α-氨基酸及其酯化物侧链对其β-环糊精复合物稳定常数的影响

摘要

为了探索α-氨基酸及其酯化物的侧链R基团对其与环糊精非共价复合物结合强度的影响, 将一定摩尔比的β-环糊精(β-CD)分别与L型正缬氨酸(n-Val)、 亮氨酸(Leu)、 苯丙氨酸(Phe)、 天冬氨酸(Asp)、 天冬氨酸-4-苄酯(Asp-4-benzyl ester)和天冬氨酸-4-叔丁酯(Asp-4-t-butyl ester)在室温下混合, 反应平衡后采用电喷雾电离质谱进行竞争反应检测, 并以改进的质谱滴定结合曲线拟合法计算结合常数.结果表明, 它们均可形成摩尔比为1∶1的非共价复合物.在2组竞争反应中, 复合物的结合强度顺序分别为[β-CD∶Asp-4-benzyl ester+H]+>[β-CD∶Asp-4-t-butyl ester+H]+>[β-CD∶Asp+H]+以及[β-CD∶Phe+H]+>[β-CD∶Leu+H]+> [β-CD∶n-Val+H]+. 质谱滴定曲线拟合法测得[β-CD∶n-Val+H]+, [β-CD∶Asp+H]+, [β-CD∶Asp-4-t-butyl ester+H]+, [β-CD∶Asp-4-benzyl ester+H]+, [β-CD∶Leu+H]+和[β-CD∶Phe+H]+的稳定常数(lgKst)分别为1.81, 2.54, 3.14, 3.26, 3.36和3.67, 结合强度依次增强.竞争反应的定性分析结果与质谱滴定定量法测得结合强度结果的趋势一致.由于所选用的α-氨基酸及其酯化物客体的羧基端 (-COOH)和氨基端 (-NH2)均相同, 且都为亲水基团, 仅有侧链R基团不同, 因此在溶液中客体分子受疏水驱动与β-CD主体靠近并结合时, 侧链R基团的疏水力和极性2个因素起重要作用.由于客体分子体积小, 其碳端的羧基还可与β-CD大口或小口边缘的羟基形成氢键, 使复合物更加稳定.%For exploring effects of side chain R groups on binding strength of non-covalent complexes of β-cyclodextrin(β-CD) with α-amino acids and esters, a stoichiometry of β-CD with L-Norvaline(n-Val), L-Phenylalanine(Phe), L-Leucine(Leu) L-Aspartic acid(Asp), L-Aspartic acid-4-tert-butyl ester(Asp-4-t-butyl ester) and L-Aspartic acid-4-benzyl ester(Asp-4-benzyl ester) were mixed respectively and incubated at room temperature for 12 h to reach the equilibrium.In positive mode, electrospray ionization mass spectrometry(ESI MS) results indicated that β-CD could form non-covalent complexes with n-Val, Phe, Leu, Asp, Asp-4-t-butyl ester and Asp-4-benzyl ester, respectively.By comparison, two group competition reactions showed that the binding strength order were [β-CD∶Asp-4-benzyl ester+H]+>[β-CD∶Asp-4-t-butyl ester+H]+>[β-CD∶Asp+H]+ and [β-CD∶Phe+H]+>[β-CD∶Leu+H]+>[β-CD∶n-Val+H]+, respectively.The stability constants of the complexes of β-CD with α-amino acids and esters were determined via an improved mass spectrometric titration coupled with curve fitting methodology.The obtained lgKst values were 1.81, 2.54, 3.14, 3.26, 3.36, 3.67 for [β-CD∶n-Val+H]+, [β-CD∶Asp+H]+, [β-CD∶Asp-4-t-butyl ester+H]+, [β-CD∶Asp-4-benzyl ester+H]+, [β-CD∶Leu+H]+ and [β-CD∶Phe+H]+, respectively.The obtained lgKst values further confirmed competition reactions results.Because all α-amino acids or esters have the same C-termini(-COOH) and the N-termini(-NH2), which are both hydrophilic, and different side chain R group.It can be concluded that the guest molecules were driven to approach β-CD by the hydrophobic force in the formation of the complexes in solution.Therefore, hydrophobic effect and polarity of the side chain R group can play an important role in the formation of the inclusion complexes.Moreover, the formed inclusion complexes can be stabilized by hydrogen-bonding interactions.