The distance at which FRET efficiency is exactly 50%–the physical basis for the molecular ruler that enables nanometer-scale protein interaction measurement.
FRET efficiency follows the relationship E = R0^6 / (R0^6 + r^6), where r is the donor-acceptor separation. This sixth-power dependence creates a steep sensitivity curve:
At r = 0.5 × R0 (~3 nm): E 98%
At r = R0 (5.83 nm): E = 50%
At r = 1.5 × R0 (~9 nm): E 10%
At r = 2 × R0 (~12 nm): E 1.5%
This relationship means FRET effectively measures distances from ~0.5 R0 to ~2 R0–the 1–10 nm range that precisely matches protein-protein interactionLoading... distances.
What R₀ Means: The Förster radius is the distance at which FRET efficiency is exactly 50%. For typical fluorophore pairs, this is 4-7 nanometers.
Practical Meaning:
• Closer than R₀ → strong FRET (most energy transfers)
• At R₀ → 50% FRET
• Further than R₀ → weak FRET (little energy transfers)
This creates a very sensitive distance detector in the 1-10nm range—perfect for protein interactions.
