spectral and thermal properties of 2-PEC and 4-PEC

R O

NH2

the contents of monosubstituted amino groups ≥60 and 50% mol, respectively. Earlier we showed that selectively monosubstituted products of chitosan modification with acrylic acid in gel can be obtained only at DS below 0.6 independently of the reaction conditions [29]. This suggests the existence of intramolecular factor, which prevents complete substitution of chitosan aminogroups. Most likely, this factor is related to unavailability of a certain share of aminogroups localized in internal domains of macromolecules coiled in concentrated gels. The comparison of primary, secondary, and tertiary aminogroups ratios in 2-PEC and 4-PEC shows that 2-vinylpyridine is more selective reagent compared to more reactive 4vinylpyridine (Table 1). Thus, at high DS (>0.8) 2-PEC is a selectively monosubstituted derivative of chitosan, while 4-PEC, independently of modification conditions, contains mono- and disubstituted aminogroups in the ratio close to 1:1, and can be considered as a statistical polymer.

The reaction medium is a gel, which is held without stirring at an optimal temperature of 70 ◦ C. Lower conversion rate was observed at lower temperature, while the increase of temperature above 70 ◦ C promoted decoupling of 4-vinylpyridine and destruction of macromolecular backbone and, thus, resulted in lower DS and reduced yield of product, as was observed previously in 2-PEC synthesis [22]. Chitosan in protonated form (e.g. chitosan HCl) is required for gelation; besides, the released protons catalyze the addition reaction. One can see that the increase of molar excess of 4vinylpyridine in the reaction medium provides that of DS (Table 1). The peculiarity of the synthetic approach used is conducting the reaction between mono- and polymeric components in a physical gel. As was previously reported for the reaction of chitosan with 2-vinylpyridine [22] or carbonic acid derivatives [16,28], the gel-synthesis effect is manifested by the existence of an optimal chitosan concentration in gel, at which the highest DS is obtained [29]. In case of chitosan reaction with 4-vinylpyridine, the optimal chitosan concentration was in the range 8–10%. According to the data summarized in Table 1 (Nos. 1 and 2), the activity of 4-vinylpyridine in reaction with chitosan is higher compared to 2-vinylpyridine in diluted gels. However, with the increase of the chitosan gel concentration 2-vinylpyridine acquires higher reaction activity (Table 1, Nos. 3–7). Indeed, the gel synthesis effect is observed in gels with chitosan concentration 15% [22] and 8% (Table 1) for the reaction with 2-vinylpyridine and 4-vinylpyridine, respectively. 4-Vinylpyridine is known to manifest higher activity in reactions with sterically hindered amines [30], although its activity to a great extent depends on the nucleophile structure [31]. This fact can explain the difference in the reactivity of vinylpyridines in interaction with chitosan, which can be considered as a highly sterically hindered polyamine. One can see that the excess of 4- and 2-vinylpyridines up to 3 molar equivalents does not affect DS of the chitosan derivatives and assures obtaining selectively monosubstituted products (Table 1, Nos. 3 and 5). When vinylpyridine excess reached 4 molar equivalents, disubstituted glucosamine units were identified in the reaction products with 2-vinylpyridine and 4-vinylpyridine at

HOCH 2

Content of amino groups, % mol

R N

CH2 CH2 O

m

O HOCH2

d

S.Yu. Bratskaya et al. / International Journal of Biological Macromolecules 62 (2013) 426–432

HOCH2

O O

HO H N

N

CH2 CH2

Scheme 2. Intramolecular hydrogen bond formation in 2-PEC structure.

