Chemical Dynamics Simulations of Thermal Desorption of Protonated Dialanine from a Perfluorinated Self-Assembled Monolayer Surface.

Classical chemical dynamics simulation results are presented for the thermal desorption kinetics and energetics of protonated dialanine ions (ala2 -H+ ) physisorbed on/in a perfluorinated self-assembled monolayer (F-SAM) surface. Previously developed analytic potentials were used for the F-SAM and the ala2 -H+ /F-SAM intermolecular interaction, and the AMBER valence force field was used for ala2 -H+ . The activation energy, Ea = 13.2 kcal/mol, determined from the simulations is consistent with previous simulations of the ala2 -H+ /F-SAM binding energy. The A-factor, 7.8 × 1011 s-1 , is about an order of magnitude lower than those representative of small molecule desorption from metal and semiconductor surfaces. This finding is consistent with the decreased entropies of ala2 -H+ and the F-SAM upon desorption. Using the Arrhenius parameters for ala2 -H+ desorption from the F-SAM, the lifetime of ala2 -H+ adsorbed on the F-SAM at 300 K is 5 × 10-3 s. Larger peptide ions are expected to have longer adsorption lifetimes.