Topology-Dependent Chain Stiffness and Local Helical Structure of Cyclic Amylose Tris(3,5-dimethylphenylcarbamate) in Solution

Conformational properties of rigid and semiflexible cyclic chains are still unclear owing to few experimental researches on dilute solution properties. Five cyclic amylose tris(3,5-dimethylphenylcarbamate) (cADMPC) samples ranging in the weight-average degree of polymerization from 23 to 150 were prepared from enzymatically synthesized cyclic amylose. Light scattering and small-angle X-ray scattering measurements were made on the samples to determine the weight-average molar mass Mw, the particle scattering function P(q), and the z-average mean-square radius of gyration ⟨S2⟩z in methyl acetate (MEA), 4-methyl-2-pentanone (MIBK), and tetrahydrofuran (THF) at 25 °C. The obtained P(q) and ⟨S2⟩z data were analyzed on the basis of the cyclic wormlike chain model to determine the wormlike chain parameters, that is, the helix pitch (or helix rise) per residue h and the Kuhn segment length λ–1 (the stiffness parameter or twice of the persistence length) as a function of Mw. Although the chain stiffness parameter λ–1 for the corresponding linear polymer was reported to be 22 and 73 nm in MEA and MIBK, respectively, those for cADMPC in the three solvents were determined to be about 20 nm, this value being still significantly larger than that for cyclic amylose in aqueous sodium hydroxide. On the other hand, the former parameter h is somewhat larger than those for the linear ADMPC. The extended main chain of cADMPC by the topological constraint does not retain the chain stiffness as high as the corresponding linear chain. This phenomenon only becomes significant when the corresponding linear polymer behaves as a stiff chain with a small value of the Kuhn segment number NK. The threshold value of NK to achieve the significant difference in NK between the linear and cyclic chains is about less than 1.0–1.5, at which the probability to link the both ends (ring closure probability) of the linear wormlike chain significantly decreases with decreasing NK.