Chitosan–cobalt(II) and nickel(II) chelates as antibacterial agents
Introduction
Chitosan is a powerful chelating agent which easily forms complexes with transition metals and heavy metals [1], [2], [3]. Most researches on chitosan–metal complexes focused on their applications in the sequestration or removal of metal ions, dyeing, catalysis, water treatment and other industrial processes [4], [5], [6], [7], [8], [9], [10]. However, a few researchers paid attention to their biological activities [1]. Antimicrobial activity of chitosan was observed against a wide variety of microorganisms including fungi [11], [12], [13], [14], algae [15] and some bacteria strains [1], [2], [16], [17], [18], [19]. The advantage of chitosan over other type of disinfectants is demonstrated in its higher antibacterial activity [2] plus the fact that it possesses a lower toxicity towards mammalian cells. Many attempts such as structural modification, adjustment of molecular factors and forming various derivatives [13], [20], [21], [22] have been taken up to improve the antimicrobial activity of chitosan. Interestingly, the few available research effort has shown that chitosan–metal complexes are much better antimicrobial agents than free chitosan and metal salts [23].
Metal ions such as Ag+, Cu2+, Zn2+, Co2+ and Ni2+ etc., form an important group of antimicrobial agents which have different active target from most bacteriostatic polymers [24], [25], [26], [27]. In order to explain the phenomenon of chitosan–metal complexes’ antimicrobial activity, the complex reaction between chitosan and metal ions may be described according to the Lewis acid–base theory [28]. Metal ions are referred to as “super acid” as they are stronger acceptor of electrons than H+. More so, chitosan possesses a lot of poly-cationic amines in low pH medium which interact readily with negatively charged substances such as proteins, phospholipids and fatty acid on the cell surface of bacteria, and in turn inhibit the growth of microorganisms [29]. Chitosan–metal ion chelation will increase the positive charge density of chitosan. This is expected to lead to enhanced adsorption of polycation onto the negatively charged cell surface causing enhanced growth inhibition.
This work therefore, focused on the chelation of chitosan with two closely related bivalent metal ions (Co2+ and Ni2+) and subsequent growth inhibitory studies against standard bacteria, Staphylococcus aureus ATCC 4533, S. faecalis ATCC 8043 and Escherichia coli ATCC 25923. The results of this finding which depict the effects of the metal coordination to chitosan antibacterial activity is herein reported.
Section snippets
Materials
The chitosan sample preparation (involving sample procurement, deproteinization, demineralization and N-deacetylation of the chitin) was carried out using literature methods of Adewuyi et al. [3]. The viscosity average molecular weight was determined by measuring the relative viscosity with an Ostwald viscometer. The solvent system used was 0.10 M CH3COOH/0.20 M NaCl. Molecular weight was calculated from the intrinsic viscosity based on the Mark–Houwink equation. All chemicals were of analytical
Characterization
The metal ion analysis results showed that the chelate ratios of complexes increased with metal ions, although not all the ions (Co(II) and Ni(II)) involved in chelation (Table 1). The FT-IR Spectra of chitosan, chitosan–Co and chitosan–Ni chelates are shown in Fig. 1. The amide 1 band (υCO) around 1637 cm−1 stretching frequency, characteristic of chitosan with acetylated units is present in all the spectra. However, in chitosan–metal chelates, a new band around 1625–1635 cm−1 appears. These
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2018, International Journal of Biological MacromoleculesCitation Excerpt :CS can be cross-linked easily through its reactive amino groups which enhances its mechanical strength and metal ion affinity. The metal complexes of CS are reported as better antimicrobial agents than free CS and free metal ions [8–10]. Schiff’sbases of CS have been proven to be superior in many aspects as compared to chitosan [11,12].
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