Diffusional Behavior of Poly(β-benzyl l-aspartate) in the Rodlike, Random-Coil, and Intermediate Forms as Studied by High Field-Gradient ¹H NMR Spectroscopy

The diffusion coefficients of poly(β-benzyl l-aspartate)(PBLA) with the α-helix (rodlike) form in chloroform and the random-coil form in a mixture of chloroform and trifluoroacetic acid have been measured as a function of the PBLA concentration (C_PBLA) by using high field-gradient ¹H NMR. The PBLA concentration range for the former is from 0.11 to 9.20 wt % and for the latter it is from 0.20 to 25.2 wt %. From these experimental results, it is found that the diffusion coefficient of PBLA in the rodlike form is much smaller than that of PBLA in the random-coil form at the same PBLA concentration. This implies that the small diffusion coefficients of PBLA in the rodlike form as compared with those in the random-coil form come from the relatively large radius of gyration R_g. Further, it is found that the diffusional behavior of PBLA in the rodlike form is different in the three PBLA concentration regions, that is, region 1 (C_PBLA = 0.110.39 wt %), region 2 (C_PBLA = 0.390.86 wt %), and region 3 (C_PBLA = 0.869.20 wt %). In the random-coil form, the diffusion is different in the two PBLA concentration regions, that is, region I (C_PBLA = 0.201.03 wt %) and region II (C_PBLA = 1.0325.2 wt %). These diffusional behaviors in the rodlike form and in the random-coil form can be reasonably explained by Tinland−Maret−Rinaudo theory and de Gennes theory, respectively. Further, transitional change from the rodlike form to the random-coil form is discussed on the basis of the diffusional behavior.