- 學位論文
以密度泛函理論探討微水合對於天門冬胺酸的影響
Microsolvation of Aspartic Acid: A DFT Study
摘要
天門冬胺酸(Asp)在氣相中的結構已經被廣泛和大量的探討過，在天門冬胺酸內部許多單鍵的旋轉下，總共有1296種可能的結構。我們利用hyperchem軟體進行分子動力學的模擬來尋找初始結構，並在wB97XD/ 6-31+G(d) 結構最佳化下找出106種穩定的結構。接著以DFT方法計算天門冬胺酸與水的複合物Asp-(H2O)n，n=1, 2, 3, 4，的結構與能量，並以微水合 (Microsolvation) 的方式探討水分子對天門冬胺酸中性和兩性離子在相對穩定性上的影響。計算結果顯示，與CCSD(T)和MP2方法比較，wB97XD/ 6-311++G(d,p)// wB97XD/ 6-31+G(d) 方法在預測相對能量能有很好的表現。而一個水分子可以使兩性離子穩定存在氣相中，四個水分子能使中性和兩性離子有相同的穩定度。
並列摘要
Systematic and extensive conformational searches of aspartic acid in gas phase have been performed. For the gaseous aspartic acid, a total of 1296 trial canonical structures were generated by allowing for all combinations of internal single-bond rotamers [1]. We use hyperchem and run the MD simulations to find initial structures. The possible structures were optimized at wB97XD/ 6-31+G(d) level and a total of 106 stable conformers were determined. Aspartic acid-water clusters were computed by DFT method to examine the effects of microsolvation on the relative stability of the zwitterionic and neutral forms of aspartic acid. wB97XD/6-311++G(d,p)//wB97XD/ 6-31+G(d) is found to yield a good prediction on relative energy compared to CCSD(T) and MP2 methods. It is found that one water molecule could make the zwitterion stable in gas phase. We predict that four water molecules can make neutral and zwitterion isoenergetic.