Effects of Peptide Backbone Amide-to-Ester Bond Substitution on the Cleavage Frequency in Electron Capture Dissociation and Collision-Activated Dissociation

Probing the mechanism of electron capture dissociation on variously modified model peptide polycations has resulted in discovering many ways to prevent or reduce $ {\text{N}} - {{\text{C}}_α } $ {\text{N}} - {{\text{C}}_α } bond fragmentation. Here we report on a rare finding of how to increase the backbone bond dissociation rate. In a number of model peptides, amide-to-ester backbone bond substitution increased the frequency of $ {\text{O}} - {{\text{C}}_α } $ {\text{O}} - {{\text{C}}_α } bond cleavage (an analogue of $ {\text{N}} - {{\text{C}}_α } $ {\text{N}} - {{\text{C}}_α } bonds in normal peptides) by several times, at the expense of reduced frequency of cleavages of the neighboring $ {\text{N}} - {{\text{C}}_α } $ {\text{N}} - {{\text{C}}_α } bonds. In contrast, the ester linkage was only marginally broken in collisional dissociation. These results further highlight the complementarity of the reaction mechanisms in electron capture dissociation (ECD) and collision-activated dissociation (CAD). It is proposed that the effects of amide-to-ester bond substitution on fragmentation are mainly due to the differences in product ion stability (ECD, CAD) as well as proton affinity (CAD). This proposal is substantiated by calculations using density functional theory. The implications of these results in relation to the current understanding of the mechanisms of electron capture dissociation and electron transfer dissociation are discussed.