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Introduction: The aim of this work was to investigate the coordinative interaction of microcrystalline chitosan in aqueous solution with Ca2+ and Mg2+ ions by using the pH glass electrode potentiometry as well the IR spectrometric method. Thus, such a knowledge might be usefull for instance in finding the possibility of calcium (II) as well as Mg(II) delivery via complexes with microcrystalline chitosan, the well known biodegradable excipient. Material and methods: Microcrystalline chitosan (weight-average molecular weight = 2.105 Da, Institute of Biopolymers and Chemical Fibers, Łódź, Poland) was used in the form of hydrogel of definite polymer content (2.68 wt %) at average degree of deacetylation: 90 %. Molar mass of each constitutional unit (mer) amounted to 161 g•mol-1. Potentiometric equilibria studies were carried out by means of a Molspin automatic titration kit with combined microelectrode OSH 10-10. Overall concentration formation constants: betamlh = [MmLlHh]/[M]m[L]l[H]h were calculated by SUPERQUAD. Thin chitosan polymer films were used in IR measurements on a ATI Mattson Infinity Series FTIR spectrophotometer. Results and discussion: From among the possible formation equilibria (also formation of deprotonated species with participation of the chitosan hydroxyl group) only one of them, that is to say, the ML species has been accepted by SUPERQUAD both for Ca2+ and Mg2+. It follows from the FTIR investigations that the distinct shift of the II amide –NH band towards lower wave numbers with lowering pH from 1577 cm-1 towards 1521 and 1511, respectively, indicates bond weakening and may suggest the formation of dative bonds between the chitosan –NH2 and the metal ion. Conclusions: The experimental results confirm the ML complexation via –NH2 for both the cations. The stability constants log K = 3,35(5) with Ca(II) and log K = 3,29(6) with Mg(II) are comparable with the typical mean values reported in the literature for bioligands.

Keywords: microcrystalline chitosan, Ca(II), Magnesium, Mg(II)