Separation, Characterization And Stabilization Of Photosystem I From Spirulina Platensis

Life on earth depends on photosynthesis, the conversion of light energy from the sun to chemical energy or most negative redox potential. Solar energy based on photosynthesis could help human to resolve depletion of fossil fuel resource and aggravation of the environmental pollution. However, there are many techinal hurdles to be solved to develop such a photosynthetic membrane protein-based biological solar cell. In this paper, we mainly focus on high active photosystem I (PSI) complex preparation and how to maintain its long-term stability, which are two key points to develop such a promising solar cell device.Firstly, intact PSI was successfully separated from Spirulina platensis using sucrose density gradient ultracentrifugation method, and the key factor of solubilizing ratio was optimized. Maximum yield of PSI could be obtained at a solubilizing ratio of 30. Isolated PSI was proved to be a mixture of monomer and trimer analyzed with absorption spectra, low temperature fluorescence spectra and circular dichroism. The purity and activity of isolated PSI was determined using SDS-PAGE and oxygen consumption, which laid an important foundation to further study the stability of PSI and PSI-based biological solar cell device.Secondly, stability of photosynthetic membrane protein complex determines the lifespan of solar cell devices. Several conventional chemical surfactants and peptide surfactants were investigated to improve thermal stability of isolated PSI in solution. Cationic (CTAB) and non-ionic (DM) showed better effect on the stability of PSI than anionic (SDS) and zwitterionic surfactant (FC-16), which maybe correlate with the surface charges of PSI. Cationic peptide I5K2 could enhance denatured temperature of PSI from 48.2℃to 52.9℃, which was much better than conventional chemical surfactants. I5K2 could also significantly extend half-life of PSI at 48.2℃and showed an obvious concentration effect. Anionic A6D even exhibited converse effect on the thermal stability of PSI. In order to further explore the mechanism of surfactant stabilizing effect on PSI, the number of hydrophobic group I and hydrophilic group K was changed systematically to study its influence on the thermal stability of PSI. All synthesized IxKy series of peptide surfactants could enchanc the thermal stability of PSI to some extent, but no obvious rule was found with changing the number of two groups on the stability of PSI, which was influenced by hydrogen bonds or hydrophobic interactions between the complex system containing PSI, surfactants and water molecules. AFM analysis showed that I5K2 together with PSI and Triton X-100 could form large vesicular structures. The hydrophobic region of PSI was tightly wrapped up to prevent the attack of water molecules, which maybe partly explain how peptide surfactants to enhance the thermal stability of PSI. This process was also simulated in this paper.