Vibrationally Resolved Photoabsorption Spectroscopy of Red Fluorescent Protein Chromophore Anions

Photoabsorption studies of red fluorescent protein chromophore anions have been performed at the ELISA electrostatic heavy-ion storage ring. The broad absorption band due to electronic excitation of the chromophores is tuned to a longer wavelength (redshifted) by extending the electronic conjugation of the molecule. A clear vibrational progression is resolved with $E_{\mathrm{v}\mathrm{i}\mathrm{b}}\sim 380$ and $520{\mathrm{c}\mathrm{m}}^{-1}$ for two different forms of the chromophore. The vibrational modes correspond to collective motions of the entire molecular structure. It is argued that the excited electronic state has an equilibrium configuration far from that of the electronic ground state, i.e., poor Franck Condon overlap.