Guest-to-host proton transfer in melatonin-beta-cyclodextrin inclusion complex by ionspray, fast atom bombardment and tandem mass spectrometry

Ionspray (IS) and fast atom bombardment (FAB) positive ionisation mass spec trometry (MS) of 1:1 beta -cyclodextrin (beta -CD)-melatonin (MLT) host-gue st complex allowed the detection of gaseous protonated 1:1 beta -CD-MLT. Ta ndem MS collision-induced dissociation (CID} of such protonated 1:1 beta -C D-MLT species showed the proton (charge} to be retained to a significant ex tent by the host and by its cage fragmentation products, in spite of the hi gher 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 a ssociation. Collisional activation could be accounted for by the promotion of such an endothermic process; however, the proton affinity decrease of th e guest determined by the loss of the elements of acetamide, which is a dom inant MS dissociation reaction of pure protonated MLT, could also provide a rationale for such an endothermic guest-to-host proton transfer. This prop osal parallels the reaction scheme we had previously formulated for the ana logous MS and tandem MS behaviour of 1:1 beta -CD-5-methoxytryptamine inclu sion complex with the protonated 5-methoxytryptamine guest undergoing deami nation. Copyright (C) 2001 John Wiley & Sons, Ltd.