Mass spectrometric behavior of functionalized calix[4]arenes: the screening ability of host–guest complex formation with amino acid methyl esters

The ESI–MS and MS/MS behavior of functionalized
calix[4]arenes (1–5) has been studied in both
positive and negative-ion mode. Liquid chromatography
coupled to ESI–MS has been successfully used for separation
of the byproducts issuing from the functionalization
pathways, through the application of a simple reversed
phase mechanism. The ability of (1–5) to host methyl esters
of amino acids, tyrosine, tryptophan, phenylalanine, cysteine,
valine, serine, leucine, isoleucine, and threonine has
been evaluated by means of MS identification of the host–
guest resulting in protonated molecular ions. The direct
infusion within the ESI source of the solutions containing
the two partners (i.e., calixarene and amino acid derivative)
could act as a fast screening means for the evaluation of
hosting capability. Only positive ionization may offer
information about the host–guest complexes being formed.
The influence of the excess of a partner in the infused
solution strongly alters ionization yields, making quantitative
approaches meaningless. Attempts to chromatographically
isolate the host–guest complexes failed, probably due
to the fact that interactions of the partners with the mobile
and stationary phases are higher than the inclusion interactions.
Structures consisting of combined fragments of the
host–guest partners resulting from the collisional induced
dissociation have not been observed.