Relative binding affinities of molecular capsules investigated by ESI-Mass spectrometry.
ESI-MS(/MS) was used as a method which allows the fast, unambiguous and sensitive simultaneous detection and relative stability approxn. of supramol. assemblies in mixts. In spite of the obvious fundamental differences between soln. and gas phase, ESI-MS in the case of self-assembled mol. capsules was shown to produce very similar results to single binding expts. monitored by NMR titrns. as well as conformational searches performed by Monte-Carlo simulations. MS/MS expts. reveal the same relative order of gas phase stabilities as previously found in soln. Also, proton transfer reactions which lead to new mol. capsules, are not detectable in the time-averaged NMR spectrum. However, the newly produced species are found in the complex mixts. by ESI-MS and can be conveniently characterized by subsequent MS/MS expts.: in a collision-induced dissocn. the single half-spheres are easily discovered and structurally assigned. Thus, ESI-MS has worked as a valuable tool for the rapid screening of complex supramol. mixts. and in combination with MS/MS expts. elucidated both the path of unexpected side reactions as well as the thermodn. gas-phase stabilities of all components in the mixt.
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