Optical dephasing in red fluorescent protein

The excited state dynamics of the red fluorescent protein DsRed have been studied by spectral hole-burning at low temperature. The homogeneous line width Γhom of the S1←S0 0–0 transition of the red (R-) form was obtained from MHz-resolution hole widths that were extrapolated to a burning fluence of zero at each temperature. The temperature dependence of Γhom follows a T1.3 power law between 1.2 and 4 K. For T→0, Γhom extrapolates to a value determined by the fluorescence lifetime τfl∼3.5 ns. The spectral distribution of hole depths in the red wing of the excitation spectrum is interpreted in terms of ‘trap’ states for ‘downhill’ energy-transfer within the DsRed tetramer. Furthermore, from simulations of experimentally determined zero-phonon holes and phonon-side holes we obtain for the R-form an electron–phonon coupling strength S of ∼1.25 and a mean phonon frequency νph of ∼50 cm−1.