Side-Chain-Dependent Helical Conformation of Amylose Alkylcarbamates: Amylose Tris(ethylcarbamate) and Amylose Tris(<i>n</i>‑hexylcarbamate)

Eight amylose tris­(ethylcarbamate) (ATEC) samples ranging in the weight-average molar mass <i>M</i>_w from 1.0 × 10⁴ to 1.1 × 10⁶ g mol^–1 and five amylose tris­(<i>n</i>-hexylcarbamate) (ATHC) samples of which <i>M</i>_w varies from 4.9 × 10⁴ to 2.2 × 10⁶ 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 (1PrOH) to determine <i>M</i>_w, 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 <i>h</i> 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 <i>h</i> value significantly depends on the alkyl side groups in a complex fashion, that is, <i>h</i> = 0.36 nm for ATEC, <i>h</i> = 0.29 nm for ATHC, and <i>h</i> = 0.26 nm for amylose tris­(<i>n</i>-butylcarbamate) (ATBC). This is likely related to the bulkiness of side groups packed inside the amylosic helices. The solvent dependence of <i>h</i>, λ^–1, and the fraction <i>f</i>_hyd of intramolecular hydrogen bonds for ATEC can be explained by a current model as is the case with ATBC [Terao, K.; Macromolecules 2010, 43, 1061], in which each contour point along the chain takes loose helical and rigid helical sequences independently