Importance of shattering fragmentation in the surface-induced dissociation of protonated octaglycine.

A QM + MM direct chemical dynamics simulation was performed to study collisions of protonated octaglycine, gly(8)-H(+), with the diamond {111} surface at an initial collision energy E(i) of 100 eV and incident angle theta(i) of 0 degrees and 45 degrees. The semiempirical model AM1 was used for the gly(8)-H(+) intramolecular potential, so that its fragmentation could be studied. Shattering dominates gly(8)-H(+) fragmentation at theta(i) = 0 degrees, with 78% of the ions dissociating in this way. At theta(i) = 45 degrees shattering is much less important. For theta(i) = 0 degrees there are 304 different pathways, many related by their backbone cleavage patterns. For the theta(i) = 0 degrees fragmentations, 59% resulted from both a-x and b-y cleavages, while for theta(i) = 45 degrees 70% of the fragmentations occurred with only a-x cleavage. For theta(i) = 0 degrees, the average percentage energy transfers to the internal degrees of freedom of the ion and the surface, and the energy remaining in ion translation are 45%, 26%, and 29%. For 45 degrees these percentages are 26%, 12%, and 62%. The percentage energy-transfer to DeltaE(int) for theta(i) = 0 degrees is larger than that reported in previous experiments for collisions of des-Arg(1)-bradykinin with a diamond surface at the same theta(i). This difference is discussed in terms of differences between the model diamond surface used in the simulations and the diamond surface prepared for the experiments.

An analytical potential energy function to model protonated peptide soft-landing experiments. The CH3NH3+/CH4 interactions.

To model soft-landing of peptide ions on surfaces, it is important to have accurate intermolecular potentials between these ions and surfaces. As part of this goal, ab initio calculations at the MP2/aug-cc-pVTZ level of theory, with basis set superposition error (BSSE) corrections, were performed to determine both the long-range attractive and short-range repulsive potentials for CH(4) interacting with the -NH(3)(+) group of CH(3)NH(3)(+). Potential energy curves for four different orientations between CH(4) and CH(3)NH(3)(+) were determined from the calculations to obtain accurate descriptions of the interactions between the atoms of CH(4) and those of -NH(3)(+). A universal analytic function was not found that could accurately represent both the long-range and short-range potentials for collision energies as high as those obtained in surface-induced-dissociation (SID) experiments. Instead, long-range and short-range analytic potentials were developed separately, by simultaneously fitting the four ab initio potential energy curves with a sum of two-body interactions between the atoms of CH(4) and -NH(3)(+), and then connecting these long-range and short-range two-body potentials with switching functions. Following a previous work [J. Am. Chem. Soc., 2002, 124, 1524], these two-body potentials may be used to describe the interactions of the N and H atoms of the -NH(3)(+) group of a protonated peptide ion with the H and C atoms of alkane-type surfaces such as alkyl thiol self-assembled monolayers and H-terminated diamond. Accurate short-range and long-range potentials are imperative to model protonated peptide ion soft-landing experiments. The former controls the collision energy transfer, whereas the latter describes the binding of the ion to the surface. A comparison of the ab initio potential energy curves for CH(3)NH(3)(+)/CH(4) with those for NH(4)(+)/CH(4) shows that they give nearly identical two-body interactions between the atoms of -NH(3)(+) and those of CH(4), showing that the smaller NH(4)(+)/CH(4) system may be used to obtain the two-body potentials. A comparison of the four ab initio potential energy curves reported here for CH(3)NH(3)(+)/CH(4), with those given by the AMBER and CHARMM molecular mechanical potentials, show that these latter potentials "roughly" approximate the long-range attractions, but are grossly in error for the short-range repulsions. The work reported here illustrates that high-level ab initio calculations of intermolecular potentials between small model molecules may be used to develop accurate analytical intermolecular potentials between peptide ions and surfaces.