Herzberg-Teller Vibronic Contribution to Mesomeric Dipole Moment Determines Two-Photon Absorptivity of Fluorescent Proteins

Abstract

Fluorescent proteins (FPs) are widely used in two-photon microscopy as genetically-targeted bio-probes. The physical basis of large variability of their two-photon absorption (2PA) brightness is however not understood. We have recently demonstrated that the mFruits series of FPs, having the same red anionic chromophore, show the 2PA band in the region of S0 - S1 electronic transition, corresponding to 900 - 1200 nm of laser wavelength. In this 2PA band, the vibronic 0-1 transition is stronger than the 0-0 transition, in contrast to one-photon absorption spectrum where the 0-0 transition dominates. It is also intriguing that the strength of the dominant vibronic 2PA transition strongly depends on the surrounding of the chromophore. Here we perform a comprehensive analysis of the 2PA spectral profiles of Fruits FPs. We show that a crucial factor which drives their optical properties is the local electric field at the chromophore site (varying from one mutant to another). Variation of the field promotes the shift of equilibrium between the two resonating forms of the chromophore π-conjugation structure, which, in turn, results in systematic changes of mesomeric dipole moment (the difference between the dipole moments in the excited and ground states, Δμ) and of the single-to-double bond-length alternations (BLA). Because the two-photon tensor of the S0 - S1 transition is proportional to Δμ2, wesuggest an interesting physical effect implying strong Herzberg-Teller coupling of Δμ with the BLA coordinate. This effect can only be observed in 2PA spectrum. Our model quantitatively explains the vibronic enhancement in 2PA spectrum of Fruits FPs and also provides the upper limit estimation for the 2PA peak cross section of any FP possessing red anionic chromophore. It also can guide mutagenesis efforts toward improvement of two-photon brightness of FPs.