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

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(CH) 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).