Investigation of the characteristics, synthesis methods, and structural-phase properties of zinc complex compounds

Abstract

This study investigated various methods for obtaining zinc complexes with amino acids, including Zn(Gly)₂ synthesized in aqueous medium and Zn(Pro)₂ synthesized both in aqueous and organic media. The elemental composition of the obtained compounds was determined using SEM-EDX analysis, which revealed the presence of reaction precursors in the compounds synthesized with ZnCl₂ as a starting material. X-ray structural analysis confirmed the formation of phase-pure products containing only minor amounts of precursor impurities. FTIR spectroscopy enabled the identification of functional groups, and the spectra indicated that hydrogen bonding with oxygen was replaced by coordination bonding between zinc and oxygen. Furthermore, shifts in the wave numbers of the bands provided evidence for the formation of coordination bonds between zinc and nitrogen atoms. SEM images allowed for the assessment of particle shape and size, revealing that the zinc–proline complex synthesized in organic medium had significantly smaller particles than those synthesized in water. Thermogravimetric analysis showed the presence of crystallization water in the Zn(Gly)₂ compound, whereas the Zn(Pro)₂ complex was anhydrous. The Zn(Gly)₂ complex was incorporated as a preservative into a commercial cosmetic serum. The zinc–glycine complex demonstrated effective antimicrobial protection of the cosmetic product against Pseudomonas aeruginosa, Staphylococcus aureus, E. coli, and Candida albicans, even at a minimum concentration of 0.3%. However, no significant activity was observed against Aspergillus brasiliensis, indicating either no reduction or insufficient reduction of fungal colonies. These results suggest the potential for developing preservative-free cosmetic products by adding an additional skin-beneficial ingredient with fungicidal properties to the zinc–glycine-based cosmetic compositions.