Sub-picosecond fluorescence spectroscopy of the M intermediate in the photocycle of bacteriorhodopsin by using up-conversion fluorometry

Initial photophysical characterization of the proteorhodopsin optical proton sensor (PROPS)

Fluorescence is not frequently used as a tool for investigating the photocycles of rhodopsins, largely because of the low quantum yield of the retinal chromophore. However, a new class of genetically encoded voltage sensors is based upon rhodopsins and their fluorescence. The first such sensor reported in the literature was the proteorhodopsin optical proton sensor (PROPS), which is capable of indicating membrane voltage changes in bacteria by means of changes in fluorescence. However, the properties of this fluorescence, such as its lifetime decay components and its origin in the protein photocycle, remain unknown. This paper reports steady-state and nanosecond time-resolved emission of this protein expressed in two strains of Escherichia coli, before and after membrane depolarization. The voltage-dependence of a particularly long lifetime component is established. Additional work to improve quantum yields and improve the general utility of PROPS is suggested.

Ultrashort-lived excited states of aminophthalimides in fluid solution

The Sn-->S0 ultraviolet fluorescence spectra (270-380 nm) of 4-aminophthalimide (4AP) and its N-methyl derivative 4-amino-N-methylphthalimide (4ANMP) are reported, following the absorption of two laser pulses. In polar but non-hydrogen-bonding solvents, both molecules exhibit a principal emission maximum near 290 nm, whereas solutions in hydrogen-bonding solvents display two prominent emission bands, near 300 and 350 nm. The relative intensities of these bands depend on solvent type and, in a pump-probe experiment, on the wavelengths and temporal spacing of two ultrashort pulses. Experiments covering the range approximately 0.1-100 ps showed evidence for at least two distinct ultrashort relaxation processes, the rates of which depend on solvent. Fluorescence upconversion experiments at < 0.2 ps resolution have shown that the longer-duration process correlates with the fluorescence Stokes shift, and provide evidence that the solvent-dependent shift of the S1-->S0 fluorescence spectrum is reflected in the fluorescence quantum efficiency of a nearby electronic state.