| State of the art frequency domain instrumentation was developed to measure the natural fluorescence decay of single tryptophan residue proteins. The instrumentation uses mode-locked, synchronously pumped, cavity dumped and frequency doubled dye laser excitation. The detectors are modulated phase coherently with the mode-locked laser pulses to accurately determine the phase delay and modulation of the fluorescence response. The instrument achieves 1 ps time resolution. The resolvability provided by the experimental data obtained with such instrumentation is determined. It is shown that the discrete exponential analysis of the fluorescence decay overestimates the resolvability of the data. It is shown that large sets of exponentially decaying components with lifetimes distributed continuously in lifetime space can be recovered, within the limits of resolvability provided by the frequency domain data, using probability density functions. It is also shown based on the dynamic nature of the macromolecules that the fluorescence decay of proteins can arise from distributions of exponentially decaying components. Lifetime distribution functions are derived based on (i) conformational interconverting models with distributions of activation energies and (ii) statistical mechanical models of the protein. The fluorescence lifetime distribution analysis of data from four single tryptophan residue proteins show that the shape of the distribution is determined by the structure of the protein. Such distributions become narrower and shifted to shorter lifetime values with increasing temperature. These results can be explained based on the dynamic origin of the fluorescence lifetime distribution in macromolecules. The inadequacy of single conformational model to account for the fluorescence lifetime distribution suggests that proteins fluctuate in a multitude of conformational sub-states. |
