Preserving the native environment of photosystem I within styrene maleic acid lipid particles (SMALPs) for applications in biohybrid solar devices.

In order to meet the growing demand for energy in our society and the environmental need to reduce carbon dioxide pollution in our atmosphere, it is imperative that we devise a scalable strategy to convert sunlight to electricity without the use of Earth-limited resources and rare elements. Biohybrid solar devices (BHSDs), described here, which incorporate photosynthetic proteins onto electrode surfaces to facilitate the capture and conversion of sunlight to electricity offer such a strategy, though the efficiencies of these devices remain very low at present. This work is focused on the innovation of retaining the native environment of photosystem I (PSI) within polymer bound nanodiscs, which demonstrate enhanced photochemistry in vitro, and the downstream implications for this innovation to be used in biohybrid solar devices. Here, we demonstrate that the efficiency of BHSDs is significantly reduced at elevated temperature (40 °C), and these effects can be partially mitigated by the introduction of osmolytes such as glycine-betaine and sucrose at 0.5 - 1 M. Further, we present data that suggest retaining the native thylakoid membrane environment of PSI within nanodiscs leads to enhanced photochemistry in vitro. In particular, PSI encapsulated within styrene maleic acid lipid particles (SMALPs) display a 1,000-fold faster conversion of photons to electrons compared to detergent solubilized PSI. To investigate what may be causing this discrepancy, we proceeded to characterize these PSI-SMALPs using a number of biochemical and biophysical analyses. Using small angle neutron and X-ray scattering techniques we determined the PSI-SMALP is approximately 30% larger than detergent solubilized PSI, surrounded by a tightly bound polymer belt with little protrusion from the particle. Mass spectrometry suggests that this increase in size is due to the retention of a lipid annulus surrounding the protein that is highly enriched in one particular lipid, sulfoquinovosyldiacylglycerol (SQDG), compared to the bulk thylakoid membrane, suggesting lateral heterogeneity in both the proteins and lipids in cyanobacterial thylakoid membranes. We also present neutron reflectivity data that suggests that these polymers are most disruptive to SQDG containing lipid monolayers. Lastly, we determined that the esterification of SMA copolymers leads to greatly enhanced solubilization of galactolipid membranes.