  In this study, we explore the electron transition mechanism and optical
properties of the popular red fluorescent protein mCherry. By examining the
charge transfer spectrum and combining it with the mCherry hole-electron
distribution, we identify that the charge transfer between the phenolate and
imidazolinone loops significantly contributes to the absorption spectrum.
Quantitative analysis of charge transfer shows that, overall, the electrons are
transferred to the C16 atom in the middle of phenolate and the imidazolinone
loops during absorption. We speculate that C16 may also absorb protons to
enable the photoconversion of mCherry in the excited state, similar to the
blinking mechanism of IrisFP. In addition, we further investigated the optical
properties of mcherry in the external field by polarizability
(hyperpolarizability), showing the anisotropy of the polarization, the first
hyperpolarization and the second hyperpolarization by unit spherical
representation. Our results suggest that significant polarization and second
hyperpolarizability occur when the field direction and electron transfer
direction are aligned. We also analyzed the polarizability and first
hyperpolarizabilities for different external fields. The polarizability mutated
when the external field satisfies the S_0,min-> S_1 transition. Finally, the
study of the first hyperpolarizability shows that adjusting the appropriate
field can lead to a linear photoelectric effect or second harmonic generation
of mCherry. These studies have certain reference values for various red
fluorescent protein correlation simulations and experiments because of the
similarity of the red fluorescent protein.
