らせん高分子の水和過程における構造ゆらぎ: 水和ポリエチレンイミンモデルを用いた量子化学計算による配座効果の見積もり

摘要

ポリエチレンイミソ（PEI）の5量体水和モデルを用いた量子化学計算により，水和過程におけ る配座の会合エネルギー（Ec）に及ぼす配座効果（torsionfacter：f（の影響）の見積もりを行った．非水モデ ルとして構造最適化された4種の配座モデル［（ttt）5，（ttg＋）5，（g＋g＋g＋）5および（tg＋t）5，f．＝1．00～1．35］ を準備し，水和モデルは水和率（力）を0～1とし水分子をイミノ基に対し非連続または連続的に配置し た・島はちの増加とともに水の配置形式の影響を強く受けた．（tgt）5配座では，水和率の増加（力＞0．5）と ともにキンク（trans→gauChe）を伴う大きい構造ゆらぎが認められEcほ増加傾向を示した．Ecの計算結果 はPEI結晶の水和過程での構造転移（tgt→ttt）に関する観測結果にはば対応し，構造転移の推進力の一つ としてらせん構造の構造ゆらぎが関係することが推察された．%The effect of the chain torsion factor (f_t) on the association energy of conformer (E_c) of a helical polymer during hydration was examined using hydrated ethyleneimine oligomer models for polyethyleneimine (PEI) by quantum chemical calculations. Nonhydrate models optimized for four conformers [(ttt)_5, (ttg + )_5, (g+g + g+)_5 and (tg+t_)5, repeated units of N-C, C-C and C-N bonds. f_t= 1.00- 1.35] were prepared. In the hydrate models, water molecules were discontinuously (α) or continuously (β) located at imino groups, and hydrate ratios (h) were set from 0 to 1. Ec values were strongly affected by hydrate types with higher f_t, and those of β-type were larger than a-type. In (tgt)_5 conformers, with increasing h (h≥0.5), large structural fluctuations were found with kink conformations (trans→gauche), and the Ec values increased. These results correlate well to experimental results that show that the PEI structure transformes from helix (tgt) to planar zigzag (ttt) by hydration. It was estimated that such structural fluctuation results in molecular crowding effects, and is one of the driving forces in the structural transition of PEI.