Ping-Pong Protons: How Hydrogen-Bonding Networks Facilitate Heterolytic Bond Cleavage in Peptide Radical Cations

Electron
capture and electron transfer dissociation (ECD/ETD) tandem
mass spectrometry (MS/MS) are commonly employed techniques for biomolecular
analysis. The ECD/ETD process predominately cleaves N–Cα peptide backbone bonds, leading to primary sequence
information complementary to other mass spectrometry techniques. Despite
frequent laboratory use, the mechanistic underpinnings surrounding
N–Cα bond cleavage remain debated. While the
majority of mechanisms assume a homolytic bond rupture, we recently
showed that heterolytic cleavage is also thermodynamically viable.
For a cleavage of this type to be feasible, the charge separation
created upon breaking of the N–Cα backbone
bond must be quickly annihilated. In this work, we show, using density
functional computations, that specific hydrogen-bonding motifs and
structural rearrangements involving proton transfers stabilize the
transition state associated with heterolytic cleavage and eliminate
the ensuing charge separation from the final product fragments. The
movement of protons can occur either directly from the z- to c-fragment or in a more complex manner including
a ping-pong-type mechanism. The nature of these diverse hydrogen-bonding
motifs reveals that not only those functional groups proximate to
the bond rupture site, but also the entire global chemical environment,
play important roles in backbone cleavage characteristic of ECD/ETD
MS/MS. For doubly charged systems, both conformation and electron
localization site dictate which of the two fragments retains the final
positive charge.