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Dr. Ramakrishna Guda



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Green fluorescent protein (GFP), from Aequorea coerulescens, has been expressed as a His-tag fusion protein in E. coli cells and purified using metal chelate affinity and gel filtration chromatography. The protein fluoresces at 510 nm due to its natural chromophore that is spontaneously formed after properly folding and remains stable and intact after protein unfolding. The chromophore is buried in a beta barrel and has a permanent dipole moment in both the ground state and the excited state. The difference between permanent dipole moments Δμge is highly proportional to the electrostatic environment around the chromophore. Since Δμge is related to the two photon absorption (TPA) cross section, δ, and the TPA cross section is related to the electric field, the change of electric field through unfolding of the protein was monitored by the measurement of the two photon absorption cross-section. The fact that the TPA cross section is sensitive to the electric field makes it a powerful tool to study metal binding, protein-protein interactions, and protein misfolding or aggregation.

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