This work describes a stroboscopic optical boxcar technique for the determination of fluorescence lifetimes which achieves performance comparable to techniques such as time-correlated single photon counting or phase modulation. The stroboscopic technique is based on the use of a traveling wave injected into a delay line connecting the dynodes of a photomultiplier tube. The transient potential difference created between two adjacent dynodes results simultaneously in significant amplification and the generation of a ‘‘gate’’ for the amplification process. Accurate control of the timing between the flashing of the gated lamp and the computer controlled delayable triggering of the photomultiplier tube pulser allows the gate to be placed at any time position within the range of the digital delay generator. The intensity of the fluorescence emission can thus be measured as a function of time relative to the excitation flash yielding data which is very similar to that from time-correlated single photon counting. Data analysis is done by iterative reconvolution in a procedure very similar to the analysis of time-correlated single photon counting data. Fluorescence lifetimes from a few hundred picoseconds to hundreds of nanoseconds can be determined to an accuracy and precision of better than ±2%.

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The time-resolved fluorescence of the lone tryptophanyl residue of ribonuclease T1 was investigated by using a mode-locked, frequency-doubled picosecond dye laser. The fluorescence decay could be characterized by a single exponential function with a lifetime of 3.9 ns. The fluorescence was readily quenched by uncharged solutes but was unaffected by iodide ion. These observations are interpreted in terms of the electrostatic properties of the amino acid residues at the active site of the protein, which would appear to restrict the access of solute species to the tryptophanyl residue. The temperature dependence of the fluorescence lifetime and anisotropy decay time could be rationalized in terms of a model which postulates a significant ordering of the solvent layer immediately surrounding the surface of the protein.