3. Formamide as the Model Compound for Photodissociation Studies of the Peptide Bond

Dynamics simulations are an essential step in exploring ultrafast phenomena in photochemistry and photobiology. In this chapter we present results of photodynamics studies for some model compounds for the peptide bond using the on-the-fly surface hopping method. The mechanism of photodissociation of formamide, its protonated forms and methyl substituted derivatives in their lowest singlet excited states in the gas phase is discussed in detail. Merits and demerits of using these simple molecules as models in exploring photochemical and photophysical properties of more complex systems, like peptides and proteins, are emphasized. It is found that in all examined model molecules the major deactivation process after excitation to the S₁ state is dissociation of the peptide C–N bond. The same holds for the deactivation path from the S₂ state, with exception of the O- protonated formamide in which C–O dissociation becomes the major deactivation process. Furthermore, it is shown that substitution by the methyl group(s), as well as protonation, strongly influence the lifetime of both excited states. In the last section application of the newly developed hybrid nonadiabatic photodynamics QM/MM approach in calculating photodissociation of formamide in argon matrix is illustrated.