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

We report kinetic UV resonance Raman measurements of the pH dependence of t he activation barrier for the trans-cis isomerization of glycylglycine (Gly -Gly). We determined the charge-state dependence of the trans-cis ground-st ate 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 measur ed value of Delta G for cationic Gly-Gly at pH 3.0 (3.6 +/- 0.4 kcal/mol) l ies between the zwitterion and anion values. The measured trans-to-cis acti vation barriers of 13.3 +/- 1.0, 12.6 +/- 0.9, and 13.9 +/- 1.1 kcal/mol fo r the cation, the zwitterion, and the anion, respectively, are identical wi thin 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 acti vation barriers are somewhat surprising since the zwitterionic form can uni quely form ion pairs, which should impact the trans, the cis, and the activ ated complex energies.