Bioconjugation strategy to covalently bind photosynthetic reaction centers (RCs) to hydrogen-bonded organic semiconductors

Photosynthesis represents one of the most important biological reactions in the biosphere, since all life on Earth, directly or indirectly, depends on it as a source of energy. Nature performs the photosynthetic process using specialized protein - pigments complexes organized to ensure up to 98% conversion of the absorbed photons in stable, long - living charge separated states. A proper combination of the photosynthetic core , the reaction center (RC), with engineered materials, i.e. metals or inorganic semiconductor electrodes, has attracted great attention for the building of new versatile hybrid devices for solar energy conversion 1 . Here we propose a covalent approach able to stably anchor RCs onto evaporated thin films of the hydrogen - bonded pigments epindolidione (EPI) and quinacridone (QNC) 2 , a well - known class of organic colorants which have recently emerged as promising semiconductors, demonstrating hole mobility in the range of 0.1 - 1 cm 2 /Vs and outstanding operatio nal stability in both air and aqueous media with pH 3 - 10. Due to low - toxicity, biocom patibility and potential low - cost, EPI and QNC substrates are envisioned to go where other traditional semicond uctors simply cannot be applied, resulting to be amenable to direct surface bioconjuga tion. T he NH functional group of these molecules in thin film reacts spontaneously with N - hydroxysuccinimide functionalized linke rs as disuccinimidyl sub erate. The protruding linker s are then used to covalently bind the lysines residues of the Rhodobacter sphaeroides reaction center, by forming an amide linkage. Our pro tocol is shown to preserve the semiconducting properties of the pigment s while maintaini ng the protein's photoactivity. Multiple - ref lection infrared spectroscopy and atomic force microscopy demonstrated the effective covalent binding and the robustness of the protein anchoring even after buffer washing procedures compared to the weakness of the physisorbed RC interactions. Furthermore, RC charge recombination kinetic measurements confirmed the fully functionality of bioconjugated proteins and ruled out any possible hindering effect from the organic films. As key results of our work, we have shown that semiconductors preserve their favo rable electrical properties: the proposed photoconductor devices operate under water, before and after RCs anchoring . These are enabling steps for usin g hydrogen - bonded pigments as a platform for multifunct ional bioelectronics devices, paving the way in designing and realizing new photosynthetic protein - based hybrid systems.