agreement with the literature data for chitosan and its derivatives. The chemical shifts of H-1 proton in N-mono-4-pyridylethyl and N,N-di-4-pyridylethyl glucosamine units are 5.04 and 5.12 ppm, respectively. In case of the isomeric shifts were found to be 5.18 and 5.37 ppm, respectively, showing, in comparison with 4-PEC, the signal displacement to the weak field owing to the difference in the structure of isomeric pendant groups. One of the possible reasons for this difference in 1 H NMR spectra of 2-PEC and 4-PEC is ability of 2-PEC to form intramolecular hydrogen bond (Scheme 2). Thermogravimetry analysis of 4-PEC in the form of a free base with IR-spectroscopy identification of gaseous destruction products has shown that polymer starts to lose water and CO2 at 50 ◦ C, the substantial dehydration resulted in polymer carbonization starts at 150 ◦ C (Fig. 1). Further thermal degradation associated with destruction of the side chain, 4-pyridylethyl pendant group, occurs with the

Fig. 1. TGA investigation of 2-PEC (1) and 4-PEC (2) with DS = 0.8.

1

429

formation of products similar to those obtained during thermodestruction of 2-PEC [27]: pyridine, picoline, and 4-vinylpyridine. There was no difference in products of thermal destruction of 4-PEC varying in DS. The comparison of thermal properties of 2-PEC and 4-PEC (Fig. 1) shows that the thermal stability of 2-PEC is 200 ◦ C, while that of 4-PEC is 150 ◦ C, which is lower than that of chitin and chitosan (DA = 0.84) – 350 and 284 ◦ C, respectively [32]. The possible reason for higher thermal stability of 2-PEC, as compared to its regioisomer 4-PEC, is the formation of hydrogen bond, which contributes to stabilization of the pendant group in chitosan macromolecular backbone. Thus, the presence of 2-(4-pyridyl)ethyl group in the 4-PEC structure corroborated by 1 H NMR spectroscopy resulted in significant changes in thermal properties compared to 2-PEC. 3.3. Sorption properties of 2-PEC and 4-PEC To reduce the solubility of 2-PEC and 4-PEC in acidic media for sorption application, these derivatives were cross-linked with epichlorohydrin. The comparison of cross-linking degrees (DCL) shows that 4-PEC (DCL = 0.28) is more reactive in interactions with cross-linker compared to its regioisomer 2-PEC (DCL = 0.16). This difference in reactivity in reactions of nucleophilic substitution is, most likely, related to lower sterical hindrances of 4-PEC, whose pyridine nitrogen can act as more active nucleophile center (Scheme 3), compared to the sterically hindered nitrogen of 2-pyridylethyl fragment (Scheme 2). As a result of cross-linking 2-PEC and 4-PEC with epichlorohydrine, the content of hydroxyl groups in polymers is slightly increased [33–35] (DS(OH) ≤ 0.1). Besides, in case of 4-PEC a small amount of strongly basic ammonia centers (DS ≤ 0.06) can be also formed (Scheme 3). It is known that even minor structural changes with introduction of new types of functional group after crosslinking can affect sorption capacity and selectivity of metal uptake [33,36,37]. Figs. 2 and 3 illustrate sorption properties of isomeric derivatives 2-PEC and 4-PEC with DS ∼ 0.8 toward noble and transition metal ions. Sorption isotherms were fitted with the Langmuir equation; the parameters obtained are summarized in Table 2. One can see that, as compared to plain chitosan, introduction of 2pyridylethyl and 4-pyridylethyl groups in all cases lead to the increase of the sorption capacity. However, isomeric derivatives 2-PEC and 4-PEC significantly differ in affinity toward all studied metal ions. Using the affinity coefficient, which was calculated as KL × Qmax [38], the following

2 Scheme 3. Possible structures of cross-linked fragments of 2-PEC (1, 2) and 4-PEC (1–3).

3

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Fig. 2. Sorption isotherms of noble metal ions on 2-PEC and 4-PEC from HCl solutions of different concentrations (solid lines correspond to fits of isotherms using Langmuir equation).

C(Ag), mmol/l

Ni(II) sorption, mmoI/g

Ag(I) sorption, mmoI/g

Cu(II) sorption, mmoI/g

2-PEC 4-PEC Chitosan

C(Cu), mmol/l

C(Ni), mmol/l

Fig. 3. Sorption isotherms of transition metal ions on chitosan, 2-PEC and 4-PEC from 1 M NH4 NO3 solution, pH = 5.3 (solid lines correspond to fits of isotherms using the Langmuir equation).

