UV resonance Raman studies of cis-to-trans isomerization of glycyglycine derivatives

We report kinetic UV resonance Raman measurements of the pH dependence of the activation barrier for the trans-cis isomerization of glycylglycine (Gly-Gly). We determined the charge-state dependence of the trans-cis ground-state energy difference, Delta G, and the trans-cis isomerization activation barrier, E-a. ac for zwitterionic Gly-Gly at pH 5.7 (3.0 +/- 0.5 kcal/mol) is below that of pH 10.5 anionic Gly-Gly (4.4 +/- 0.7 kcal/mol). The measured value of Delta G for cationic Gly-Gly at pH 3.0 (3.6 +/- 0.4 kcal/mol) lies between the zwitterion and anion values. The measured trans-to-cis activation barriers of 13.3 +/- 1.0, 12.6 +/- 0.9, and 13.9 +/- 1.1 kcal/mol for the cation, the zwitterion, and the anion, respectively, are identical within experimental error. However, the larger zwitterionic Gly-Gly (13 +/- 1.9 s(-1)) cis-to-trans isomerization rate constant over those of the cation (3 +/- 0.4 s(-1)) and anion (2 +/- 0.3 s(-1)) suggests a similar to 1 kcal/mol decreased activation barrier. The small differences in energy and activation barriers are somewhat surprising since the zwitterionic form can uniquely form ion pairs, which should impact the trans, the cis, and the activated complex energies.