Effect of L-Histidine Substitution on Sol-Gel of Transition Metal Coordinated Poly Ethyleneimine: Synthesis and Biochemical Characterization

The study was aimed to investigate the effect of chemical modification of branched poly ethyleneimine (PEI) on chelation of transition metal ions (Me2+) including Zn2+, Cu2+ or Ni2+ and sol-gel conversion thereof. To modulate chelation property of PEI, imidazole moieties were introduced into the polymer backbone by carbodiimide chemistry at different molar ratios of fmoc-protected l-histidine. The synthesis was characterized by H-1-NMR spectroscopy and size exclusion chromatography. Potentiometric titration of PEI/Me2+ aqueous dispersions showed formation of stable complexes at pH above 5 depending on the degree of l-histidine substitution. FT-IR spectroscopy showed the imidazole ring of l-histidine was involved in the coordination interactions between PEI and Me2+. Addition of Zn2+ to PEI solution induced sol-gel conversion at a critical molar ratio decreasing by a higher degree of l-histidine modification. The gelation process led to formation of stable globular nanostructures as confirmed by atomic force microscopy with projected mean diameters less than 200 nm. Cellular experiment showed that l-histidine substitution enhanced cyto-compatibility of PEI, moreover cytotoxicity decreased significantly upon coordination of Zn2+ with the polymers. Conclusively, the coordination complexes of Zn2+ and l-histidine substituted PEI could serve as a nano system for biomedical applications.