Potential Energy Landscape of the Electronic States of the GFP Chromophore in Different Protonation Forms: Electronic Transition Energies and Conical Intersections

Abstract

We present the results of quantum chemical calculations of the transition energies and conical intersection points for the two lowest singlet electronic states of the green fluorescent protein chromophore, 4'-hydroxybenzylidene-2,3-dimethylimidazolinone, in the vicinity of its cis conformation in the gas phase. Four protonation states of the chromophore, i.e., anionic, neutral, cationic, and zwitterionic, were considered. Energy differences were computed by the perturbatively corrected complete active space self-consistent field (CASSCF)-based approaches at the corresponding potential energy minima optimized by density functional theory and CASSCF (for the ground and excited states, respectively). We also report the EOM-CCSD and SOS-CIS(D) results for the excitation energies. The minimum energy S0/S1 conical intersection points were located using analytic state-specific CASSCF gradients. The results reproduce essential features of previous ab initio calculations of the anionic form of the chromophore and provide an extension for the neutral, cationic, and zwitterionic forms, which are important in the protein environment. The S1 PES of the anion is fairly flat, and the barrier separating the planar bright conformation from the dark twisted one as well as the conical intersection point with the S0 surface is very small (less than 2 kcal/mol). On the cationic surface, the barrier is considerably higher (∼13 kcal/mol). The PES of the S1 state of the zwitterionic form does not have a planar minimum in the Franck-Condon region. The S1 surface of the neutral form possesses a bright planar minimum; the energy barrier of about 9 kcal/mol separates it from the dark twisted conformation as well as from the conical intersection point leading to the cis-trans chromophore isomerization.

Concerted Asynchronous Proton Transfer in H-Bonding Relay Model: An Implication of Green Fluorescent Protein

“…This HOMO-LUMO transition could induce partial charge transfer to the CH group, which can cause the C2-C3 and C3-C4 bond length to change. 31 In addition, HOMO and LUMO of the complex are localized within the chromophore and therefore proton transfer would hardly be affected by the electronic transition, which is consistent with the fact that proton transfer in the system follows the same pathway in both ground and excited states.…”

Analytical Derivative Coupling for Multistate CASPT2 Theory

“…Anionic pHBDI is considered to be an emitting species of the green fluorescent protein (GFP) and its variants. 6,[66][67][68][69][70][71][72][73][74][75][76] While GFP exhibits strong fluorescence with a lifetime on the order of nanoseconds, 6,66,[68][69][70][71][72][73] the nonadiabatic transition is known to occur in about a few picoseconds when the chromophore is not embedded in the protein environment. 77 As a resonant monomethine dye, it is widely accepted that the anionic GFP chromophore undergoes nonadiabatic transitions when the chromophore is twisted along the bridge.…”

A two-state model of twisted intramolecular charge-transfer in monomethine dyes

“…several cases, electronic structure computations [33][34][35][36][37][38] have predicted the existence of low-energy conical intersection seams with significant twist, which would be expected to facilitate ultrafast deactivation. The conical intersections themselves arise from the crossing of diabatic states with distinct charge localization, and simulations indicate that coupling of solvation behavior to the charge-transfer behavior can significantly affect the rate of decay.…”