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 activat ion barriers of E(a) = 13.8 +/- 0.8 kcal/mol for aqueous solutions of N-methylacetamide (NMA) and E(a) = 11.0 +/- 0.7 kcal/mol for glycylgly cine (Gly-Gly). These activation barriers were determined from the tem perature dependence of the ground state isomerization rates, which wer e measured by using UV resonance Raman to monitor the relative populat ions of the cis and trans amides as the sample solutions were translat ed through a 206.5 nm CW laser beam. Photon absorption causes the norm ally trans form to isomerize to the cis form. We developed a photochem ical model to relate the measured relative Raman intensities to the gr ound state isomerization rates. We also measured the photochemical iso merization quantum yields, the yields for photochemical degradation, a nd the cis and trans Raman cross sections and their molar absorptiviti es. We also measured the temperature dependence of the cis and trans R aman intensities to determine the Gibbs free energy gap between the gr ound state cis and trans forms of NMA (2.6 +/- 0.4 kcal/mol) and Gly-G ly (3.1 +/- 0.5 kcal/mol).