Characterization of Starburst Dendrimers by the EPR Technique. Copper(II) Ions Binding Full-Generation Dendrimers

Abstract

Cu(II) was used as a probe to investigate the structure and ion binding ability of full-generation poly(amidoamine) starburst dendrimers ( n SBDs). Computer-aided analysis of the EPR spectra provided information on the formation of copper complexes in various internal or external locations of the dendrimers, as well as the n SBD structure as a function of the size (generation) of the dendrimers, pH, temperature, Cu(II) concentration, and aging of the samples. At low pH, Cu 2+ competes with protons for binding with external amino groups, which become available for complexation at pH > 3.5. The Cu(H 2 O) 6 2+ complexes are localized in proximity to the SBD surface, where structural modifications of the water solution prevented the occurrence of a freezing transition. A portion (about 20%) of Cu(H 2 O) 6 2+ resides in the water pools in the open structure of the earlier generation dendrimers ( G < 4) and are capable of undergoing a freezing transition. Progressive penetration of Cu 2+ ions into the SBD structure occurs with an increase in pH and gives rise to the formation of complexes with various amino groups of the SBD structure. Between pH 4 and 5, the EPR spectra clearly show the superposition of three components corresponding to (a) a Cu(H 2 O) 6 2+ complex, (b) a complex with two surface NH 2 groups, and (c) a complex with two surface NH 2 groups and two internal NR 3 groups (Cu−N 4 ). A fraction of the third component increases with an increase in generation. At pH ≥ 6 the Cu−N 4 complex is the only species present in the Cu(II)− n SBD solution. The low mobility of this complex supports the hypothesis that this complex is located in the external layers of the SBDs, which become densely packed in the later generations. At higher pH, Cu 2+ migrates to the internal SBD structure and the complexes show higher mobilities. The dendrimers decomposed upon aging and decomposition was almost complete for the earlier generation dendrimers at pH > 5.5 and at 60 days after preparation.