DNA tetrahedra as structural frameworks for catalytic centers

The need for a renewable and sustainable light-driven energy source is the motivation for this work, which utilizes a challenging, yet practical and attainable bio-inspired approach to develop an artificial oxygen evolving complex, which builds upon the principles of the natural water splitting mechanism in oxygenic photosynthesis. In this work, a stable framework consisting of a three-dimensional DNA tetrahedron has been used for the design of a bio-mimic of the Oxygen-Evolving Complex (OEC) found in natural Photosystem II (PSII). PSII is a large protein complex that evolves all the oxygen in the atmosphere, but it cannot be used directly in artificial systems, as the light reactions lead to damage of one of Photosystem II's core proteins, D1, which has to be replaced every half hour in the presence of sunlight. The final goal of the project aims to build the catalytic center of the OEC, including the Mn4CaCl metal cluster and its protein environment in the stable DNA framework of a tetrahedron, which can subsequently be connected to a photo-stable artificial reaction center that performs light-induced charge separation. Regions of the peptide sequences containing Mn4CaCl ligation sites are implemented in the design of the aOEC (artificial oxygen-evolving complex) and are attached to sites within the tetrahedron to facilitate assembly. Crystals of the tetrahedron have been obtained, and X-ray crystallography has been used for characterization. As a proof of concept, metal-binding peptides have been coupled to the DNA tetrahedron which allowed metal-containing porphyrins, specifically Fe(III) meso-Tetra(4-sulfonatophenyl) porphyrin chloride, to be encapsulated inside the DNA-tetrahedron. The porphyrins were successfully assembled inside the tetrahedron through coordination of two terminal histidines from the orthogonally oriented peptides covalently attached to the DNA. The assembly has been characterized using Electron Paramagnetic Resonance (EPR), optical spectroscopy, Dynamic Light Scattering (DLS), and x-ray crystallography. The results reveal that the spin state of the metal, iron (III), switches during assembly from the high-spin state to low-spin state.