One- and two-photon excited fluorescence lifetimes and anisotropy decays of Green Fluorescent Proteins

We have used one- (OPE) and two-photon (TPE) excitation with time-correlate d single-photon counting techniques to determine time-resolved fluorescence intensity and anisotropy decays of the wild-type Green Fluorescent Protein (GFP) and two red-shifted mutants, S65T-GFP and RSGFP. WT-GFP and S65T-GFP exhibited a predominant similar to 3 ns monoexponential fluorescence decay , whereas for RSGFP the main lifetimes were similar to 1.1 ns (main compone nt) and similar to 3.3 ns, The anisotropy decay of WT-GFP and S65T-GFP was also monoexponential (global rotational correlation time of 16 +/- 1 ns), T he similar to 1.1 ns lifetime of RSGFP was associated with a faster:rotatio nal depolarization, evaluated as an additional similar to 13 ns component, This feature we attribute tentatively to a greater rotational freedom of th e anionic chromophore. With OPE, the initial anisotropy was close to the th eoretical limit of 0.4; with TPE it was higher, approaching the TPE theoret ical limit of 0.57 for the colinear case. The measured power dependence of the fluorescence signals provided direct evidence for TPE, The general inde pendence of fluorescence decay times, rotation correlation times, and stead y-state emission spectra on the excitation mode indicates that the fluoresc ence originated from the same distinct excited singlet states (A*, I*, B*), However, we observed a relative enhancement of blue fluorescence peaked at similar to 440 nm for TPE compared to OPE, indicating different relative e xcitation efficiencies. We infer that the two lifetimes of RSGFP represent the deactivation of two substates of the deprotonated intermediate (I*), di stinguished by their origin (i.e,, from A* or B*) and by nonradiative decay rates reflecting different internal environments of the excited-state chro mophore.