电喷雾质谱研究气相中环糊精与锂离子的非共价复合物

Investigation on Non-covalent Complexes of Cyclodextrins with Li＋ in Gas Phase by Mass Spectrometry

为了探索环糊精和锂离子的非共价相互作用，一定计量比的α-环糊精（α-CD）、β-环糊精（β-CD）、七-（2，6-二-氧-甲基）-β环糊精（DM-β-cD）和七-（2，3，6-三-氧-甲基）-β-环糊精（TM-β-CD）分别与氯化锂混合，在室温下反应10min使达到平衡．溶剂组成为含70％乙腈的水溶液，甲酸含量为0．1％．正离子模式下，电喷雾质谱的结果表明环糊精可以和锂离子结合，并生成1：1化学计量比的非共价复合物．碰撞诱导解离观察到一系列带有锂的环糊精的碎片，表明碰撞后复合物失去了葡萄糖单元，进一步确认了该复合物的生成．质谱滴定法测定了四种复合物Li＋α-CD、Li＋β-CD、Li＋DM-β-CD、Li＋TM-β-CD的解离常数Kd1分别为1．87×10^-5、2．67×10^-5、3．36×10^-5、3．05×10^-5mol／L．Li＋和β-CD复合物的解离常数小于Li＋和DM-β-CD复合物，其原因为部分甲基化的DM-β-CD具有较大的空间位阻．Li＋和TM-β-CD复合物的Kd1接近于Li＋和DM-β-CD复合物，表明β-CD疏水腔的大口完全甲基化的TM-β-CD，与β-CD疏水腔的大口部分甲基化的DM-β-CD与锂离子结合能力相接近，该结果意味着锂离子与DM-β-CD疏水腔的结合位点很可能位于DM-β-CD疏水腔的小口．密度泛函理论计算结果表明，-个Li＋可以和α-CD的四个相邻的氧原子结合，四个相邻的氧原子分别来自于伯羟基和糖苷键，它确认了Li＋位于α-CD疏水腔的小口．对[α-CD＋Li]＋复合物而言，一个Li＋可以通过静电作用，在小口与α-CD疏水腔作用，并形成非特异性结合的主客体复合物．

To investigate the non-covalent interaction between cyclodextrins （CD） and lithium ion, a stoichiometry of α-CD, β-CD, heptakis（2,6-di-O-methyl）-β-CD （DM-β-CD）, or heptakis（2,3,6-tri-O-methyl）-β-CD （TM-β-CD） was mixed with lithium salt, respectively, and then incubated at room temperature for 10 min to reach the equilibrium. In posi- tive mode, the electrospray ionization mass spectrometry （ESI-MS） results demonstrated that lithium ion can conjugate to α-, β-, DM-β- or TM-β-CD and form 1：1 stoichiometric non-covalent complexes. The binding of the complexes was further confirmed by collision- induced dissociation. The dissociation constants Kdl of four complexes （Li＋α-CD, Li＋β- CD, Li＋DM-β-CD, and Li＋TM-β-CD） were determined by mass spectrometric titration. The results showed Kdl were 18.7, 26.7, 33.6, 30.5 μmol/L for the complexes of Li＋ with α-CD, β-CD, DM-β-CD, and TM-β-CD, respectively. Kdl for the Li＋ complexes of/3-CD is smaller than that of DM-β-CD due to its steric effect of the partial substituted -CH3. The Kdl for the Li＋ complexes of DM-β-CD is nearly in agreement with that of TM-β-CD, indicating Li＋ is more likely to locate in the small rim of DM-β-CD＇s hydrophobic cavity. The DFT results showed through electrostatic interaction, one Li＋ can strongly conjugate to four neighboring oxygen atoms. For the （α-CD＋Li）＋ complex, one Li＋ may also situate the small rim of α-CD＇s hydrophobic cavity to form a non-specific host-guest complex.