Analysis of carrageenans by matrix-assisted laser desorption/ionization and electrospray ionization mass spectrometry. I. kappa-carrageenans

Matrix-assisted laser desorption/ionization (MALDI) and electrospray ioniza tion (ESI) mass spectra of small kappa-carrageenans are reported and discus sed. 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 at tachment of one Na+ ion to the carrageenan, whereas for negative ions one N at ion is detached from the sulfate group. Multiply charged species are not observed in MALDI, Intense ESI spectra can be obtained in negative ion mod e and now multiply charged species are seen. Alkali exchange experiments sh ow that in these small carrageenan anions one, but only one, alkali metal i on is bound in a bidentate coordination with two ionic sulfate groups. G2-t ype ab initio calculations on model ions HO- [M+] OH (M = Li, Na, K, Cs), a s well as arguments based on a simple Coulombic interaction model, show tha t the bidentate stabilization energy drops rapidly as the size of the alkal i cation increases. Exchange of Naf with Li+ leads to expulsion of the Nat ion generating, in ESI, intense multiply charged anions. An attempt is made to rationalize this behavior in terms of hydration effects. Copyright (C) 2001 John Wiley & Sons, Ltd.