Table 2 Parameters of Langmuir model for metal ions sorption on chitosan and its derivatives, KL – Langmuir constant (l/mmol), Qmax – maximum sorption capacity (mmol/g). Polymer

Pt(IV) 0.1 N HCl

Chitosana 2-PEC 4-PEC a

Pd(II) 0.1 N HCl

Cu(II), 1 M NH4 NO3

Ag(I), 1 M NH4 NO3

Ni(II), 1 M NH4 NO3

KL

Qmax

R

KL

Qmax

R

KL

Qmax

R

KL

Qmax

R

KL

Qmax

R

2.31 3.44 4.92

0.55 2.70 1.61

0.98 0.86 0.90

3.52 5.42 3.21

0.89 3.65 2.68

0.98 0.98 0.98

3.67 1.25 2.23

0.34 1.55 0.63

0.96 0.92 0.97

5.94 2.59 6.52

0.445 1.34 0.477

0.85 0.99 0.95

0.51 3.18 2.30

0.32 0.48 0.32

0.82 0.97 0.90

Values for noble metal sorption on chitosan are taken from [27].

row of affinity can be summarized: 2-PEC > 4-PEC > chitosan. One can see that the sorption capacity of 2-PEC is in general 1.5–2.5 higher compared to 4-PEC that can be related to possibility of 2pyridyl derivatives to form chelates (Scheme 4). Indeed, a significant difference in sorption properties of chelating and non-chelating isomers was earlier reported for low molecular weight reagents [39] and pyridylmethyl derivatives of chitosan [20,40]. The sorption capacities increase toward all noble metals owing to the increased number of electron-donor nitrogen atoms in 2-PEC and 4-PEC. Besides, changes in sorption constants (KL ) show that introduction of pyridyl group leads to a significant increase of the stability of complexes with multivalent ions – [PtCl6 ]2− and [PdCl4 ]2− (Table 2). This fact supports the assumption that pyridyl-containing derivatives can form stable

HOCH2 O

HOCH2

O

O

O HO

HO Cu2+

NH CH2

Cl

CH2

Cl

N

Pd

Cl

NH2 CH2

CH2 Cl HN

Scheme 4. Possible chelates formation in 2-PEC structure.

quasi-chelates with anionic species of noble metals (Scheme 4) [40]. Another informative method for evaluation of the sorption mechanism and the stability of complexes formed between functional polymers and metal ions is desorption with different eluents [41]. A notable decrease of Pt(IV) and Pd(II) ions elution with HCl solution for 2-PEC and 4-PEC derivatives (Table 3) can be related to decrease of the ion-exchange contribution to sorption mechanism in comparison with plain chitosan. However, one can notice that desorption of metal species with 1 M HCl from 4-PEC is somewhat lower than that from 2-PEC showing apparently stronger interactions of Pt(IV) and Pd(II) with the former PEC isomer. This incidental contradiction can be cleared up by the dependences of sorption properties of 2-PEC and 4-PEC on HCl concentration (Fig. 2). As typically observed for chitosan and its derivatives, the sorption capacity and affinity decrease dramatically with the increase of the concentration of competitor Cl− ions [40,42,43], but 4-PEC is significantly less sensitive to the HCl concentration compared to 2-PEC (Fig. 2). The most probable reason for this behavior is not higher affinity of 4-PEC to noble metal ions, but its higher cross-linking degree, which was also evident from a significant difference in the swelling degree of 2-PEC and 4-PEC in 0.1 M HCl solution – 400% and 150%, respectively. The increase of the cross-linking degree not only reduces polymer swelling but also decreases the number of functional groups available for metal ions binding. We have earlier shown using imidazol-containing

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431

Table 3 Efficiency of Pt(IV) and Pd(II) ions desorption from chitosan and its pyridyl-derivatives. Eluent

