A method for the measurement of a degree of fluorescence resonance energy transfer taking place between a donor and acceptor system by the steps of: irradiating a combined donor-acceptor system with a beam of intensity modulated excitation energy of a first wavelength; receiving fluorescence emissions from the donor and acceptor molecules having overlapping spectra; simultaneously determining a modulation lifetime ( tau mod) and a phase lifetime ( tau phi ) of the combined emitted fluorescence of the donor-acceptor system; and determining a degree of acceptor ingrowth by comparison of tau mod and tau phi . The method is particularly suited to measurements of a degree of FRET in a biological system.
FLUORESCENCE ASSAY FOR BIOLOGICAL SYSTEMS