Role of zwitterions in kindling fluorescent protein photochemistry

Kindling fluorescent protein (KFP), one of the chronologically first members of photoswitchable colored proteins from the green fluorescent protein (GFP) family, increasingly attracts efforts from experimental and theoretical sides. Ambiguous conclusions in solving puzzles of photochemistry of KFP and of its parent natural protein asFP595 are partially explained by lack of reliable theoretical data on chromophore properties in the electronically excited state. We report the results of state-of-the-art quantum chemistry calculations of the structure and energy of the KFP chromophore, 2-acetyl-,4-(p-hydroxybenzylidene)-1-methyl-5-imidazolone (AHBMI), both in the ground and excited states. Ground state equilibrium structures of anionic and zwitterionic protonation states of AHBMI were computed by the conventional MP2 method while excited state structures were characterized by the extended multireference perturbation theory method XMCQDPT2 including optimization of geometry parameters at this level. In particular, the computational results demonstrate that the basicity of the N2 nitrogen atom of the imidazolinone ring should noticeably increase upon electronic excitation, thus affecting excited state proton transfer events in proteins. The results of these simulations as well as of quantum mechanical-molecular mechanical (QM/MM) calculations for model protein systems evidence that KFP conformations with the zwitterionic chromophore are hardly expected to occur in the ground state, but may be populated upon excitation.