PHYSICS GRADUATE STUDENT SYMPOSIUM<br>Measuring the Stark Shift in the Two-Photon Cross Section of Fluorescent Proteins

Two-photon fluorescence imaging has vast improvements over its one-photon counter part for imaging biological systems, including reduced contributions due to scattering, z-axis dependent focusing, ease of detection geometry and reduced out of focus photobleaching.  Central to the development and versatility of fluorescence imaging is green fluorescent protein (GFP) and its ability to be expressed in various mammalian systems without the need for species-specific cofactors.  Manipulation of GFP’s chromophore amino acid structure has also enabled the creation of many GFP mutants with a range of emissions wavelengths spanning from the blue to the red.  Stark spectroscopy has revealed that GFP mutant emission wavelengths are dependent on the local electric field produced by the chromophore’s amino acid sequence, suggesting a dependence on the chromophore’s difference dipole moment. Coincidentally, two-photon absorption is also dependent on the difference dipole moment, suggesting that changing the electric field environment of the protein can shift the emissions spectrum of the protein’s two-photon response.  If it is possible to characterize and monitor the shift in two-photon emissions from an applied electric field, this effect offers many potential improvements to two-photon fluorescence imaging of biological processes, like neuron action potential events, which involve the movement of ions across the neuronal membrane.

Lunch will be served in the Don Meyer Common Room at 11:50 a.m.