Desorption efficiency, %

HCl (mol/l)/thiourea (mol/l)

Pt(IV)

1.0/0 0.1/0.1 0.1/0.2 0.1/0.5

74 77 77 75

Pd(II)

Chitosan ± ± ± ±

2-PEC

5 4 2 4

14 43 40 37

± ± ± ±

2 5 1 6

derivatives of chitosan cross-linked with different agents that the increase of the polymer matrix stiffness results in the decrease of sorption capacity but increase of selectivity in the presence of Cl− ions [44]. Taking into account that neither Pd(II) nor Pt(IV) can be desorbed from 2-PEC and 4-PEC by HCl solutions, one can assume that coordination of metals with chitosan functional fragments makes a significant contribution to the sorption mechanism in all cases. The data presented in Table 3 show that the efficiency of Pd(II) and Pt(IV) desorption with eluents, containing strong complexing agent for noble metals – thiourea, is significantly higher for 4-PEC indicating that this derivative forms less stable complexes compared to isomeric 2-PEC. Indeed, the Pt(IV) and Pd(II) desorption efficiency from PEC-4 ranges from 92 to 100% even at low concentration of thiourea, while the increase of thiourea concentration from 0.1 to 0.5 M has no positive effect on the desorption efficiency from 2-PEC, which remains below 50% in case of Pt(IV). Summarizing the data on noble metals sorption of 2-PEC and 4-PEC derivatives, one can conclude that 2-PEC has higher sorption capacity which results, on the one hand, from its ability to form more stable chelates with multivalent [PdCl4 ]2− and [PtCl6 ]2− ions, and, on the other hand, from its lower cross-linking degree due to its lower reactivity compared to 4-PEC. Despite lower sorption capacity, 4-PEC shows higher selectivity toward noble metal ions in the presence of Cl− ions that makes it promising for noble metals recovery from highly acidic solution for analytical preconcentration. Another advantage of 4-PEC for this application is high efficiency of platinum and palladium desorption with thiourea-containing

OH NH2

O

OH HO

+ Ag

O

Ag+

OH

HO

HO

[Ag-PEC-4(aq)] (3)

OH +

[Ag-PEC-4(aq)] (4)

NH2 HO Ag+ NH2 OH

O

O O

OH

OH OH

NH

N Ag+

HO

OH OH OH

NH

HO HO

OH

NH

N

N H2O Ag+ HO

O N

NH

HO OH

O OH

O

HO +

[Ag-Chit 2] (5)

+

[Ag-PEC-2 2] (6)

Ag+ OH2

OH OH OH

OH

18 92 91 92

NH

O

OH2 +

[Ag-PEC-2(aq)] (2)

4-PEC

6 5 4 4

N

OH

OH OH +

+

± ± ± ±

N HN

H2O

OH OH

[Ag-Chit(aq)] (1)

34 73 80 77

O

+ Ag

OH

OH

2-PEC

65 ± 9 83 ± 1 100 98 ± 2

OH

N

HN

Chitosan

5±2 100 100 100

± ± ± ±

3 3 4 4

solutions which allows metals determination after elution without sorbent decomposition. A significant difference in sorption properties of 2-PEC and 4PEC derivatives toward transition metal ions is illustrated by Fig. 3 and Table 2. The pH dependence of sorption capacity for 4-PEC was analogous to that reported previously for 2-PEC [27]: in pH range from 4 to 7.5 sorption capacity was nearly independent on pH. One can see that introduction of both 2-pyridylethyl and 4-pyridylethyl functionalities resulted in the increase of sorption capacities of chitosan toward Cu2+ , Ni2+ , and Ag+ ions. A gradual increase of the sorption capacities for Cu2+ and Ni2+ and the affinity coefficient in the row chitosan < 4-PEC < 2-PEC is expected and can be related to the increase of electron donor nitrogen atoms content in 4-PEC, as compared to chitosan, and to chelating properties of 2-PEC, as compared to non-chelating 4-PEC isomer. However, a nearly negligible increase of the 4-PEC sorption capacity for Ag+ , as compared to plain chitosan, is somewhat surprising. To elucidate the origin of this difference in affinity, we have considered two possible mechanisms of ion binding to chitosan: “bridge model”, which describes intramolecular or intermolecular coordination of metal ions with several amine groups [45], and “pendant model”, which describes binding of the metal ion to a single amine group as a pendant [46]. Although the most favorable coordination model – “bridge” or “pendant” – is still disputable, there are experimental [47] and theoretical [48,49] evidences that realization of both models is possible depending on conditions. It is worth mentioning that all theoretical studies of chitosan complexation were mainly focused on Cu(II) ion binding, whereas investigation of chitosan and its derivatives interaction with Ag(I) have been never reported.

