Green fluorescent protein variant S65T/H148D has been reported to host a photocycle involving the photoinduced proton transfer reaction between the chromophore and residue Asp148 under 50 fs and without a measurable kinetic isotope effect, and experimental evidence is suggestive of the existence of a highly delocalized proton between these residues. The blinding speed at which this biological system undergoes proton transfer has been ascribed to the extreme increase of acidity of the GFP chromophore in the electronic excited state where proton transfer takes place. This work strives to present a coherent, complete, and balanced description of the dynamics of this specific variant of GFP in which it will be shown that this increase of acidity is insufficient to explain the behavior observed. This study tracks the behavior of this photosystem to the delicate interplay between structure and dynamics shown in the presence of solvent. In this way, it has been found that the dynamics of this protein intertwines its structure with the intervening solvent to give rise to effectively degenerate situations in what concerns the reactants and products of the proton transfer reaction in ground and, most importantly, photoexcited state, in terms of potential energy profiles associated with the proton migration. Under these conditions, proton transfer can occur in accordance with the experimental data available. This set of characteristics is possibly common to a host of other proton transfer based fluorescent proteins, and helps promoting GFP S65T/H148D to a case of archetypal significance. Thus, our results can be useful to understand the way many fluorescent proteins work and, more generally, the molecular basis for proton transfer reactions in proteins.

Extensive MD simulations combined with QM/MM calculations have been performed on mNeptune1 and mCardinal red fluorescent proteins to establish the reasons behind the red shift of the excitation wavelength of mCardinal with respect to mNeptune1. In both cases, it is seen that Arg197 stabilizes the chromophore but cannot be described as stabilizing preferentially the excited state because of the anchor point of the interaction. The interactions of the linking bonds to the α-helix of both proteins to the chromophore have been analyzed. It has been found that, besides the presence of a strategically placed residue Gln41 in mCardinal, solvation water molecules play an active role in the energetics of the stabilization of the excited state, which is preferentially stabilized in the case of mCardinal in contrast to mNeptune1.

The green fluorescent protein (GFP) variant S65T/H148D recovers the A-band fluorescence lost in the single mutant S65T, and it has been established that Asp148 is the alternate proton acceptor for the excited state proton transfer (ESPT). This mutant has been widely studied and presents unique spectroscopic properties, such as an ultrafast rise in the fluorescence (<50 fs). Also it exhibits a red-shift of the A absorption band of 20 nm with respect to wt-GFP's. The double mutant E222Q/H148D presents a very similar behaviour, at least within the experimental data available (which is scarcer than those of S65T/H148D). By means of dynamic theoretical studies we have been able to (1) reproduce and thoroughly analyse the red-shifted absorption spectra of both mutants and (2) predict the structure that the variant E222Q/H148D (for which there is no X-ray-resolved structure available) most probably adopts in water at room temperature. Our results deepen the understanding of the way GFP variants work and give some new insights into the rational design of fluorescent proteins and biological photosystems in general.

We report quantum mechanical/molecular mechanical non-adiabatic molecular dynamics simulations on the electronically excited state of green fluorescent protein mutant S65T/H148D. We examine the driving force of the ultrafast (τ < 50 fs) excited-state proton transfer unleashed by absorption in the A band at 415 nm and propose an atomistic description of the two dynamical regimes experimentally observed [Stoner Ma et al., J. Am. Chem. Soc., 2008, 130, 1227]. These regimes are explained in terms of two sets of successive dynamical events: first the proton transfers quickly from the chromophore to the acceptor Asp148. Thereafter, on a slower time scale, there are geometrical changes in the cavity of the chromophore that involve the distance between the chromophore and Asp148, the planarity of the excited-state chromophore, and the distance between the chromophore and Tyr145. We find two different non-radiative relaxation channels that are operative for structures in the reactant region and that can explain the mismatch between the decay of the emission of A* and the rise of the emission of I*, as well as the temperature dependence of the non-radiative decay rate.

Green fluorescent protein (GFP) mutant S65T/H148D has been proposed to host a photocycle that involves an excited-state proton transfer between the chromophore (Cro) and the Asp148 residue and takes place in less than 50 fs without a measurable kinetic isotope effect. It has been suggested that the interaction between the unsuspected Tyr145 residue and the chromophore is needed for the ultrafast sub-50 fs rise in fluorescence. To verify this, we have performed a computer-aided mutagenic study to introduce the additional mutation Y145F, which eliminates this interaction. By means of QM/MM molecular dynamics simulations and time-dependent density functional theory studies, we have assessed the importance of the Cro-Tyr145 interaction and the solvation of Asp148 and shown that in the triple mutant S65T/H148D/Y145F a significant loss in the ultrafast rise of the Stokes-shifted fluorescence should be expected.