Ag⁺ labeling: A convenient new tool for the characterization of hydrogen-bonded supramolecular assemblies by MALDI-TOF mass spectrometry

Herein we describe our results on the characterization of a wide variety of different hydrogen-bonded assemblies by means of a novel matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique with Ag⁺ labeling. The labeling technique with Ag⁺ ions is extremely mild and provides a nondestructive way to generate charged assemblies that can be detected by mass spectrometry. Up to now more than 25 different single (1₃·2₃), double (3₃·2₆), and tetrarosettes (4₃·2₁₂) have been successfully characterized by the use of this method. The success of the method entirely depends on the presence of a suitable binding site for the Ag⁺ ion. A variety of functionalities has been identified that provide strong binding sites for Ag⁺, either acting in a cooperative way (π-arene and π-alkene donor functionalities) or individually (cyano and crown ether functionalities). The method works well for assemblies with molecular weights between 2000 and 8000 Da, and most likely far beyond this limit.