OH

OH2

4-PEC

+

[Ag-PEC-4 2(aq)] (7)

Scheme 5. Possible structures of “pendant” (top line) and “bridge” complexes of chitosan and its derivatives with Ag+ .

OH OH

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Table 4 Relative energy (E) of formation of Ag+ complex with chitosan and its derivatives. Complex

Ag+ -chitosan Ag+ -2-PEC Ag+ -4-PEC Ag+ -4-PEC

Relative energy of complex formation, kJ/mol “Pendant” model

“Bridge” model

−1 {1} −36 {2} −7 {3} 15 {4}

−52 {5} −81 {6} −32 {7}

The geometries of studied complexes are presented above on Scheme 5: structures (1–4) and (5–7) represent “pendant” and “bridge” models, respectively. Water molecules were added to provide additional coordination sites and take into account the solvent influence. Both 2-PEC and 4-PEC have additional to chitosan coordination site N-atom of pyridyl ring but chelate-formation is possible only for 2-PEC. PEC-4 can bind Ag+ ion either with its amino or heterocyclic nitrogen atom (structures 3 and 4, respectively). Analysis of relative formation energies of complexes summarized in Table 4 leads to several important conclusions. First of all, in contrast to Cu(II) binding to chitosan, “pendant” model of complex with Ag(I) is energetically favorable only for 2-PEC derivative, in all other cases only “bridge” complexes can be formed. This explains why 2-PEC shows higher increase of sorption capacity for Ag+ ions then it could be expected from the trends observed for Cu(II) and Ni(II) uptake by all derivatives. Secondly, since the relative formation energy of Ag-4-PEC complex (structure 5, Scheme 5) is positive, only the amino group nitrogen of 4-PEC can participate in Ag+ binding that makes 4PEC and chitosan virtually indistinguishable from their sorption constants and sorption capacities (Table 2). Finally, the “bridge” model was also more favorable for Ag+ binding to 2-PEC. However, at high metal loading degrees, the share of less stable “pendant” complexes increases owing to the lack of available functional groups. This results in the decrease of 2-PEC sorption constant in comparison with chitosan and 4-PEC, although affinity coefficient (KL × Qmax ) of 2-PEC remains the highest. 4. Conclusion A new chitosan derivative – N-2-(4-pyridyl)ethylchitosan was synthesized using the “synthesis in gel” method of polymeranalogous transformations. The synthesis conditions were optimized to yield derivatives varying in the substitution degree. The comparative investigation of two isomeric derivatives – N-2(2-pyridyl)ethylchitosan (2-PEC) and N-2-(4-pyridyl)ethylchitosan (4-PEC) by 1 H NMR spectroscopy confirmed the structural difference and revealed that 2-PEC was a selectively monosubstituted derivative, while 4-PEC is a statistical polymer formed by differently substituted glucosamine units. Higher thermal stability observed for 2-PEC was related to the formation of an intramolecular hydrogen bond (Scheme 2). Changes in the pendant group structure showed a significant effect on sorption properties of PEC derivatives toward anionic species of noble metals and cations of transition metal ions. In general, the affinity of derivatives changes in the following row 2-PEC > 4-PEC > chitosan. The increase of 2-PEC sorption capacities is related to its ability to form 6-membered chelates and, additionally, to lower cross-linking degree in comparison with 4PEC cross-linked under the same conditions. Higher stability of 2-PEC complexes with Pd(II) and Pt(IV) ions was corroborated by desorption experiments using thiourea-containing eluents. DFT calculations were used to prove that difference in the preferable complexation model –“pendant” or “bridge” – also can significantly contribute to difference of sorption properties of isomeric 2-PEC and 4-PEC derivatives.

