Studies of ultrafast non-radiative deactivation (NRD) mechanisms of electronically excited states of small model peptide systems in gas phase, such as N-acetylphenylalaninylamide (NAPA), can provide key insights into the rich photochemistry of proteins, especially the mechanisms responsible for their photostability. We focus on elucidating the NRD pathways of the vibrationless ππ* excited state in three NAPA conformers (βL(a), γL(g+) and γL(g–)) and their conformer dependent excited state lifetime (70, 1.5 and 35 ns, respectively) as observed experimentally.1 Potential NRD pathways were obtained from non-adiabatic molecular dynamics simulations based on modified Tully’s fewest switch algorithm and on-the-fly construction of potential energy surfaces and non-adiabatic couplings using time dependent density functional theory.2 Two mechanisms exhibiting excitation transfer from the phenyl ππ* excited state to nπ* excited states located on each amide group were further refined for all three conformers. This includes the computation of the energy barriers and conical intersection geometries on the coupled cluster (CC2) level of theory for the two mechanisms. By comparing the sizes of the vibrational zero point energy accessible part of the corresponding conical intersection seams, as well as the effects of methyl substitution on the primary amino group, we were able to pinpoint the specific mechanism responsible for the fast NRD processes in the NAPA conformers. In a broader sense, we disclose the structural motifs/motions that control the relaxation dynamics in peptides and hence contribute to preventing their irreversible photochemical damage.3 1. M. Mališ, Y. Loquais, E. Gloaguen, H. S. Biswal, F. Piuzzi, B. Tardivel, V. Brenner, M. Broquier, C. Jouvet, M. Mons, N. Došlić, I. Ljubić J. Am. Chem. Soc. 134 (2012) 20340 2. R. Mitrić, U. Werner, V. Bonačić-Kouteck. J. Chem. Phys. 129 (2008) 164118 3. M. Mališ, Y. Loquais, E. Gloaguen, C. Jouvet, V. Brenner, M. Mons, I. Ljubić, N. Došlić Phys. Chem. Chem. Phys. 16 (2014) 2285