Application of peptide gemini surfactants as novel solubilization surfactants for Photosystems i and II of cyanobacteria

We designed novel peptide gemini surfactants (PG-surfactants), DKDKC ₁₂K and DKDKC₁₂D, which can solubilize Photosystem I (PSI) of Thermosynecoccus elongatus and Photosystem II (PSII) of Thermosynecoccus vulcanus in an aqueous buffer solution. To assess the detailed effects of PG-surfactants on the original supramolecular membrane protein complexes and functions of PSI and PSII, we applied the surfactant exchange method to the isolated PSI and PSII. Spectroscopic properties, light-induced electron transfer activity, and dynamic light scattering measurements showed that PSI and PSII could be solubilized not only with retention of the original supramolecular protein complexes and functions but also without forming aggregates. Furthermore, measurement of the lifetime of light-induced charge-separation state in PSI revealed that both surfactants, especially DKDKC₁₂D, displayed slight improvement against thermal denaturation below 60 C compared with that using β-DDM. This degree of improvement in thermal resistance still seems low, implying that the peptide moieties did not interact directly with membrane protein surfaces. By conjugating an electron mediator such as methyl viologen (MV²⁺) to DKDKC₁₂K (denoted MV-DKDKC ₁₂K), we obtained derivatives that can trap the generated reductive electrons from the light-irradiated PSI. After immobilization onto an indium tin oxide electrode, a cathodic photocurrent from the electrode to the PSI/MV-DKDKC₁₂K conjugate was observed in response to the interval of light irradiation. These findings indicate that the PG-surfactants DKDKC ₁₂K and DKDKC₁₂D provide not only a new class of solubilization surfactants but also insights into designing other derivatives that confer new functions on PSI and PSII.