Acknowledgements The work was financially supported by a cooperative grants (12-II-UB-04-008 and 12-S-3-1003) between Ural and Far Eastern Branches of the Russian Academy of Sciences. References [1] W. Jaeger, J. Bohrisch, A. Laschewsky, Prog. Polym. Sci. 35 (2010) 511–577. [2] D.-J. Liawa, K.-L. Wang, Y.-C. Huang, K.-R. Lee, J.-Y. Lai, C.-S. Ha, Prog. Polym. Sci. 37 (2012) 907–974. [3] D. Vyprachticky, K.W. Sung, Y. Okamoto, J. Polym. Sci. A 37 (1999) 1341–1345. [4] K.-I. Kitahara, S. Okuya, I. Yoshihama, T. Hanada, K. Nagashima, S. Arai, J. Chromatogr. A 1216 (2009) 7409–7414. [5] L.C. Cesteros, J.L. Velada, I. Katime, Polymer 36 (1995) 3183–3189. [6] L.A. Belfiore, A.T.N. Pires, Y. Wang, H. Graham, E. Ueda, Macromolecules 25 (1992) 1411–1419. [7] G.K. Bekturganova, T.K. Dzhumadilov, E.A. Bekturov, Macromol. Chem. Phys. 197 (1996) 105–111. [8] B. Tamami, M. Hatam, D. Mohadjer, Polymer 32 (1991) 2666–2670. [9] R. Jayakumar, M. Prabaharan, S.V. Nair, S. Tokura, H. Tamura, N. Selvamurugan, Prog. Mater. Sci. 55 (2010) 675–709. [10] M.N.V. Ravi Kumar, R.A.A. Muzzarelli, C. Muzzarelli, H. Sashiwa, A.J. Domb, Chem. Rev. 104 (2004) 6017–6084. [11] B. Krajewska, Sep. Purif. Technol. 41 (2005) 305–312. [12] M. Rinaudo, Pham Le Dung, M. Milas, Int. J. Biol. Macromol. 14 (1992) 122–128. [13] H. Baumann, V. Faust, Carbohydr. Res. 331 (2001) 43–57. [14] K. Kurita, Mar. Biotechnol. 8 (2006) 203–226. [15] Y. Shigemasa, A. Ishida, H. Sashiwa, H. Saimoto, Y. Okamoto, S. Minami, A. Matsuhashi, Chem. Lett. (1995) 623–624. [16] A.V. Pestov, N.A. Zhuravlev, Y.G. Yatluk, Russ. J. Appl. Chem. 80 (2007) 1154–1159. [17] G. Paradossi, E. Chiessi, M. Venanzi, B. Pispisa, Int. J. Biol. Macromol. 14 (1992) 73–80. [18] Y.A. Skorik, C.A.R. Gomes, N.V. Podberezskaya, G.V. Romanenko, L.F. Pinto, Y.G. Yatluk, Biomacromolecules 6 (2005) 189–195. [19] G. Kogan, Y.A. Skorik, Toxicol. Appl. Pharmacol. 201 (2004) 303–310. [20] C.A. Rodrigues, M.C.M. Laranjeira, V.T. Favere, E. Stadler, Polymer 39 (1998) 5121–5126. [21] A.I. Gamzazade, V.M. Slimak, A.M. Skljar, E.V. Stykova, S.A. Pavlova, S.V. Rogozin, Acta Polymerica 36 (1985) 420–424. [22] A.V. Pestov, S.Y. Bratskaya, Y.A. Azarova, M.I. Kodess, Yu.G. Yatluk, Russ. J. Appl. Chem. 84 (2011) 678–683. [23] M.S. Gordon, M.W. Schmidt, Theory