Intermolecular charge transfer enhances two-photon absorption in yellow fluorescent protein

Abstract

We present a quantum chemical study of the two-photon absorption (TPA) properties of yellow fluorescent protein (YFP), a mutant of the extensively studied green fluorescent protein. The aromatic chromophore of YFP has a π-stacking interaction with the aromatic ring of a tyrosine residue (Tyr203) in a parellel-displaced structure with a distance of about 3.4Å. We study the TPA spectrum of the π-stacking system of YFP using the well-established Coulomb-attenuated B3LYP density functional (CAM-B3LYP) and the second-order approximate coupled-cluster model CC2. This work presents both the first comprehensive study of the two-photon absorption spectrum of YFP and the largest-scale coupled-cluster calculation of two-photon absorption that has ever been performed. We analyze the intermolecular charge-transfer (ICT) transitions in this stacked system and show that the ICT transitions are an important mechanism for enhancing the TPA cross sections in YFP. We investigate the distance dependence of the ICT transitions and show that their TPA cross sections are strongly dependent on the separation of the aromatic moieties. This provides a means for tuning the TPA properties of YFP and other structurally related fluorescent proteins through molecular engineering.

Benchmarking two-photon absorption cross sections: performance of CC2 and CAM-B3LYP

“…This is in part related to the lack of efficient publicly available codes to compute coupled-cluster TPA strengths. 17 CC2/aug-cc-pVDZ TPA cross sections were found to be much higher than CAM-B3LYP/ aug-cc-pVDZ cross sections. Paterson et al 16 included different density functionals in their comparison to the coupled-cluster hierarchy of methods on small molecules of up to six atoms.…”

“…15 All these different implementations span the spectrum from computationally efficient to very accurate, with TDDFT allowing for efficient calculations on large molecules and the coupled-cluster hierarchy of methods allowing for systematic improvement of the accuracy. 17 TDDFT TPA cross sections have been reported for molecular systems with more than 100 atoms, such as the fullerene-buckycatcher complex in ref. 16 Benchmarking CC2 and TDDFT TPA strengths against higher-level methods is important in order to assess the accuracy of these methods in work on larger molecules, for which more accurate calculations are not yet feasible.…”

“…13 The suggested explanations for this discrepancy were a strong overestimation of TPA strengths for CC2 or a mistake in the conversion to macroscopic units. 17 The aim of this study is to assess the quality of CC2 and TDDFT/CAM-B3LYP TPA strengths by comparing with the recent benchmark study by Nanda and Krylov 13 and to inspect whether CC2 calculations do indeed strongly overestimate the TPA strength. 17 The aim of this study is to assess the quality of CC2 and TDDFT/CAM-B3LYP TPA strengths by comparing with the recent benchmark study by Nanda and Krylov 13 and to inspect whether CC2 calculations do indeed strongly overestimate the TPA strength.…”

Open-Ended Recursive Approach for the Calculation of Multiphoton Absorption Matrix Elements

“…This finding is also based on the finding from several studies showing the reliability of CAM-B3LYP for the description of two-photon absorption. 21,56 These studies feature a comparison between a CAM-B3LYP and a coupled cluster treatment of two-photon absorption. As we found the behavior of the different j -photon absorption properties to be reasonably similar, we consider these results to be transferable to 3PA and 4PA.…”

Resolving Electronic Transitions in Synthetic Fluorescent Protein Chromophores by Magnetic Circular Dichroism

“…The PE approximation was thoroughly tested and found very suitable for fast estimations of the environmental effects on the chromophore . Effects of chemical modifications to the green FP chromophore were studied using TDDFT and molecular dynamics (MD), and intermolecular charge transfer in yellow FP was estimated using TDDFT and CC2 approaches . Ground‐ and excited‐states vibrations and dielectric response to photoexcitation were estimated at the complete active space self‐consistent field (CASSCF) level .…”