NH tautomerism in the natural chlorin derivatives

NH tautomerism of five Mg-free chlorophyll a and b derivatives 2-6 was studied utilizing NMR spectroscopy and molecular modeling. The results from the dynamic NMR measurements of the chlorins revealed that substituent effects contribute crucially to the free energy of activation (Delta G(double dagger)) in the NH tautomeric processes. An intermediate tautomer for the total tautomeric NH exchange in a chlorin was observed for the first time, when the H-1 NMR spectra of chlorin e(6) TME (3) and rhodin g(7) TME (4) (TME = trimethyl ester) were measured at lower temperatures. The lower energy barriers (Delta G(1)(double dagger)) obtained for the formation of the intermediate tautomers of 3 and 4, assigned to the N-22-H, N-24-H trans-tautomer, were 10.8 and 10.6 kcal/mol, respectively. The energy barrier (Delta G(2)(double dagger) value) for the total tautomeric NH exchange in the five chlorins was found to vary from 13.6 kcal/mol to values higher than 18 kcal/mol. The lowest Delta G(2)(double dagger) value (13.6 kcal/mol) was obtained for rhodochlorin XV dimethyl ester (2), which was the only chlorophyll derivative lacking the C-15 substituent. In the case of chlorins 4 and 5, the steric crowding around the methoxycarbonylmethyl group at C-15 raised the Delta G(2)(double dagger) activation free-energy to 17.1 kcal/mol. However, the highest energy barrier with Delta G(2)(double dagger) > 18 kcal/mol was observed for the NH exchange of pyropheophorbide a methyl ester (6), possessing the macrocycle rigidifying isocyclic ring E. Our results demonstrate that the steric strain, arising either from the steric crowding around the bulky substituent at C-15 or the macrocycle rigidifying isocyclic ring E, slows down the NR tautomeric process. We suggest that deformations in the chlorin skeleton are closely connected to the NH tautomeric exchange and that the exchange occurs by a stepwise proton-transfer mechanism via a hydrogen bridge.