Dipole and Coulomb Forces in Electron Capture Dissociation and Electron Transfer Dissociation Mass Spectroscopy

Ab initio electronic structure calculations were performed on a doubly charged polypeptide model H+-Lys(Ala)(19)-CO-CH(NH2)-CH2-SS-CH2-(NH2)CH-CO-(Ala)(19)-Lys-H+ consisting of a C-terminal protonated Lys followed by a 19-Ala alpha-helix with a 20th Ala-like unit whose side chain is linked by a disulfide bond to a corresponding Ala-like unit connected to a second 19-Ala alpha-helix terminated by a second C-terminal-protonated Lys. The Coulomb potentials arising from the two charged Lys residues and dipole potentials arising from the two oppositely directed 72 D dipoles of the alpha-helices act to stabilize the SS bond's sigma* orbital. The Coulomb potentials provide stabilization of 1 eV, while the two large dipoles generate an additional 4 eV. Such stabilization allows the SS sigma* orbital to attach an electron and thereby generate disulfide bond cleavage products. Although calculations are performed only on SS bond cleavage, discussion of N-C-alpha bond cleavage caused by electron attachment to amide pi* orbitals is also presented. The magnitudes of the stabilization energies as well as the fact that they arise from Coulomb and dipole potentials are supported by results on a small model system consisting of a H3C-SS-CH3 molecule with positive and negative fractional point charges to its left and right designed to represent (i) two positive charges ca. 32 angstrom distant (i.e., the two charged Lys sites of the peptide model) and (ii) two 72 D dipoles (i.e., the two alpha-helices). Earlier workers suggested that internal dipole forces in polypeptides could act to guide incoming free electrons (i.e., in electron capture dissociation (ECD)) toward the positive end of the dipole and thus affect the branching ratios for cleaving various bonds. Those workers argued that, because of the huge mass difference between an anion donor and a free electron, internal dipole forces would have a far smaller influence over the trajectory of a donor (i.e., in electron transfer dissociation (ETD)). The present findings suggest that, in addition to their effects on guiding electron or donor trajectories, dipole potentials (in combination with Coulomb potentials) also alter the energies of SS sigma* and amide pi* orbitals, which then affects the ability of these orbitals to bind an electron. Thus, both by trajectory-guiding and by orbital energy stabilization, Coulomb and dipole potentials can have significant influences on the branching ratio of ECD and ETC in which disulfide or N-C-alpha bonds are cleaved.