Effect of pH on Water Proton NMR Relaxation in Agmatine-Containing Poly(amidoamine) Hydrogels

Transverse and longitudinal relaxation times of water protons were measured and magnetization transfer experiments were performed on cross-linked amphoteric poly(amidoamine)s containing different proportions of agmatine side groups swollen in buffer media with pH values in the 1.4-8.5 range in order to obtain information on water/polymer interactions as a function of pH and polymer composition. The experimental results indicated that water interacts with polymer chains mainly via hydrogen bonding to carboxylate groups and via proton exchange with amino groups. The introduction of agmatine in the polymer disfavors water/polymer interactions possibly because of polymer structural changes and competing electrostatic interactions with carboxylate groups.