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

Abstract

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.