Fine-Grained Spatial and Temporal Resolution of Water and Protein Contributions to Ultra-Fast and Slower Fluorescence Shifts from MD + QM Simulations

Abstract

Vibrant discussion persists as to the origin of the time dependent fluorescence spectral (Stokes) shift (TDFSS) in the range 100 fs to 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 (< 2ps) component in proteins are also of interest. Two questions of interest are: (1) the relative contributions of protein and water; and (2) what length scales characterize these contributions. To help answer these ongoing questions we have performed molecular dynamics simulations in conjunction with semiempirical quantum mechanics (MD + QM) for the proteins STNase, GB1, and monellin, each of which has been the subject of ultrafast experiments. We have examined the spatial contributions to shifts at 1pm intervals of distance, which has revealed that only 5-10 waters less than 0.8nm from Trp contribute to the TDFSS, while ∼100 waters out to 1.5 nm often contribute blue shifts that are the same in the ground and excited state. These simulations raise a further question as to the mechanism of fast anti-correlated fluctuations of protein and water contributions to the fluorescence shift.