ja9b07152_si_001.pdf (9.21 MB)
Unified Model for Photophysical and Electro-Optical Properties of Green Fluorescent Proteins
journal contribution
posted on 2019-09-11, 18:33 authored by Chi-Yun Lin, Matthew G. Romei, Luke M. Oltrogge, Irimpan I. Mathews, Steven G. BoxerGreen
fluorescent proteins (GFPs) have become indispensable imaging
and optogenetic tools. Their absorption and emission properties can
be optimized for specific applications. Currently, no unified framework
exists to comprehensively describe these photophysical properties,
namely the absorption maxima, emission maxima, Stokes shifts, vibronic
progressions, extinction coefficients, Stark tuning rates, and spontaneous
emission rates, especially one that includes the effects of the protein
environment. In this work, we study the correlations among these properties
from systematically tuned GFP environmental mutants and chromophore
variants. Correlation plots reveal monotonic trends, suggesting that
all these properties are governed by one underlying factor dependent
on the chromophore’s environment. By treating the anionic GFP
chromophore as a mixed-valence compound existing as a superposition
of two resonance forms, we argue that this underlying factor is defined
as the difference in energy between the two forms, or the driving
force, which is tuned by the environment. We then introduce a Marcus–Hush
model with the bond length alternation vibrational mode, treating
the GFP absorption band as an intervalence charge transfer band. This
model explains all of the observed strong correlations among photophysical
properties; related subtopics are extensively discussed in the Supporting
Information. Finally, we demonstrate the model’s predictive
power by utilizing the additivity of the driving force. The model
described here elucidates the role of the protein environment in modulating
the photophysical properties of the chromophore, providing insights
and limitations for designing new GFPs with desired phenotypes. We
argue that this model should also be generally applicable to both
biological and nonbiological polymethine dyes.