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Energy transfer, unfolding, and fragmentation dynamics in collisions of N-protonated octaglycine with an H-SAM surface.

Results are reported for PM3 and RM1 QM+MM direct dynamics simulations of collisions of N-protonated octaglycine (gly(8)-H(+)) with an octanethiol self-assembled monolayer (H-SAM) surface. Detailed analyses of the energy transfer, fragmentation, and conformational changes induced by the collisions are described. Extensive comparisons are made between the simulations and previously reported experimental studies. Good agreement between the two semiempirical methods is found regarding energy transfer, while differences are seen for their fragmentation time scales. Trajectories were calculated for 8 ps with collision energies from 5 to 110 eV and incident angles of 0 degrees and 45 degrees. A linear relationship is found between the collision energy and key parameters of the final internal energy distributions of both gly(8)-H(+) and the H-SAM. In general wider distributions are seen for the H-SAM than for the peptide ion. An incident angle of 45 degrees leads to more energy transfer to the peptide, with wider distributions. The average percentage energy transfer to gly(8)-H(+) is nearly independent of the collision energy, while the average percentage transfer to the surface increases with collision energy. For normal incidence, we find an average percentage energy transfer to gly(8)-H(+) which is in excellent agreement with the experimentally measured value 10.1 +/- 0.8% for the octapeptide des-Arg(1)-bradykinin [J. Chem. Phys. 2003, 119, 3414]. At each collision energy dramatic conformational changes of gly(8)-H(+) are seen. The initial folded structure rearranges to form a beta-sheet like structure showing that the collision induces peptide unfolding. This process is more pronounced at an incident angle of 45 degrees. Following the conformation change, nonshattering fragmentation, promoted by proton transfer, is observed at the highest collision energies. Substantially more fragmentation occurs for the RM1 simulations.