Materials for FRET donors and acceptors
Authors from the US Naval
Research Laboratory in Washington, from George Mason University in Manassas, Va.,
and from the National Research Center on nanoStructures and bioSystems at Surfaces
in Modena, Italy, recently wrote a review in which they discuss the benefits and
limitations of various fluorophores that can be used as donors and acceptors in
bioanalytical Förster resonance energy transfer (FRET) techniques.
FRET uses an excited donor fluorophore
to transfer energy to an acceptor in a nonradiative process that takes place through
dipole-dipole interactions. Optimal interactions usually occur in the 10 to 100
Å range. Because the FRET process is distance-dependent on the nanoscale,
it has proved to be a powerful technique for monitoring myriad biomolecular interactions.
Traditional FRET dyes are organic compounds
that emit in the ultraviolet to near-infrared range. Another organic dye is the
photochromic type that can be switched between two different structural forms when
illuminated with certain wavelengths. This changes the dye’s acceptor functionality
and allows it — essentially — to be turned on and off.
Naturally occurring biomolecules such
as tryptophan and fluorescent proteins are increasingly being used as donors and
acceptors in FRET. They are easily introduced into proteins and can be used in vivo
for cell labeling. However, fluorescent proteins can be large and susceptible to
changes in environmental conditions.
Inorganic materials also are increasingly
being used as donors and acceptors. For example, gold nanoparticles have been shown
to exhibit increased sensitivity over some other dye combinations for FRET-based
DNA sensing. Silicon nanoparticles are difficult to produce but are being considered
as a nontoxic alternative to semiconductor materials for in vivo imaging. Quantum
dots have versatile excitation wavelengths with emissions ranging from the ultraviolet
to the infrared and may have applications in photodynamic cancer therapy.
It may even be possible to produce
unique emissions using multilabeled DNA structures with different combinations of
molecules and a single excitation wavelength. This technology could have applications
in information encoding.
The authors conclude that the growing
array of materials available for FRET will expand its applications. They point to
the study of protein- and peptide-folding kinetics, elucidation of macromolecular
interactions and multicolor analysis as some applications that may particularly
benefit from FRET advances. (Angewandte Chemie Int. Ed. 2006, 45, pp. 4562-4588.)
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