FTIR spectroscopic characterization of Cu(II) coordination compounds with exopolysaccharide pullulan and its derivatives

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Abstract

Pullulan is a water-soluble, extracellular neutral polysaccharide with a linear flexible chain of α-(1  6)-linked maltotriose units, the structure of which is intermediate between pullulan and amylose structures because of the co-existence of both α-(1  6) and α-(1  4)-glycosidic linkages in single compounds. In alkali solutions Cu(II) ion forms complexes with reduced low-molar pullulan (RLMP). The metal content and the solution composition depends on pH. The complexing process begins in a weak alkali solution (pH > 7), and involves OH groups in C(2) and C(3) or C(6) pullulan monomer units (α-d-glucopyranose). Complexes of Cu(II) ion with reduced low-molar pullulan were synthesized in the water solutions, at the boiling temperature and at different pH values (7.512). Fourier-Transform Infrared (FTIR) spectroscopic data of synthesized complexes are rare in literature. FTIR spectroscopic characterization (FTIR, LNT-FTIR, ATR-FTIR, and FTIR microspectroscopy) of Cu(II) ion complexes with RLMP (Mw 6000 g mol−1) was carried out in this work. The similarities of the γ(Csingle bondH) range in a part of FTIR spectra indicate that there is no difference in the conformation of the C1 glucopyranose (Glc) unit in the RLMP and synthesized Cu(II) complexes. The complexing Cu(II) ion with RLMP in the dependence from the pH form different types of complex (pH 7–8: Cu(II)(Glc)2(H2O)2, pH 8–10: Cu(II)(Glc)2(H2O)(OH), pH 10–12: Cu(II)(Glc)2(OH)2).

Introduction

Pullulan is a linear exopolysaccharide of α-d-glucopyranose that is often described as a α-(1  6) linked polymer of maltotriose subunits. This unique linkage pattern gives pullulan with distinctive physical properties. A number of potential applications have been reported for this biopolymer as a result of its good film-forming properties; pullulan can form thin films which are transparent, oil resistant and impermeable to oxygen. Pullulan may be used as a coating and packaging material, as a sizing agent for paper, as a starch replacer in low-calorie food formulations, in cosmetic emulsions, and in other industrial and medicinal applications [1]. Pullulan is derivatized easily to control its solubility or provide reactive groups. Consequently, pullulan and its derivatives have numerous potential food, pharmaceutical, and industrial applications.

Bernier [2] isolated water-soluble polysaccharides from the cultures of Aureobasidium pullulans and reported that α-d-glucopyranose is the major product of acid hydrolysis. Based on the positive optical rotation and IR spectrum of pullulan, Bender et al. [3] concluded that the polymer is a α-glucan in which α-(1  4) linkages predominate. Subsequent studies using IR, periodate oxidation, and methylation analysis established that pullulan is essentially a linear glucan containing α-(1  4) and α-(1  6) linkages in a ratio of 2:1 [4]. Partial acid hydrolysates of pullulan include isomaltose, maltose, panose, and isopanose [5]. The discovery of the enzyme pullulanase provided a critical tool for the analysis of the structure of pullulan [6]. Pullulanase specifically hydrolyzes the α-(1  6) linkages of pullulan and converts the polymer almost quantitatively to maltotriose [7]. Based on this result, pullulan is frequently described as a polymer of α-(1  6) linked maltotriose subunits (Fig. 1).

However, pullulan can also be viewed as a polymer of panose or isopanose subunits, which may reflect the biosynthetic origins of the molecule more accurately. Indeed, a number of enzymes that produce panose or isopanose from pullulan have been described since. Catley et al. [8] established that pullulan contains maltotetraose subunits (Fig. 2) in addition to the predominant maltotriose subunits.

The frequency of maltotetraose subunits appears to vary on a strain-specific basis, from about 1% to 7% of total residues [9]. The evidence suggests that maltotetraose subunits are distributed randomly throughout the molecule [10]. Unlike the maltotriose subunits in pullulan, maltotetraose residues are substrates for many α-amylases, and it has been proposed that hydrolysis of pullulan at these sites accounts for the decrease in molecular weight commonly observed in late cultures.

Many types of carbohydrate derivatives (reduced or oxidized) have been synthesized for biomedical applications. In addition, polysaccharides such as chitin [11], chitosan [12], [13], heparin [14], [15], alginate [16], [17], inulin [18], dextran [19], [20], [21] and pullulan [22], [23], [24] have been derivatized for biomedical applications. Pullulan is a polysaccharide that has been used in a drug delivery because of its solubility and biocompatibility. In addition, although the polysaccharides have many ionic groups, both anionic and cationic, pullulan is nonionic [25]. Reduced low-molar pullulan (RLMP), was chosen as a new material for complexing, and the subsequent interactions with Cu(II) ions were investigated in this study. The complexing process begins in a weak alkaline solution (pH > 7), and involves OH groups in C(2) and C(3) or C(6) pullulan monomer units (α-d-glucopyranose). Complexes of Cu(II) ion with reduced low-molar pullulan were synthesized in the water solutions, at the boiling temperature and at different pH values, ranging from 7.5–12. Cu(II) complexes were prepared from sodium salts, and investigated in the solid state. Fourier transform Infrared spectroscopic data of synthesized complexes are rare in literature. FTIR spectroscopic characterization is now widely used to study the composition of the complex carbohydrate systems, the molecular interactions, a molecular orientation and conformational transitions of polysaccharides [26], [27], [28], [29], [30].

The major goal of this work is to use different FTIR spectroscopic techniques (FTIR, LNT-FTIR, ATR-FTIR, and FTIR microspectroscopy) as the main tools to verify the conformation and the structure of this type of ligand around the Cu(II) ions. The additional characterization was provided by pH-measurements and AAS analyses.

Section snippets

Materials

Pullulan of average molar mass 2 × 105 g mol−1 and reduced low-molar pullulan of average molar mass 6000 g mol−1 was obtained from PCI “Zdravlje Actavis Co.” (Leskovac, Serbia). NaOH, ethanol (96%) and CuCl2 × 2H2O was purchased from Merck (Darmstadt, Germany). Redistilled water from a PCI “Zdravlje Actavis Co.” (Leskovac, Serbia) was used in the preparation of all solutions.

Complex synthesis

Cu(II) ion complex synthesis with RLMP have been described in detail by Nikolić et al. [31].

Preparation of samples

For FTIR sample preparation the KBr

Results and discussion

FTIR spectroscopy opens up new possibilities for the fine structural analysis of polysaccharides and its derivatives, the establishment of the type of bonding between the elementary links and their rotational isomerism. Weak intermolecular interactions have a significant influence on the specifically valuable properties of biological molecules and polymers. We had to restrict ourselves to a few examples of wide potentialities of the method of FTIR spectroscopy in investigating the relationships

Conclusions

The complexing process begins in a weak alkali solution (pH > 7.5), and involves OH groups in C(2) and C(3) or C(6) pullulan monomer unit (α-d-glucopyranose).

A part of FTIR spectra, in the range on 1000–700 cm−1 of Cu(II) ion complexes with RLMP, indicates no influence of complexing process on the conformation change of C1 glucopyranose units.

The IR band δ(HOH) at the frequency of 1640 cm−1 indicated the existence of water molecules in a complex structure.

From LNT-FTIR it follows that

Acknowledgements

This study was supported by the Ministry of Science of the Republic of Serbia Grant TR-19048 and Grant TR-19035.

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