Through-space and through-bond electron transfer within positively charged peptides in the gas phase

Abstract

Processes by which an electron, initially attached to a protonated or fixed-charge side chain, can (i) undergo through-bond transfer to migrate to an S S σ* orbital and cleave a disulfide linkage, or (ii) undergo through-space transfer from that charged site to another positively charged side chain are examined using ab initio electronic structure methods. Earlier work from this group along these lines is extended in two directions: 1. The spacer units along which through-bond electron transfer occurs are extended to include olefinic as well as aliphatic units (because polypeptide backbones and side chains contain π as well as σ bonds). 2. The electron binding energies of positively charged side chains can vary substantially. In earlier work, protonated amine NH 3 + and fixed-charge N(CH 3) 3 + units were used as model systems. In this work, two positive units with more similar electron binding strengths are employed to see whether more facile electron transfer occurs when two such groups collide. The primary findings are 1. That through-bond electron transfer through combined olefinic–aliphatic linkages is not qualitatively different (in rate) than through aliphatic linkages. 2. That through-bond transfer can occur to S S bonds at experimentally relevant rates but only over ca. 7 intervening bonds. 3. That through-space transfer from one positive side chain to another can occur but only if the electron binding strengths of the two side chains’ positive sites are similar.