Direct dynamics simulations of collision- and surface-induced dissociation of n-protonated glycine. Shattering fragmentation

Direct dynamics classical trajectory simulations are used to study energy transfer and unimolecular dissociation in collisions of N-protonated glycine, (gly-H)⁺, with an argon atom and a hydrogenated diamond {111} surface. The (gly-H)⁺ potential is represented by the AM1 semiempirical electronic structure theory and analytic potentials developed previously are used for the diamond surface and the (gly-H)⁺/Ar and (gly-H)⁺/ diamond intermolecular potentials. The AM1 potential for (gly-H)⁺ gives the same collisional energy transfer distributions as does the AMBER empirical force field. For (gly-H)⁺ + diamond {111} at a collision energy and angle of 70 eV and 45°, the average percent energy transfer to (gly-H)⁺ vibration/rotation, to the surface, and to final ion translation are 12, 38, and 50, respectively. A distribution of (gly-H)⁺ dissociation products are observed in these collisions, with ∼55% of the dissociations occurring while (gly-H)⁺ collides with the surface, i.e., shattering fragmentation. Shattering is initiated when the orientation of (gly-H)⁺ and the "hardness" of the collision "drives" a H-atom from CH₂ to the carbonyl carbon or a H-atom from NH₃ to the carbonyl oxygen or ejects a H₂ molecule from NH₃. Shattering is not important in (gly-H)⁺ collisions with Ar at 13 eV and an impact parameter of zero, but as found for the surface collisions, the Ar collision may "force" H-atom transfer. The simulations suggest that nonstatistical fragmentation dynamics may be important in the collisional dissociation of protonated amino acids and peptides. The collision may directly "drive" the ion to a fragmentation transition state structure.