Side-chain-dependent helical conformation of amylose alkylcarbamates: amylose tris(ethylcarbamate) and amylose tris(n-hexylcarbamate).

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 (1PrOH) to determine M(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 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. ; Macromolecules 2010 , 43 , 1061 ], in which each contour point along the chain takes loose helical and rigid helical sequences independently.

Citation

Ken Terao, Fumihiro Maeda, Keiko Oyamada, Takaaki Ochiai, Shinichi Kitamura, Takahiro Sato. Side-chain-dependent helical conformation of amylose alkylcarbamates: amylose tris(ethylcarbamate) and amylose tris(n-hexylcarbamate). The journal of physical chemistry. B. 2012 Oct 25;116(42):12714-20

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PMID: 23039368

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