ON THE CONTRIBUTION OF ELECTRON-TRANSFER REACTIONS TO THE QUENCHING OF TRYPTOPHAN FLUORESCENCE

We present a theoretical calculation of rates for excited state electr on transfer (ET) reactions in zwitterionic tryptophan for each of its six rotameric conformers. Within the Born-Oppenheimer approximation, w e calculate the electronic part of the rate, using the renormalized pe rturbation expansion (RPE) method. The vibrational part is extracted f rom experimental data, assuming that at neutral pH, the observed fluor escence decay profile is composed of two exponentials corresponding to two competing populations in the sample; one in which fluorescence is accompanied by ET and other that is not. Complementary data, namely, ionization potentials and charge distributions are obtained using a co mbination of classical molecular mechanics simulation to determine the geometry of the six rotamers, and semiempirical (INDO/S) molecular or bital calculation. Our results indicate that conformers perp chi(2)g(- ) and anti chi(2)g(-) give the main contribution to ET in the time sca le of fluorescence. (C) 1995 American Institute of Physics.