Can Förster Theory Describe Stereoselective Energy Transfer Dynamics in a Protein–Ligand Complex?

Abstract

Förster resonance energy transfer (FRET) reactions involving ligands and aromatic amino acids can substantially impact the fluorescence properties of a protein-ligand complex, an impact intimately related to the corresponding binding mode. Structural characterization of such binding events in terms of intermolecular distances can be done through the well-known R distance-dependent Förster rate expression. However, such an interpretation suffers from uncertainties underlying Förster theory in the description of the electronic coupling that promotes FRET, mostly related to the dipole-dipole orientation factor, dielectric screening effects, and deviations from the ideal dipole approximation. Here, we investigate how Förster approximations impact the prediction of energy transfer dynamics in the complex between flurbiprofen (FBP) and human serum albumin (HSA), as well as a model FBP-Trp dyad, in which recent observation of enantioselective fluorescence quenching has been ascribed to energy transfer from FBP to Trp. To this end, we combine classical molecular dynamics simulations with polarizable quantum mechanics/molecular mechanics calculations that allow overcoming Förster approximations. On the basis of our results, we discuss the potential of structure-based simulations in the characterization of drug-binding events through fluorescence techniques. Overall, we find an excellent agreement between theory and experiment both in terms of enantioselectivity and FRET times, thus strongly supporting the reliability of the binding modes proposed for the (S) and (R) enantiomers of FBP. In particular, we show that the dynamic quenching arises from a small fraction of drug bound to the secondary site of HSA at the interface between subdomains IIA and IIB, whereas the enantioselectivity arises from the larger flexibility of the (S)-FBP enantiomer in the binding pocket.

Electronic energy transfer in biomacromolecules

“…Although these fluctuations have been found to be small for photosynthetic complexes, large fluctuations on s values in the range 0.4-0.8 have been observed for the drug flurbiprofen (FBP) interacting with Trp214 of human serum albumin (HSA) (Figure 2a) due to a remarkable flexibility in the binding pocket. 83 Overall, these results indicate that particular orientations, distances, and local environments can lead to important deviations from the 1/ε opt screening factor assumed in Förster theory and thus also in its distance and mutual D/A orientation dependence. In biomacromolecules, where the environment is significantly heterogeneous, correct screening effects can only be obtained with a polarizable and atomistic model for the environment.…”

“…Indeed, the increased flexibility of (S)-FBP compared with (R)-FBP in the binding pocket was shown to explain the observed $40% faster EET dynamics for that enantiomer. 83 On the other hand, the distribution of SEET couplings found for the FBP/HSA complex displays a less Gaussian shape compared with the CP29 case, and the coupling values oscillate at slightly smaller values at longer simulation times, an indication of potential static coupling fluctuations. 83 (b) triplet-triplet energy transfer (TEET) coupling between two adenines in a polyA-polyT DNA sequence, 98 (c) SEET coupling between chlorophylls a603 and a609 in the CP29 photosynthetic complex, 79 (d) TEET coupling between chlorophyll a603 and violaxanthin in the CP29 photosynthetic complex.…”

“…83 On the other hand, the distribution of SEET couplings found for the FBP/HSA complex displays a less Gaussian shape compared with the CP29 case, and the coupling values oscillate at slightly smaller values at longer simulation times, an indication of potential static coupling fluctuations. 83 (b) triplet-triplet energy transfer (TEET) coupling between two adenines in a polyA-polyT DNA sequence, 98 (c) SEET coupling between chlorophylls a603 and a609 in the CP29 photosynthetic complex, 79 (d) TEET coupling between chlorophyll a603 and violaxanthin in the CP29 photosynthetic complex. 101 In FRET systems where conformational transitions and fluorophore orientational dynamics can drastically change the mutual distance and orientation between the D/A chromophores, MD simulations are particularly useful in disentangling dynamic and static coupling fluctuations in the prediction of EET efficiencies.…”

“…This situation is exemplified in Figure a, which shows considerable fluctuations in the coupling between FBP and Trp214 of HSA. Indeed, the increased flexibility of (S)‐FBP compared with (R)‐FBP in the binding pocket was shown to explain the observed ∼40% faster EET dynamics for that enantiomer . On the other hand, the distribution of SEET couplings found for the FBP/HSA complex displays a less Gaussian shape compared with the CP29 case, and the coupling values oscillate at slightly smaller values at longer simulation times, an indication of potential static coupling fluctuations.…”

“…16 MD simulations have also been used to analyze the role of coupling fluctuations on EET processes in biosystems where the D/A molecules are constrained to a given conformation, like photosynthetic systems, 8 DNA 98,99 and protein-ligand complexes. 83,100 Overall, simulation studies have shown that singlet and triplet EET couplings display qualitatively different fluctuations due to their different long-or short-range nature. In Figure 4, we show some examples of coupling fluctuations calculated using QM/MM methods along classical MD trajectories for different biosystems.…”