Practice question

Indole ring possesses large dipole increase upon excitation. Polar protic solvent molecules, especially water, reorient and form stronger dipolar interactions around excited state during fluorescence lifetime, stabilizing S1 more than ground state and shrinking transition energy noticeably. Resulting photon energy decreases, observed as red shift towards 350 nanometers. Hydrophobic core or protein aggregation removes water, restoring blue emission around 330 nanometers. Heme binding quenches intensity without systematic wavelength shift. Hence direct water interaction with excited dipole provides sensitive reporter of solvent accessibility, unfolding or conformational opening of protein structure.