Rational design of ultrafast PpcA-Ru(bpy)3 complexes.

Abstract

Converting light energy into its electrochemical equivalent requires precise control and fine-tuning of relevant kinetic and thermodynamic parameters of electron transfer (ET) steps. A particularly big challenge for artificial photosynthesis is to create the primary charge separation on the ultrafast (sub-nanosecond) time scale, as slower reactions are likely to require precious metal photosensitizers and produce significant amounts of reactive oxygen species. However, the structural requirements for ultrafast ET are poorly understood. The previous work from our lab has demonstrated that ET rates do not strictly depend on the distances. Similarly, we have demonstrated that short ET pathways through covalent bonds do not always result in ultrafast ET. We hypothesize that a rigid attachment of photosensitizers and efficient heat dissipation into the protein framework are needed for ultrafast ET. To test this hypothesis, we developed 13 new mutant forms of PpcA with the attachment cysteine sites showing restricted solvent exposure. We successfully labeled 11 out of 13 with Ru(bpy)3 and observed the expected complex masses in LC-MS. Analytical size-exclusion chromatography revealed the absence of aggregation and predominantly monomeric forms. Temperature-dependent circular dichroism (CD) spectroscopy showed the absence of any significant destabilization of the protein structure due to mutations and photosensitizer attachment, aside from I38CRu. Finally, we measured electron transfer rates that show that 10 of the 11 labeled PpcA-Ru(bpy)3 complexes show ultra-fast electron transfer.