以FTIR探討2-吡咯甲醛的氫鍵自結合、異結合及構型轉換現象

Abstract

2-吡咯甲醛透過分子間氫鍵的自結合被視為研究蛋白質中β-折疊片基本架構的模型。而β-折疊片在生化領域上與蛋白質中的二級結構α-螺旋體同樣重要。我們欲探討2-吡咯甲醛在鈍性、非極性及低極性的溶劑中，因氫鍵而形成自結合之單雙體平衡及因單體之構型轉換而形成的順反兩式平衡。於固定溫度及相同溶劑中，利用FTIR光譜在不同濃度下的順式單體、反式單體及雙體N-H吸收度的變化，代入我們新推導的四個公式，以求出順式單體吸收係數、反式單體吸收係數、雙體吸收係數、單雙體自結合平衡常數(Kd)及構型轉換平衡常數(Ki)。根據平衡常數隨溫度變化的關係，利用van't Hoff 理論作圖，可求得在不同溶劑系統下之單雙體自結合與構型轉換的標準反應焓及反應熵。並探討溶劑效應對相關熱力學參數的影響。另外我們也導証公式檢驗雙體的形式為環狀或線性雙體。最後討論2-吡咯甲醛與芳香族化合物透過氫鍵形成異結合，利用導証的公式求得錯體吸收係數及異結合平衡常數(Kx)。比較不同芳香族化合物對異結合的影響。

Intermolecular interaction for β-sheet protein belongs to the most important and topical biological binding motifs. The self-association of 2-pyrrolecarbaldehyde through hydrogen bonding provides a useful model for it. The coexistence of isomerization of cis-form and trans-form and dimerization of 2-pyrrolecarbaldehyde in n-octane, CCl4, C2Cl4 and C2HCl3 at four different temperatures have been studied by using FTIR spectroscopy. The respective integrated concentration-dependent absorbance of the NH fundamental stretching at a constant temperature in a given solvent for cis-form monomer, trans-form monomer, and dimer were resolved, calculated and then treated based on four derived linear equations (20), (21), (22) and (31) to extract the molar absorpitivity of cis-form monomer, of trans-form monomer, and of dimer, and the isomerization constant (Ki) as well as dimerization constant (Kd). Variation of ambient temperature allows us to obtain the temperature-dependent values of Ki and Kd in a given solvent, as a result the standard enthalpy ( Ho) and entropy ( So) of isomerization and dimerization were determined respectively via a van’t Hoff plot. Solvent effect on different values of K, Ho and So for isomerization and dimerization in various dielectric constants of solvents were analyzed. Whether the structure of dimer is cyclic or linear can be determined with equation (47). Finally, the hetero-association of 2-pyrrolecarbaldehyde with aromatic compounds like benzene or toluene in non-polar solvent C2Cl4 was also studied based on equation (58) to obtain the hetero-association constant (Kx) and molar absorpitivity of molecular complex. The influence of these two aromatic compounds on hetero-association with 2-pyrrolecarbaldehyde was also compared.