Diffusional Behavior of Polypeptides in the Isotropic, Biphasic, and Liquid Crystalline Phases As Studied by the Pulse Field-Gradient Spin−Echo 1H NMR Method

The self-diffusion coefficients (D) of α-helical poly(n-octadecyl l-glutamate) (POLG) and chloroform (CHCl3) as solvent in the isotropic, biphasic, and liquid crystalline phases were measured as a function of POLG concentration at 30 °C by the pulse field-gradient spin-echo 1H NMR method and the diffusion 1H NMR imaging method, to elucidate diffusional behavior of the polypeptide and solvent. From the experimental results, it was found that the diffusion coefficient of POLG is largely decreased in going from the isotropic phase to the liquid crystalline phase, and in the biphasic phase two kinds of fast and small diffusion components with diffusion coefficients of ∼10-6 and ∼10-7 cm2/s, respectively, exist, which correspond to the isotropic and liquid crystalline regions, respectively. Further, it was found that in the liquid crystalline phase the diffusion coefficients of chloroform as solvent in the liquid crystalline phase in the directions parallel (D ∥) and perpendicular (D ⊥) to the α-helical chain axis are different from each other and then D ∥ is much larger than D ⊥.