- 學位論文
利用密度泛函理論討論碳鏈長度對磺基甜菜鹼的吸水性、離子親和力以及pKa的影響
Carbon Spacer Length Effects to Water Affinity、Ion Affinity and pKa of Zwitterionic Sulfobetaine by Density Functional Theory Calculations
摘要
兩性離子(zwitterionic)聚合物表面有良好的親水性，防污和抗菌性能，磷酸甜菜鹼(Phosphobetaine，PB)、羧基甜菜鹼(Carboxybetaine，CB)、與磺基甜菜鹼(Sulfobetaine，SB)皆為兩性離子材料。兩性離子自身之間的相互作用以及它們與周圍環境的相互作用皆會影響其抗汙能力，像是水合能力以及離子親和力，有研究指出，兩性離子材料的分子中亞甲基(CH2)所形成的碳鏈會隨著長度不同，抗汙能力也會有所不同。因此，本研究利用密度泛涵理論討論碳鏈長度對磺基甜菜鹼與水、離子間的相互作用力，密度泛涵選擇M06-2X、B3LYP以及ωB97X，利用溶劑化模型計算各碳鏈長度下的水合能，可以發現隨著碳鏈長度上升而上升，但在四個碳鏈長度時，並不會增加他的親水性，會有持平的趨勢，也就是說碳鏈數越多親水性並不會越好，且在偶極矩計算中也會有相同的趨勢，因此偶極矩跟溶劑自由能會有關係。除此之外，對磺基甜菜鹼做pKa計算可以發現碳鏈長度越長，pKa越高，代表長鏈容易被質子化，抗汙能力會有下降的趨勢，但SB的pKa全是負值，正常狀況下不容易產生質子化，表面呈現兩性離子狀態。在離子親和力計算下，碳鏈長度的增加對離子親和力並不會有顯著的影響，只有Mg2+在越短碳鏈下作用力會有些微下降的趨勢。而離子的結合強度為Mg2+ >Li+ >Na+ >K+ >Ca2+，可以發現Mg2+、Ca2+同為二價陽離子，但是離子親和力卻相差很多，因為電荷密度的差異導致，Mg2+的電荷密度相較於Ca2+較高會與SB表面有好的相互作用。最後，為了不依靠溶劑化模型，把15顆水分子隨機放在SB周圍，計算水與單體之間的作用力，在四個碳鏈長度時上升趨勢減少，導致有持平的趨勢發生，跟前面的結果相符合。在未來，會利用更高的基底函數去計算15顆水以及單體之間的交互作用力以及其IR光譜，還會繼續討論SB跟陰離子之間的離子效應，並且覆蓋表面的接枝密度也會影響到抗汙能力，因此兩性離子彼此間的相互作用力必須要了解。
並列摘要
Zwitterionic polymers such as phosphobetaine (PB), carboxybetaine (CB), and Sulfobetaine (SB) have great potential in biomedical applications because of good anti-fouling. The interactions between zwitterionic polymers and surrounding water or ions are crucial to their anti-fouling properties. Previous studies showed that the carbon spacer length between the positive and negative moiety changes the anti-fouling capability of zwitterionic polymers. However, detailed understandings of this phenomenon is well studied. In this study, we used the density functional theory (M06-2X, B3LYP, and ωB97X) to calculate how carbon spacer length influences the interaction of sulfobetaine with water and ions. The solvation free energy increased as the increase of the carbon spacer length and reached a plateau at four carbon spacer lengths by using SMD solvation model. The computed dipole moments of SB with various carbon space length also followed the same trends. The computation result indicates that the water affinity of SB increases with the carbon space length because of the increased dipole moments. Besides, the pKa calculation showed that the pKa of SB increased with the carbon space length, indicating that the long carbon space length is much easier being protonated than short carbon space length. The increased carbon spacer length did not have a significant impact on the ion affinity of SB. The binding affinity order was Mg2+ > Li+ > Na+ > K+ > Ca2+. The ions with high charge and small ionic radius showed strong affinities to SB. In the future, we will continue to discuss the water ordering in the SB and self-association energy to complete the correlation between the carbon space length and anti-fouling capability.