Hydration structure and dynamics in pullulan aqueous solution based on 1H NMR relaxation time

Abstract

Dynamic properties of water and polymer chain proton in pullulan/H 2O systems in aqueous solutions and in frozen states were analyzed based on T 2 relaxation times in 1H-NMR and DSC. Two relaxing species with different T 2 detected in the CPMG pulse sequence were assigned to inert polymer protons with the shorter T 2 and to water protons with the longer T 2 by using deuterated pullulan solutions in D 2O. It has been proved that hydration water and free water undergoes rapid exchange in pullulan aqueous solutions. In the frozen state at −11°C, protons in ice crystals (T 2∼17 μs), protons in mobile water ( T 2>ms) and inert and labile protons in polymer chains (T 2∼0.1 ms) were distinguished in FID curves measured by using the solid echo pulse sequence. With increase in temperature, the inert protons and the labile protons show different mobility, and the inert protons are separately observable from the labile protons in the Carr–Percell–Meiboom–Gill (CPMG) method at temperatures higher than 0°C and the labile protons become mixed with water protons by rapid chemical exchange. These findings indicate the extremely high flexibility of the pullulan chain in aqueous solution.