Structural Characterization of Natural and Synthetic Macrocycles Using Charge-Transfer Dissociation Mass Spectrometry

Research in natural products (NPs) has gained interest as drug developers turn to nature to combat problems with drug resistance, drug delivery, and emerging diseases. Whereas NPs offer a tantalizing source of new pharmacologically active compounds, their structural complexity presents a challenge for analytical characterization and organic synthesis. Of particular concern is the characterization of cyclic-, polycyclic-, or macrocyclic compounds. One example of endogenous compounds as inspiration for NP development are cobalamins, like vitamin B₁₂. An example of exogenous NPs is the class of macrolides that includes erythromycin. Both classes of macrocycles feature analogues with a range of modifications on their macrocyclic cores, but because of their cyclic nature, they are generally resistant to fragmentation by collision-induced dissociation (CID). In the present work, charge-transfer dissociation (CTD) was employed, with or without supplemental collisional activation, to produce radical-driven, high-energy fragmentation products of different macrocyclic precursors. With the assistance of collisional activation of CTnoD products, CTD frequently cleaved two covalent bonds within the macrocycle cores to reveal rich, informative spectra that helped identify sites of modification and resolve structural analogues. In a third example of macrocycle fragmentation, CTD enabled an impurity in a biological sample to be characterized as a cyclic polymer of nylon-6,6. In each example, CTD spectra are starkly different from CID and are highly reminiscent of other high-energy fragmentation techniques like extreme ultraviolet dissociative photoionization (XUV-DPI) and electron ionization-induced dissociation (EID). The results indicate that CTD-MS is a useful tool for the characterization of natural and synthetic macrocycles.