Enhanced photocurrent from Photosystem I upon in vitro truncation of the antennae chlorophyll

Abstract

Current effects on climate change and dwindling fossil fuel reserves require new materials and methods to convert solar energy into a viable clean energy source. Recent progress in the direct conversion of light into photocurrent has been well documented using Photosystem I. In plants, PSI consists of a core complex and multiple light-harvesting complexes, denoted LHCI and LHCII. Most of the methods for isolating PSI from plants involve a selective, detergent solubilization from thylakoids followed by sucrose gradient density centrifugation. These processes isolate one variant of PSI with a specific ratio of Chl:P₇₀₀. In this study, we have developed a simple and potentially scalable method for isolating multiple PSI variants using Hydroxyapatite chromatography, which has been well documented in other Photosystem I isolation protocols. By varying the wash conditions, we show that it is possible to change the Chl:P₇₀₀ ratios. These different PSI complexes were cast into a PSI–Nafion–osmium polymer film that enabled their photoactivity to be measured. Photocurrent increases nearly 400 % between highly washed and untreated solutions based on equal chlorophyll content. Importantly, the mild washing conditions remove peripheral Chl and some LHCI without inhibiting the photochemical activity of PSI as suggested by SDS-PAGE analysis. This result could indicate that more P₇₀₀ could be loaded per surface area for biohybrid devices. Compared with other PSI isolations, this protocol also allows isolation of multiple PSI variants without loss of photochemical activity.