Diffusional behavior of polypeptides in the thermotropic liquid crystalline state as studied by the pulse field-gradient spin-echo $H1$ nuclear magnetic resonance method

The self-diffusion coefficients (D) and $H1$ spin–spin relaxation times $(T2)$ of poly(γ-oleyl L-glutamate)(POLLG) with long oleyl side chains and poly(γ-n-octadecyl L-glutamate)(POLG) with n-alkyl side chains have been measured by the pulse field-gradient spin-echo $H1$ nuclear magnetic resonance (NMR) method and the Carr–Purcell/Meiboom–Gill (CPMG) $H1$ NMR method, respectively, over a wide range of temperatures from 30 to 80 °C, in order to elucidate the dynamic behavior of polypeptides in the thermotropic liquid crystalline state, which take the α-helical conformation. From these experiments, it was found that the diffusion coefficients of POLLG and POLG chains in the thermotropic liquid crystalline state at 60 °C are $9.97×10−7$ and $3.53×10−7 cm2/s,$ respectively, and the former diffuses faster than the latter. The diffusion coefficients of POLG in the directions parallel $(D∥)$ and perpendicular $(D⊥)$ to the α-helical chain axis were determined over a wide range of temperatures, and the $D∥$ value was found to be much larger than the $D⊥$ value. Further, from the $H1$ $T2$ experiments, it was suggested that the mobility of the side chains relates closely to the diffusion process.