以紅外線光譜探討氮-甲基丙醯胺的自結合現象

Abstract

氫鍵對於蛋白質內胺基酸間的穩定性一直扮演著很重要的角色，但胺基酸本身很容易自結合成多體。在過去數十年間，胺基酸間氫鍵的熱力學參數並不易求得。胺基酸的衍生物之一，氮-甲基丙醯胺同樣是很容易自結合成多體的化合物，但是在高度稀釋的氮-甲基丙醯胺溶液中可視為只有單體與雙體存在。因此原本不易求得的熱力學參數能夠利用本實驗室所發展出的新方法來精確的求出。FT-IR光譜儀則是此次用來研究單雙體自結合的工具。將IR光譜中受氫鍵擾動 (N-H…O=C) 的線性雙體N-H基伸縮振動吸收鋒總吸收度數據，代入公式 (2-19) 可以得到氮-甲基丙醯胺在不同溶劑與不同溫度下的單雙體自結合平衡常數與線性雙體總莫耳吸收係數。標準焓與標準熵則利用不同溫度下的平衡常數K以van’t Hoff理論作圖求得。反式單體N-H基總莫耳吸收係數與線性雙體N-H端基總莫耳吸收係數藉由IR光譜中反式單體的N-H基伸縮振動吸收鋒總吸收度數據，代入公式 (2-36) 求得。為了確認氮-甲基丙醯胺在不同溶劑與不同溫度下的雙體是以線性雙體存在，以Am2/Ad對製備濃度[B]0作圖可發現是一條斜率為正值的直線。理論計算 (見附錄 A3) 亦表明氮-甲基丙醯胺的雙體為線性雙體。從不同溶劑下所求出標準焓、標準熵及平衡常數K可觀察到會隨著隨溶劑介電常數的上升而下降，高介電常數的溶劑明顯不利於自結合。

The hydrogen bond between amino acids in proteins plays a crucial role on its stability. In the past few decades the thermodynamic parameters for hydrogen bonding of amino acids and their derivatives have been endeavored to obtain. N-methylpropioamide, the derivatives of amino acids, is choosen for this study. In the very dilute solution, N-methylpropioamide exists trans monomer and linear dimer. And it becomes feasible to accurately study the self-association in various solvents at various temperatures. FT-IR has been employed to this purpose. Upon deconvolution of N-H stretching bands, the dimerization constant and the N-H stretching (N-H…O=C) molar absorptivity of the linear dimer N-H group can be obtained via fitting associated linear dimer N-H stretching (N-H…O=C) intergrated absorbance into equation (2-19). The enthalpy and entropy can be determined via a van’t Hoff plot from temperature-dependent dimeration constants. The N-H stretching molar absorptivities of the trans monomer N-H group and the end N-H group in linear dimer can be obtained via fitting trans monomer N-H stretching intergrated absorbances into equation (2-36). The linearity of dimer form has been confirmed by the plot of Am2/Ad against [B]0 , which appears to be linear with positive slope. Theoretical calculation (appendix A3) also indicates linearity of dimer form. Solvent effect in this study shows that as the dielectric constant of the solvent increases the dimerization constants, enthalpy and entropy will decrease.