Ackloo S, Terlouw J K, Ruttink P J, Burgers P C
Department of Chemistry, McMaster University, Hamilton, Ontario, L8S 4M1, Canada.
Rapid Commun Mass Spectrom. 2001;15(14):1152-9. doi: 10.1002/rcm.351.
Matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) mass spectra of small kappa-carrageenans are reported and discussed. MALDI spectra can be obtained in both positive and negative ion mode. In the absence of extraneous metal ions, positive ions are formed by the attachment of one Na(+) ion to the carrageenan, whereas for negative ions one Na(+) ion is detached from the sulfate group. Multiply charged species are not observed in MALDI. Intense ESI spectra can be obtained in negative ion mode and now multiply charged species are seen. Alkali exchange experiments show that in these small carrageenan anions one, but only one, alkali metal ion is bound in a bidentate coordination with two ionic sulfate groups. G2-type ab initio calculations on model ions HO(-) [M(+)] (-)OH (M = Li, Na, K, Cs), as well as arguments based on a simple Coulombic interaction model, show that the bidentate stabilization energy drops rapidly as the size of the alkali cation increases. Exchange of Na(+) with Li(+) leads to expulsion of the Na(+) ion generating, in ESI, intense multiply charged anions. An attempt is made to rationalize this behavior in terms of hydration effects.
报道并讨论了小κ-卡拉胶的基质辅助激光解吸/电离（MALDI）和电喷雾电离（ESI）质谱。MALDI谱可在正离子和负离子模式下获得。在没有外来金属离子的情况下，正离子是通过一个Na(+)离子与卡拉胶结合形成的，而负离子则是一个Na(+)离子从硫酸根基团上脱离。在MALDI中未观察到多电荷物种。在负离子模式下可获得强ESI谱，且现在能看到多电荷物种。碱交换实验表明，在这些小的卡拉胶阴离子中，一个但只有一个碱金属离子以双齿配位的方式与两个离子硫酸根基团结合。对模型离子HO(-)M(+)OH（M = Li、Na、K、Cs）进行的G2型从头算计算，以及基于简单库仑相互作用模型的论证表明，随着碱金属阳离子尺寸的增加，双齿稳定能迅速下降。用Li(+)交换Na(+)会导致Na(+)离子被逐出，在ESI中产生强多电荷阴离子。试图从水合作用的角度对这种行为作出合理的解释。
