UV Resonance Raman Ground and Excited State Studies of Amide and Peptide Isomerization Dynamics

We report the first measurements of the activation barrier for ground state trans-cis isomerization of secondary amides. We measured activation barriers of Ea ) 13.8 ( 0.8 kcal/mol for aqueous solutions of N-methylacetamide (NMA) and Ea ) 11.0 ( 0.7 kcal/mol for glycylglycine (Gly-Gly). These activation barriers were determined from the temperature dependence of the ground state isomerization rates, which were measured by using UV resonance Raman to monitor the relative populations of the cis and trans amides as the sample solutions were translated through a 206.5 nm CW laser beam. Photon absorption causes the normally trans form to isomerize to the cis form. We developed a photochemical model to relate the measured relative Raman intensities to the ground state isomerization rates. We also measured the photochemical isomerization quantum yields, the yields for photochemical degradation, and the cis and trans Raman cross sections and their molar absorptivities. We also measured the temperature dependence of the cis and trans Raman intensities to determine the Gibbs free energy gap between the ground state cis and trans forms of NMA (2.6 ( 0.4 kcal/mol) and Gly-Gly (3.1 ( 0.5 kcal/mol).