What causes red shift in Trp fluorescence?
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 provi
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.
