CHARACTERIZATION OF STARBURST DENDRIMERS BY ELECTRON-PARAMAGNETIC-RESONANCE .2. POSITIVELY CHARGED NITROXIDE RADICALS OF VARIABLE CHAIN-LENGTH USED AS SPIN PROBES

Electron paramagnetic resonance (EPR) spectroscopy has been used to in vestigate the structure and binding ability of a novel class of anioni c macromolecules: half-generation poly(amidoamine) starburst dendrimer s (n.5-SBDs), which differ systematically in size (generation), and wh ich are terminated by sodium carboxylated surfaces. The half-generatio ns in the range 1.5-7.5 have been investigated. To some extent n.5-SBD s mimic both anionic micelles, in their shape and external surface, an d biomacromolecules, such as proteins and enzymes, in their internal s tructure. Positively charged nitroxide radicals attached to carbon cha ins of different lengths were used to probe hydrophilic and hydrophobi c binding sites of the SBD structure. Mobility and polarity parameters were evaluated by means of a computer-aided analysis of the EPR spect ra of the interacting radicals. The study of these parameters as a fun ction of pH demonstrated the role of electrostatic interactions in pro moting binding at the external SBD/water interface. Such interactions are found to be weaker than those occurring with anionic micelles, due to the greater hydrophilic character of the internal SBD structure. A nalysis of the EPR parameters as a function of radical chain length, g eneration, and carboxylate concentration provided evidence of both hyd rophilic and hydrophobic interactions between the probe radicals and n .5-SBDs. The results are consistent with the hypothesis that the radic al chain may enter the SBD internal structure and interact with hydrop hobic sites. Analysis of EPR spectra at different temperatures provide d the activation energies for the rotational motion of the interacting probe.