Insensitivity of tryptophan fluorescence to local charge mutations

The steady state fluorescence spectral maximum (λmax) for tryptophan 140 of Staphylococcal nuclease remains virtually unchanged when nearby charged groups are removed by mutation, even though large electrostatic effects on λmax might be expected. To help understand the underlying mechanism of this curious result, we have modeled λmax with three sets of 50-ns molecular dynamics simulations in explicit water, equilibrated with excited state and with ground state charges. Semiempirical quantum mechanics and independent electrostatic analysis for the wild-type protein and four charge-altering mutants were performed on the chromophore using the coordinates from the simulations. Electrostatic contributions from the nearby charged lysines by themselves contribute 30-90 nm red shifts relative to the gas phase, but in each case, contributions from water create compensating blue shifts that bring the predicted λmax within 2 nm of the experimental value, 332 ± 0.5 nm for all five proteins. Although long-range collective interactions from ordered water make large blue shifts, crucial for determining the steady state λmax for absorption and fluorescence, such blue shifts do not contribute to the amplitude of the time dependent Stokes shift following excitation, which comes from nearby charges and only ∼6 waters tightly networked with those charges. We therefore conclude that for STNase, water and protein effects on the Stokes shift are not separable.