Theoretical study of through-space and through-bond electron transfer within positively charged peptides in the gas phase

As part of an on-going effort to probe mechanisms for disulfide and backbone N–Cα cleavage under electron capture or electron-transfer dissociation mass spectroscopy conditions, theoretical simulations have been carried out to consider the probabilities thata.an electron initially attached to a protonated amine site on a side chain can migrate (through-bond or through-space) to an S–S σ* orbital and thus cause disulfide cleavage;b.an electron initially attached to a protonated site might be transferred (through-bond or through-space) to another protonated site or to a fixed-charge positive site thus allowing the electron to migrate throughout charged sites in a multiply charged peptide.The primary findings of this work include:c.charged-site to S–S σ* orbital through-bond electron transfer can occur at significant probabilities but only over ca. 5 intervening bonds covering up to ca. 15 Å;d.through-space electron transfer from protonated sites to protonated sites or from fixed-charge sites to fixed-charge sites can be facile, but between protonated and fixed-charge sites transfer is very slow; to effect the transfers between equivalent sites, the two sites must come within ca. 5 Å of one another;e.through-space electron transfer from a protonated or fixed-charge site to an S–S σ* orbital can occur with reasonable probability but if the two sites come within ca. 5 Å of one another.Based on these findings, speculation is offered both to interpret recent findings of the McLuckey group on flexible, triply charged peptides and earlier data from the Marshall group on more rigid, helical, doubly charged peptides, both of which contain disulfide linkages that experiments find to be readily cleaved.