Rice University researchers have developed a method of illuminating amyloid-? (A?) peptides, which are related to the onset of Alzheimer’s, by using metallic complexes of dipyridophenazine ruthenium(II). These complexes naturally stick to beta amyloid proteins called fibrils and the combination becomes highly photoluminescent. The technique should allow for better pathophysiologic studies and tracking of effectiveness of anti-Alzheimer’s drugs in laboratory experiments.
Molecules of beta amyloid naturally aggregate in a solution, as they appear to do in the brain. Ruthenium-based molecules added to the amyloid monomers do not fluoresce, Cook said. But once the amyloids begin to aggregate into fibrils that resemble “microscopic strands of spaghetti,” hydrophobic parts of the metal complex are naturally drawn to them. “The microenvironment around the aggregated peptide changes and flips the switch” that allows the metallic complexes to light up when excited by a spectroscope, he said.
Thioflavin T (ThT) dyes are the standard sensors for detecting amyloid fibrils and work much the same way, Marti said. But ThT has a disadvantage because it fluoresces when excited at 440 nanometers and emits light at 480 nanometers — a 40-nanometer window.
That gap between excitation and emission wavelengths is known as the Stokes shift. “In the case of our metal complexes, the Stokes is 180 nanometers,” said Martí, an assistant professor of chemistry and bioengineering. “We excite at 440 and detect in almost the near-infrared range, at 620 nanometers.
“That’s an advantage when we want to screen drugs to retard the growth of amyloid fibrils,” he said. “Some of these drugs are also fluorescent and can obscure the fluorescence of ThT, making assays unreliable.”
Cook also exploited the metallic’s long-lived fluorescence by “time gating” spectroscopic assays. “We specifically took the values only from 300 to 700 nanoseconds after excitation,” he said. “At that point, all of the fluorescent media have pretty much disappeared, except for ours. The exciting part of this experiment is that traditional probes primarily measure fluorescence in two dimensions: intensity and wavelength. We have demonstrated that we can add a third dimension — time — to enhance the resolution of a fluorescent assay.”
The researchers said their complexes could be fitting partners in a new technique called fluorescence lifetime imaging microscopy, which discriminates microenvironments based on the length of a particle’s fluorescence rather than its wavelength.
Press release: Molecules ‘light up’ Alzheimer’s roots
Abstract in Journal of The American Chemical Society: Sensing Amyloid-? Aggregation Using Luminescent Dipyridophenazine Ruthenium(II) Complexes
