Tuning Maps for Two-Photon Absorption Cross Sections of Canonical and Noncanonical Fluorescent Protein Chromophores

Two-photon absorption (2PA) spectroscopy,
which involves the simultaneous
absorption of a pair of photons, is an important area of research
in chemical biology, where a diversity of biological chromophores
have been explored for 2PA applications. This work offers a perspective
on how electrostatic interactions influence the 2PA response of fluorescent
protein (FP) chromophores, demonstrating the role of their surrounding
environment in modulating 2PA optical properties. In this work, we
present electrostatic tuning maps, created by placing partial charges
along the van der Waals radii of eight different chromophores: Green
FP (neutral and anion), Red FP (neutral and anion), mBlueberry FP,
Kusabira orange FP, and two noncanonical chromophores, a hydroxyquinolone
derivative, and the chromophore of Gold FP. 2PA cross sections (σ2PA) were calculated at the CAM-B3LYP/aug-cc-pVDZ level of
theory with and without the presence of point charges. The results
show that the influence of point charges varies with their location,
leading to substantial increases or decreases in the obtained σ2PA. For the studied systems, neutral chromophores tend to
increase their σ2PA when positive point charges are
present close to the imidazolinone group, while lowering their σ2PA when they are present near the benzylidene ring. In contrast,
anionic species show an opposite trend. Using a positive or negative
point charge has the opposite effect as well, increasing or decreasing
σ2PA by relatively the same amount. These maps aid
in understanding how electrostatic interactions influence 2PA responses,
providing a framework for guiding structural modifications to the
protein environment as well as the design and optimization of 2PA
probes.