Cyclic Peptide–Polymer Conjugate Characterization Using 193 nm Ultraviolet Photodissociation Tandem Mass Spectrometry

Cyclic peptide-polymer conjugates offer a unique biocompatible system with many advantages but come at the cost of being analytically challenging. Developing further analytical techniques of complex polymer-conjugate systems is key to understanding synthetic and medicinal properties. In this contribution, a synthetic cyclic peptide-polymer conjugate is analyzed using electron capture dissociation (ECD), infrared multiphoton absorption dissociation (IRMPD), and 193 nm ultraviolet photodissociation (UVPD) on the same mass spectrometry system. IRMPD and UVPD were shown to effectively characterize unconjugated cyclic peptide species. ECD was less informative during cyclic peptide analysis due to the production of multiple sequence scrambling fragments and radical side chain losses. ECD was shown to produce extensive fragmentation and enable the characterization of conjugated side chains of cyclic species. ECD and IRMPD thus provided complementary data, enabling the target analysis of conjugated systems. UVPD effectively characterized both the cyclic peptide and the conjugating polymer in one experiment, being able to produce complete cyclic peptide fragmentation via <i>b</i>/<i>y</i> fragment pathways and polymer fragmentation via <i>a</i>/<i>x</i> poly(2-ethyl-2-oxazoline) fragment pathways.