Mechanisms of Self-Assembly and Energy Harvesting in Tuneable Conjugates of Quantum Dots and Engineered Photovoltaic Proteins

Photoreaction centers facilitate the solar energy transduction at the heart of photosynthesis and there is increasing interest in their incorporation into biohybrid devices for solar energy conversion, sensing, and other applications. In this work, the self-assembly of conjugates between engineered bacterial reaction centers (RCs) and quantum dots (QDs) that act as a synthetic light harvesting system is described. The interface between protein and QD is provided by a polyhistidine tag that confers a tight and specific binding and defines the geometry of the interaction. Protein engineering that changes the pigment composition of the RC is used to identify Förster resonance energy transfer as the mechanism through which QDs can drive RC photochemistry with a high energy transfer efficiency. A thermodynamic explanation of RC/QD conjugation based on a multiple/independent binding model is provided. It is also demonstrated that the presence of multiple binding sites affects energy coupling not only between RCs and QDs but also among the bound RCs themselves, effects which likely stem from restricted RC dynamics at the QD surface in denser conjugates. These findings are readily transferrable to many other conjugate systems between proteins or combinations of proteins and other nanomaterials.