TRYPTOPHAN FLUORESCENCE SHIFTS IN PROTEINS FROM HYBRID SIMULATIONS - AN ELECTROSTATIC APPROACH

We present hybrid quantum chemical-molecular dynamics computations on 15 partially hydrated proteins that support the view that the fluoresc ence wavelength maximum of a tryptophan (Trp) in a protein is determin ed almost entirely by the local electric field projection along the lo ng axis of the indole ring, that is to say, an internal Stark effect. The response is 1.3 x 10(6) V cm(-1)/nm, with a shift to longer wavele ngth when the field forces electrons from the pyrrole to the benzene r ing. Of 20 Trps simulated, representing a range of exposure to solvent , 15 are predicted within 10 nm of the experimental value. The finding s should greatly aid the important task of finding effective methods f or computing electrostatic fields and potentials within proteins.