FTIR spectroscopic characterization of Cu(II) coordination compounds with exopolysaccharide pullulan and its derivatives Z ˇ . Mitic ´ a, * , G.S. Nikolic ´ b , M. Cakic ´ b , P. Premovic ´ c , Lj. Ilic ´ d a Faculty of Medicine, Department of Pharmacy, University of Niš, Bul. dr Zorana Djindjic ´a 81, RS-18000 Niš, Serbia b Faculty of Technology, University of Niš, RS-16000 Leskovac, Serbia c Laboratory for Geochemistry, Cosmochemistry and Astrochemistry, University of Niš, RS-18000 Niš, Serbia d Pharmaceutical and Chemical Industry ‘‘Zdravlje Actavis Co.”, RS-16000 Leskovac, Serbia article info Article history: Received 3 October 2008 Accepted 12 January 2009 Available online 20 January 2009 Keywords: FTIR spectroscopic techniques Cu(II) ion Coordination compounds Exopolysaccharide pullulan abstract Pullulan is a water-soluble, extracellular neutral polysaccharide with a linear flexible chain of a-(1 ? 6)- linked maltotriose units, the structure of which is intermediate between pullulan and amylose structures because of the co-existence of both a-(1 ? 6) and a-(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 (a-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 (M w 6000 g mol À1 ) was carried out in this work. The similarities of the c(CAH) range in a part of FTIR spectra indicate that there is no difference in the conformation of the C 1 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 (H 2 O) 2 , pH 8–10: Cu(II)(Glc) 2 (H 2 O)(OH), pH 10–12: Cu(II)(Glc) 2 (OH) 2 ). Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Pullulan is a linear exopolysaccharide of a-D-glucopyranose that is often described as a a-(1 ? 6) linked polymer of maltotriose subunits. This unique linkage pattern gives pullulan with distinc- tive physical properties. A number of potential applications have been reported for this biopolymer as a result of its good film-form- ing 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 emul- sions, and in other industrial and medicinal applications [1]. Pullu- lan 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 cul- tures of Aureobasidium pullulans and reported that a-D-glucopyra- nose 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 a-glucan in which a- (1 ? 4) linkages predominate. Subsequent studies using IR, perio- date oxidation, and methylation analysis established that pullulan is essentially a linear glucan containing a-(1 ? 4) and a-(1 ? 6) linkages in a ratio of 2:1 [4]. Partial acid hydrolysates of pullulan include isomaltose, maltose, panose, and isopanose [5]. The discov- ery of the enzyme pullulanase provided a critical tool for the anal- ysis of the structure of pullulan [6]. Pullulanase specifically hydrolyzes the a-(1 ? 6) linkages of pullulan and converts the polymer almost quantitatively to maltotriose [7]. Based on this re- sult, pullulan is frequently described as a polymer of a-(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 maltotet- raose 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 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2009.01.019 * Corresponding author. Tel./fax: +381 18 238 770. E-mail address: zak_chem2001@yahoo.com (Z ˇ . Mitic ´). Journal of Molecular Structure 924–926 (2009) 264–273 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc