k_FRET = (1/τ_D)·(R₀/r)⁶ | E = 1/(1+(r/R₀)⁶) | R₀ = (9000·κ²·η²_D·J·ln10)/(128π⁵·n⁴·N_A))^(1/6)
FRET mechanism: Non-radiative energy transfer via dipole-dipole coupling. Rate falls as r⁻⁶: k_FRET = (1/τ_D)·(R₀/r)⁶. At r = R₀, exactly 50% efficiency. R₀ typically 20–80 Å for chlorophyll pairs.
Light-harvesting complexes: In plant LHC-II, ~300 chlorophyll molecules arranged in concentric rings transfer excitation energy to the reaction center with ~95% efficiency within ~100 ps. The Fenna-Matthews-Olson (FMO) complex shows quantum coherence in energy transfer at 77K.
Quantum biology: 2007 Fleming group (Science): wave-like quantum coherence in FMO complex at physiological temperature. Debate: does coherence enhance transfer efficiency or is it epiphenomenal? Noise-assisted quantum transport (ENAQT) — optimal dephasing rate may actually maximize efficiency.