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Commentary: Assessing FRET using spectral techniques

THE manuscript “Assessing FRET using spectral techniques” by Leavesley et al. (page XXX in this issue) addresses F€ orster resonance energy transfer (FRET) between organic molecules due to an interaction of optical transition dipole moments. Although occurring in nature for more than a billion of years, a theoretical description of this mechanism has only been given in 1948 (1). Since that time energy transfer spectroscopy has been used to probe molecular distances and mechanisms in the nanometre range (2–4), profiting from the fact that the energy transfer rate depends on the sixth power of molecular distance (kET r) and is, therefore, a very sensitive parameter. FRET experiments gained considerable importance, when in the 1990s green fluorescent protein [GFP, naturally occurring in the jellyfish Aequorea Victoria, (5)] and its mutants could be cloned and fused with almost any protein of a cell. So, nonradiative energy transfer from cyan fluorescent protein (CFP) to yellow fluorescent protein (YFP) or from GFP to red fluorescent protein was used to probe changes of molecular conformation, for example, upon binding of calcium (6), or interactions between adjacent molecules, for example, protein2protein interactions playing an important role in regulation of apoptosis (7) or in pathogenesis of M. Alzheimer and further neurodegenerative diseases (8,9). F€ orster energy transfer commonly arises, if a so-called donor molecule is excited by light and transfers its excitation energy to an adjacent acceptor molecule via dipole–dipole interaction. The latter commonly fluoresces at longer wavelength. Generally, two approaches are used to quantify this process: