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, gly₈-H⁺, with the diamond {111} surface at an initial collision energy E_i of 100 eV and incident angle θ_i of 0° and 45°. The semiempirical model AM1 was used for the gly₈-H⁺ intramolecular potential, so that its fragmentation could be studied. Shattering dominates gly₈-H⁺ fragmentation at θ_i = 0°, with 78% of the ions dissociating in this way. At θ_i = 45° shattering is much less important. For θ_i = 0° there are 304 different pathways, many related by their backbone cleavage patterns. For the θ_i = 0° fragmentations, 59% resulted from both a-x and b-y cleavages, while for θ_i = 45° 70% of the fragmentations occurred with only a-x cleavage. For θ_i = 0°, 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° these percentages are 26%, 12%, and 62%. The percentage energy-transfer to ΔE_int for θ_i = 0° is larger than that reported in previous experiments for collisions of des-Arg¹-bradykinin with a diamond surface at the same θ_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.