Refinements to the Utah-Washington mechanism of electron capture dissociation.

Ab initio electronic structure calculations on a rather geometrically constrained doubly positivley charged parent peptide ion are combined with experimental data from others on three similar ions to refine understanding of the mechanistic steps in the Utah-Washington model of electron-capture and electron-transfer dissociation. The primary new findings are that (i) the electron need not first attach to a Rydberg orbital and subsequently be extracted by an SS σ* or amide π* orbital (rather, it can be guided directly into the SS σ* or amide π* orbital by the Rydberg orbital) and (ii) Coulomb and dipole potentials within the parent ion alter both the electron binding strengths and radial ranges of Rydberg orbitals located on the positively charged sites, which, in turn, alters the ranges over which the electron can be guided. These same potentials, when evaluated at disulfide or backbone amide sites, determine which disulfide σ* and amide π* orbitals are and are not susceptible to electron attachment leading to SS and N-Cα bond cleavage. Additional experiments on the same parent ions discussed here are proposed to further test and refine the UW model.