Ultrafast dielectric response of proteins from dynamics stokes shifting of coumarin in calmodulin

The contributions of protein motions to the dielectric response in solvent-inaccessible regions of calmodulin are examined. Two-pulse and three-pulse stimulated-emission experiments are used to examine the Stokes shift dynamics of coumarins in their ground and excited states. The results allow an evaluation of linear response theory. For coumarin 153, the ground- and excited-state solvation dynamics are similar to those for acetonitrile and methanol solvents. The solvation of coumarin 343 peptide in calmodulin is noticeably different in ground and excited states, indicating that the linear response picture is not appropriate in this case. The major difference is a more prominent fast component of solvation in the ground-state solvation dynamics. The ground-state energy gap correlation function is conjectured to be the best representation of protein relaxation, while that for the excited state seems to be influenced by the coupling of the protein modes to the large dipole created by excitation.