Characterization of the Oxazolone and Macrocyclic Motifs in the b n ( n = 2-5) Product Ions from Collision-Induced Dissociation of Protonated Oligoglycine Peptides with Isomer-Selective, Cryogenic Vibrational Spectroscopy.

Collision-induced dissociation (CID) of small, protonated peptides leads to the formation of b-type fragment ions that can occur with several structural motifs driven by different covalent intramolecular bonding arrangements. Here, we characterize the so-called "oxazolone" and "macrocycle" b _n ion structures that occur upon CID of oligoglycine peptides (G _n ) ions ( n = 2-6). This is determined by acquiring the vibrational band patterns of the cryogenically cooled, D ₂ -tagged b _n ions obtained using isomer-selective, two-color IR-IR photobleaching and analyzing them with predicted (DFT) harmonic spectra for the candidate structures. Both oxazolone and macrocyclic isomers are formed by b ₄ , whereas only oxazolone species are created for b ₂ and b ₃ and the macrocycle is created for b ₅ . As such, n = 4 corresponds to the minimum size where both Oxa and MC forms are present.