Rational Donor Placement into a Native Exciton Manifold: Cavity-Anchored Dyes Drive Sub-Picosecond Energy Transfer in Light-Harvesting Complex 2

Abstract

Achieving ultrafast excitation-energy transfer (EET) through rational structural design remains a major challenge in artificial exciton-based architectures. Purple bacterial light-harvesting complex 2 (LH2) from Rhodobacter sphaeroides provides a protein-scaffolded acceptor assembly: the B850 ring of 18 strongly coupled bacteriochlorophyll a (BChl) pigments forming a broad exciton manifold. Here, we engineer supramolecular biohybrid LH2 complexes by covalently anchoring ATTO647N donors inside the intrinsic hydrophobic cavity via cysteine substitutions. Two in-cavity conjugates and an out-of-cavity reference were prepared. Femtosecond transient absorption spectroscopy of membrane-reconstituted complexes reveals markedly accelerated donor-to-B850 EET for the in-cavity designs, with sub-picosecond components (0.4–0.6 ps) and position-dependent rates up to 4.5-fold higher than the out-of-cavity reference. The enhanced kinetics are quantitatively reproduced by generalised Förster theory combined with molecular dynamics simulations, which capture the behaviour of cavity-anchored ATTO. These results establish the LH2 cavity as a chemically addressable site for targeted donor placement, providing a design principle for exciton-based supramolecular donor–acceptor architectures.