Rationale: Electrospray ionization (ESI) of an aqueous solution of α-cyclodextrin (α-CD) gave a protonated molecule, [α-CD + H] . The fragmentation behavior of the protonated molecule, including direct decomposition and ring cleavage, was investigated by varying the source fragmentor voltage. The possible chemical structures of the product ions were also examined using electronic structure calculations.
Methods: An aqueous α-CD solution was ionized by ESI and the source fragmentor voltage was varied to examine changes of the product ions depending on collision energy. The structures and energies of the precursor and product ions were obtained by electronic structure calculations using Spartan'10 and Gaussian09.
Results: The major product ions were [M + H - 162m] (where m = 1-5), with the most abundant being [M + H - 162 × 4] . The product ions had two chemical structures of the cationic site in the ether linkage ([DPDn - OH] ) and in the terminal oxonium ion ([DPn - OH] ) formed by direct decomposition of [α-CD + H] and fragmentation of the open structure ([DP6 - OH] ), respectively. The [DP6 - OH] ion is more stable than the [α-CD + H] ion.
Conclusions: The fragmentation behavior of protonated α-CD was characterized by two pathways: direct decomposition of [α-CD + H] and decomposition of the [DP6 - OH] open structure. Differences in the relative abundances of product ions were explained by the different fragmentation pathways of [α-CD + H] and [DP6 - OH] .
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