Side-Chain-Dependent Helical Conformation of Amylose Alkylcarbamates: Amylose Tris(ethylcarbamate) and Amylose Tris(n-hexylcarbamate)

摘要

Eight amylose tris(ethylcarbamate) (ATEC) samples ranging in the weight-average molar mass M_w from 1.0 × 10~4 to 1.1 × 10~6 g mol~(-1) and five amylose tris(n-hexylcarbamate) (ATHC) samples of which M_w varies from 4.9 × 10~4 to 2.2 × 10~6 g mol~(-1) have been prepared from enzymatically synthesized amylose samples having narrow dispersity indices and no branching. Small-angle angle X-ray scattering (SAXS), light scattering, viscometry, and infrared (IR) absorption measurements were carried out for their dilute solutions, that is, ATEC in tetrahydrofuran (THF), 2-methoxyethanol (2ME), methanol (MeOH), and ATHC in THF and 1-propanol (IPrOH) to determine Mw, particle scattering functions, intrinsic viscosities, and IR spectra. SAXS and viscosity measurements were also made on ATEC in d- and l-ethyl lactates. The data were analyzed in terms of the wormlike cylinder model to estimate the helix pitch (or contour length) per residue h and the Kuhn segment length λ~(-1) (stiffness parameter, twice the persistence length). Both ATEC and ATHC have large λ~(-1) in THF, that is, 33 and 75 nm, respectively, and smaller λ~(-1) were obtained in alcohols, indicating that they have rigid helical conformation stabilized by intramolecular hydrogen bonds in THF. On the contrary, the helical structure estimated from the h value significantly depends on the alkyl side groups in a complex fashion, that is, h - 0.36 nm for ATEC, h = 0.29 nm for ATHC, and h - 0.26 nm for amylose tris(n-butylcarbamate) (ATBC). This is likely related to the bulkiness of side groups packed inside the amylosic helices. The solvent dependence of h, λ~(-1), and the fraction f_(hyd) of intramolecular hydrogen bonds for ATEC can be explained by a current model as is the case with ATBC [Terao, K.; et al. Macromolecules 2010, 43, 1061], in which each contour point along the chain takes loose helical and rigid helical sequences independently.