Guest-to Host Proton Transfer in Melatonin—β-cyclodextrin Inclusion Complex by Ionspray, Fast Atom Bombardment and Tandem Mass Spectrometry

Ionspray (IS) and fast atom bombardment (FAB) positive ionisation mass spectrometry (MS) of 1:1 beta -cyclodextrin (beta -CD)-melatonin (MLT) host-guest complex allowed the detection of gaseous protonated 1:1 beta -CD-MLT. Tandem MS collision-induced dissociation (CID} of such protonated 1:1 beta -CD-MLT species showed the proton (charge} to be retained to a significant extent by the host and by its cage fragmentation products, in spite of the higher proton affinity of MLT with respect to that of beta -CD. This requires an endothermic guest-to-host proton transfer to occur within the gaseous association. Collisional activation could be accounted for by the promotion of such an endothermic process; however, the proton affinity decrease of the guest determined by the loss of the elements of acetamide, which is a dominant MS dissociation reaction of pure protonated MLT, could also provide a rationale for such an endothermic guest-to-host proton transfer. This proposal parallels the reaction scheme we had previously formulated for the analogous MS and tandem MS behaviour of 1:1 beta -CD-5-methoxytryptamine inclusion complex with the protonated 5-methoxytryptamine guest undergoing deamination.