MD+QM Investigations of the Length Scale and Forcefield Dependence of the Time Dependent Fluorescent Stokes Shift of Wild Type Staphylococcal Nuclease and Charge Mutants

Abstract

Discussion persists as to the origin of the time dependent fluorescence spectral (Stokes) shift (TDFSS) in the range 100 fs to more than 100 ps for a number of tryptophan (Trp)-containing proteins. TDFSS reports on the dynamic reorganization of the local environment around the large dipole of Trp following excitation to the 1La state. Much of the discussion centers on the ubiquitous “slow” (10 ps-5 ns) TDFSS component found only in proteins. Details of what determines the fast (<2 ps) component in proteins are also of interest. Interest focuses on two questions: (1) what are the relative contributions of protein and water; and (2) what length scales characterize these contributions? We have published extensive equilibrium MD+QM simulations on the single Trp of wild type Staph. nuclease, and four charge-changing mutants, all of which experimentally exhibit longer-decaying TDFSS (70-150 ps) than in other proteins, e.g., monellin and GB1 [D.P. Zhong, S.K. Pal, A.H. Zewail, Chem. Phys,Lett. 503 (2011), p. 1-11.] In this work, we have performed non-equilibrium, direct relaxation MD+QM molecular dynamics simulations for the wild type Staph. nuclease to investigate the origin of these long decay times, using both OPLS /TIP3P water and AMBER99sb-ildn/TIP4P water force fields. Initial results suggest that while OPLS fails to capture these longer relaxation times, AMBER99sb-ildn shows much closer agreement with experiment. We will also report results for the four charge-changing mutants and comparisons with other forcefields.