Heuristics from Modeling of Spectral Overlap in Förster Resonance Energy Transfer (FRET).

Among the photophysical parameters that underpin Förster resonance energy transfer (FRET), perhaps the least explored is the spectral overlap term ( J). While by definition J increases linearly with acceptor molar absorption coefficient (ε in M cm), is proportional to wavelength (λ), and depends on the degree of overlap of the donor fluorescence and acceptor absorption spectra, the question arose as to the value of J for the case of perfect spectral overlap versus that for representative fluorophores with incomplete spectral overlap. Here, Gaussian distributions of absorption and fluorescent spectra have been modeled that encompass varying degrees of overlap, full-width-at-half-maximum (fwhm), and Stokes shift. For ε = 10 M cm and perfect overlap, the J value (in M cm nm) ranges from 1.15 × 10 (200 nm) to 7.07 × 10 (1000 nm), is almost linear with λ (average of λ and λ), and is nearly independent of fwhm. For visible-region fluorophores with perfectly overlapped Gaussian spectra, the resulting value of J ( J) is ∼0.71 ελ (M cm nm). The experimental J values for homotransfer, as occurs in light-harvesting antennas, were calculated with spectra from a static database of 60 representative compounds (12 groups, 5 compounds each) and found to range from 4.2 × 10 ( o-xylene) to 5.3 × 10 M cm nm (a naphthalocyanine). The degree of overlap, defined by the ratio of the experimental J to the model J for perfectly overlapped spectra, ranges from ∼0.5% (coumarin 151) to 77% (bacteriochlorophyll a). The results provide insights into how a variety of factors affect the resulting J values. The high degree of spectral overlap for (bacterio)chlorophylls prompts brief conjecture concerning the relevance of energy transfer to the question "why chlorophyll".