Title: Ab Initio Determination of Tryptophan Fluorescence Quenching by Histidine Cation in HP35-N27H, Barnase, and T4Lysozyme
Abstract: The fast folding, 35-residue villin headpiece, HP35, has been at the center of numerous protein folding rate simulations. Eaton et al. have experimentally followed the folding with the N27H mutant, plausibly because the protonated His27 quenches the fluorescence from Trp23 in the folded form by electron transfer, but not when unfolded. Because of this, at least in some simulations of the folding, a major criterion for the folded form is close proximity of His27 to Trp 23. Protonated His is indeed a potent quencher of Trp fluorescence in solution and in some, but not all, proteins. For quenching to occur the energy of the Trp-to-His charge transfer(CT) state must be low enough to be in resonance with the excited state of the Trp. This resonance is dictated by the electric potential difference between the Trp and His due to local protein environment. In some proteins the electric field enables quenching, and in others it does not. We have carried out QM-MM simulations of quenching by His cation for Trp23 in in villin, Trp94 in barnase and Trp138 in Q105H T4 lysozyme using ab initio electronic coupling [Callis et al J. Phys. Chem. B; 2007; 111(35); 10335-10339] Preliminary results indicate that for villin and T4 lysozyme the lowest CT state is that of the amide backbone of Trp. In contrast, the lowest CT state for barnase has His cation as the electron acceptor. We have also computed electronic coupling matrix elements between 3-methylindole and imidazole cation for all three proteins. The average coupling is 58, 329, and 2 cm-1 for the T4 lysozyme, barnase, and villin cases, respectively, suggesting that the change of Trp fluorescence upon folding in villin is not because of queching by His27.