Energy transduction inside of amphiphilic vesicles: encapsulation of photochemically active semiconducting particles

Amphiphilic bilayer membrane structures (vesicles) have been postulated to have been abiotically formed and spontaneously assemble on the prebiotic Earth, providing compartmentalization for the origin of life. These vesicles are similar to modern cellular membranes and can serve to contain water-soluble species, concentrate species, and have the potential to catalyze reactions. The origin of the use of photochemical energy in metabolism (i.e. energy transduction) is one of the central issues in the origin of life. This includes such questions as how energy transduction may have occurred before complex enzymatic systems, such as required by contemporary photosynthesis, had developed and how simple a photochemical system is possible. It has been postulated that vesicle structures developed the ability to capture and transduce light, providing energy for reactions. It has also been shown that pH gradients across the membrane surface can be photochemically created, but coupling these to drive chemical reactions has been difficult. Colloidal semiconducting mineral particles are known to photochemically drive redox chemistry. We propose that encapsulation of these particles has the potential to provide a source of energy transduction inside vesicles, and thereby drive protocellular chemistry, and represents a model system for early photosynthesis. In our experiments we show that TiO2 particles, in the approximately 20 nm size range, can be incorporated into vesicles and retain their photoactivity through the dehydration/rehydration cycles that have been shown to concentrate species inside a vesicle.