High-Resolution Spectroscopy and Selective Photoresponse of Cryogenically Cooled Green Fluorescent Protein...

By time-resolved action spectroscopy of cryogenically cooled molecular ions, we have achieved a remarkable vibrational resolution in the photoresponse of the deprotonated green fluorescent protein (GFP) chromophore, a key molecular unit in the bioimaging of living cells. We define four characteristic spectral regions of the S₀-S₁ band with competing electronic and nuclear decay channels. We determine the energy barrier toward internal conversion to be ∼250 cm^-1. This inhibits internal conversion and hence statistical fragmentation near the S₀-S₁ band origin, which is identified at 481.51 ± 0.15 nm (20768 ± 6 cm^-1). The origin is red-shifted by only 221 cm^-1 compared to that of wild-type GFP at 77 K. This, together with a striking agreement between the vibronic profiles of the protein and its chromophore, suggests their similar photophysics. In combination with theory, the data reveal the coexistence of mutually energy-borrowing mechanisms between nuclei and electrons mediated by specific vibrational modes.