Turning on the Fluorescence from Isolated GFP Chromophore Anions at Cryogenic Temperatures

TY - JOUR

T1 - Turning on the Fluorescence from Isolated GFP Chromophore Anions at Cryogenic Temperatures

AU - Lindkvist, Thomas Toft

AU - Djavani-Tabrizi, Iden

AU - Andersen, Lars Henrik

AU - Nielsen, Steen Brøndsted

N1 - Publisher Copyright: © 2025 American Physical Society.

PY - 2025/3/4

Y1 - 2025/3/4

N2 - The chromophore anion derived from the green fluorescent protein is one of the best-studied chromophores in the gas phase, but attempts to measure fluorescence have failed at room temperature. Here we unequivocally show that the chromophore exhibits fluorescence in the gas phase when cooled to low temperatures (<150 K), thereby validating previous hypotheses. The experimental confirmation is enabled by a unique mass-spectroscopy setup, allowing for fluorescence observation near or at the 0-0 transition without inducing heat in the ions upon photon absorption. The low-temperature conditions effectively simulate the restricted motion experienced within the protein, inhibiting internal conversion via a conical intersection along a twist motion coordinate. Fluorescence-excitation experiments at 100 K reveal an absorption-band maximum at 481.6±0.2 nm, while the dispersed fluorescence spectrum shows maximum emission at 483.6±0.5 nm. Remarkably, both values closely resemble those for proteins cooled to 77 K. We estimate that after excitation at the band maximum, radiation is the only pathway back to the ground state. Franck-Condon simulations at the ωB97XD/aug-cc-pVDZ level of theory nicely reproduce the experimental spectra and identify the fluorescent form to be planar, and that an in-plane scissoring mode (80 cm-1) is active for both absorption and emission.

AB - The chromophore anion derived from the green fluorescent protein is one of the best-studied chromophores in the gas phase, but attempts to measure fluorescence have failed at room temperature. Here we unequivocally show that the chromophore exhibits fluorescence in the gas phase when cooled to low temperatures (<150 K), thereby validating previous hypotheses. The experimental confirmation is enabled by a unique mass-spectroscopy setup, allowing for fluorescence observation near or at the 0-0 transition without inducing heat in the ions upon photon absorption. The low-temperature conditions effectively simulate the restricted motion experienced within the protein, inhibiting internal conversion via a conical intersection along a twist motion coordinate. Fluorescence-excitation experiments at 100 K reveal an absorption-band maximum at 481.6±0.2 nm, while the dispersed fluorescence spectrum shows maximum emission at 483.6±0.5 nm. Remarkably, both values closely resemble those for proteins cooled to 77 K. We estimate that after excitation at the band maximum, radiation is the only pathway back to the ground state. Franck-Condon simulations at the ωB97XD/aug-cc-pVDZ level of theory nicely reproduce the experimental spectra and identify the fluorescent form to be planar, and that an in-plane scissoring mode (80 cm-1) is active for both absorption and emission.

UR - http://www.scopus.com/inward/record.url?scp=86000355433&partnerID=8YFLogxK

U2 - 10.1103/PhysRevLett.134.093001

DO - 10.1103/PhysRevLett.134.093001

M3 - Journal article

C2 - 40131035

AN - SCOPUS:86000355433

SN - 0031-9007

VL - 134

JO - Physical Review Letters

JF - Physical Review Letters

IS - 9

M1 - 093001

ER -