Directionality of two-photon absorption in representative fluorescent proteins

Molecules of fluorescent proteins (FPs) exhibit anisotropic optical properties. While the directionality of single-photon (1P) absorption and emission in the most commonly used FPs has already been characterized, directionality of their two-photon (2P) excitation remains poorly understood, even though it offers higher sensitivity to molecular orientation and promises to yield more information on molecular orientational distributions than the 1P processes. By measuring optical properties of FP crystals, we have now determined the two-photon absorptivity tensors (2PATs) of the FPs mTurquoise2, eGFP and mCherry. Our results demonstrate the feasibility of experimental 2PAT determinations, provide new insights into molecular mechanisms of 2P absorption in FPs, open a new avenue to rational development of novel biosensors and allow detailed structural interpretation of results obtained with such biosensors.