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

Abstract

Conformational properties of rigid and semiflexible cyclic chains are still unclear owing to few experimental researches on dilute solution properties. Five cyclic amylose tris(3,5dimethylphenylcarbamate) (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 zaverage mean-square radius of gyration S 2 z in methyl acetate (MEA), 4-methyl-2-pentanone (MIBK), and tetrahydrofuran (THF) at 25 C. The obtained P(q) and S 2 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  −1 (the stiffness parameter or twice of the persistence length) as a function of Mw. Although the chain stiffness parameter  −1 for the corresponding linear polymer was reported to be 22 nm 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.

Linear and cyclic amylose derivatives having brush like side groups in solution: Amylose tris(n-octadecylcarbamate)s

“…While the shape at high q region of each sample is substantially similar to that for the corresponding linear chain, a significant peak is found at the low-q range only for the cyclic chain. Similar tendency was also found for other cyclic amylose carbamates [22][23][24]37].…”

“…While P(q) of the wormlike ring cannot be calculated analytically, Tsubouchi et al [64] developed a Monte Carlo simulation method to calculate the particle scattering function Pc(q) of thin wormlike ring. Furthermore, if the chain thickness is taken into account by the touched-bead model as follows, ( ) (9) we reported that the resultant P(q) successfully reproduced the experimental data for the three amylose derivatives, that is, cATPC, cATBC, and cADMPC [24,44]. A curve fitting procedure was examined assuming log-normal molar-mass distribution with Ð = 1.05 and 1.20.…”

“…They only reproduce the S 2 z data for high Mw region and deviate upward with lowering Mw in MHK and in MTBE, suggesting that the wormlike chain parameters of the cyclic chain are different from those for the corresponding linear chain and/or they may depend on the chain length as depicted in our previous reports. [24,37,44] It should be noted that the chain thickness effect is insignificant in the Mw range investigated if we estimate it as the touched-bead model of which the contribution can be estimated as S 2  = S 2 c + 3db 2 /20 with db being the bead diameter estimated from the P(q) described below.…”

“…Meanwhile, we recently prepared three kinds of cyclic amylose carbamate derivatives with phenyl [22], n-butyl [23], and 3,5-dimethylphenyl [24] side groups from enzymatically synthesized cyclic amylose (cESA), which has substantially no linear contamination [25,26].…”

“…On the contrary, cyclic amylose tris(phenylcarbamate) (cATPC) [22,37], cyclic amylose tris(n-butylcarbamate) (cATBC) [23], and cyclic amylose tris(3,5dimethylphenylcarbamate) (cADMPC) [24] have relatively high chain stiffness in solution since the corresponding linear amylose derivatives behave as semiflexible or rigid chain of which chain stiffness depends significantly on the intramolecular hydrogen bonds [38][39][40][41] (Hbonds) and/or H-bonding interactions between polymer and solvent molecules [42,43]. A further surprising finding was that the chain stiffness and/or the local helical structure are not always the same as the corresponding linear chain [24,37,44]. It is however still unclear whether this topologically originated conformational difference depend on the chemical structure of the side groups.…”