and Applications of Computational Chemistry, the First Forty Years, Elsevier, 2005. [24] R.H. Hertwig, W. Koch, Chem. Phys. Lett. 268 (1997) 345–351. [25] W.J. Stevens, M. Krauss, H. Basch, P.G. Jasien, Can. J. Chem. 70 (1992) 612–630. [26] H. Li, C.S. Pomelli, Theor. Chim. Acta 109 (2003) 71–84. [27] S.Y. Bratskaya, Y.A. Azarova, E.G. Matochkina, M.I. Kodess, Y.G. Yatluk, A.V. Pestov, Carbohydr. Polym. 87 (2012) 869–875. [28] A.V. Pestov, Y.A. Skorik, G. Kogan, Y.G. Yatluk, J. Appl. Polym. Sci. 108 (2008) 119–127. [29] Y.A. Skorik, A.V. Pestov, M.I. Kodess, Y.G. Yatluk, Carbohydr. Polym. 90 (2012) 1176–1181. [30] G. Oeme, I. Iovel, K. Faklyam, E. Lukevits, Chem. Heterocycl. Compd. 31 (1995) 737–744. [31] A.A. Artamonov, B.A. Rosenberg, A.K. Sheinkman, React. Methods Org. Compd. 14 (1964) 173–254. [32] F.S. Kittur, K.V.H. Prashanth, K.U. Sankar, Carbohydr. Polym. 49 (2002) 185–193. [33] W.S. Wan Ngah, C.S. Endud, R. Mayanar, React. Funct. Polym. 50 (2002) 181–190. [34] A.-H. Chen, S.-C. Liu, C.-Y. Chen, C.-Y. Chen, J. Hazard. Mater. 154 (2008) 184–191. [35] R.P. Dhakal, T. Oshima, Y. Baba, React. Funct. Polym. 68 (2008) 1549–1556. [36] T. Becker, M. Schlaak, H. Strasdeit, React. Funct. Polym. 44 (2000) 289–298. [37] K.D. Trimukhe, A.J. Varma, Carbohydr. Polym. 71 (2008) 66–73. [38] E. Guibal, N.O. Sweeney, T. Vincent, J.M. Tobin, React. Funct. Polym. 50 (2002) 149–163. [39] A.M. Goemmine, Z. Eeckhaut, Bull. Soc. Chim. Belg. 80 (1971) 605–610. [40] Y. Baba, Y. Kawano, H. Hirakawa, Bull. Chem. Soc. Jpn. 69 (1996) 1255–1260. [41] K. Fujiwara, A. Ramesh, T. Maki, H. Hasegawa, K. Ueda, J. Hazard. Mater. 146 (2007) 39–50. [42] M. Ruiz, A. Sastre, Guibal E. Pd, Sep. Sci. Technol. 37 (2002) 2385–2403. [43] S.L.A. Sopena, M. Ruiz, A.V. Pestov, A.M. Sastre, Y. Yatluk, E. Guibal, Cellulose 18 (2011) 309–325. [44] A.V. Pestov, S.Y. Bratskaya, Y.A. Azarova, Y.G. Yatluk, Russ. Chem. Bull. 61 (2012) 1078–1084. [45] S. Schlick, Macromol 19 (1986) 192–195. [46] K. Ogawa, K. Oka, Chem. Mater. 5 (1993) 726–728. [47] M. Rhazi, J. Desbrieres, A. Tolaimate, M. Rinaudo, P. Vottero, A. Alagui, Polymer 43 (2002) 1267–1276. [48] R. Lue, Z. Cao, G. Shen, J. Mol. Struct. (Theochem) 860 (2008) 80–85. [49] R. Terreux, M. Domard, C. Viton, A. Domard, Biomacromol 7 (2006) 31–37.