Figure 2: Proposed ground and electronically excited states mechanism.
From: Probing water micro-solvation in proteins by water catalysed proton-transfer tautomerism
(a) The proposed protic solvent catalysed ESPT mechanism for 7-azaindole and its derivatives. The asterisk * indicates the electronically excited state. This mechanism incorporates a fast excited-state equilibrium between polysolvated and 1:1 cyclic hydrogen-bonded N1-H/H2O (or methanol) followed by proton tunnelling kpt22, which has been unambiguously proved by the kinetic deuterium isotope effect29. Under the assumption of k−1>k1 and kpt, the overall ESPT rate constant krxn can be expressed as krxn=(k1/k−1)kpt. (b) The ground-state equilibrium for 2,7-diazaindole and its derivatives between N1-H form and N2-H form isomers in water. (c) The proposed schematic diagram of a proton-transfer cycle for 2,7-diazaindole and (2,7-aza)Trp in water, in which the barrier and hence potential energy surface are arbitrarily chosen.
