NASA Astrobiology Institute (NAI)

Energy Transduction Inside of Amphiphilic Vesicles

David P. Summers, SETI Institute

Many now look beyond theories for the origin of life that rely solely on a “prebiotic soup” (capable of supporting complex “protobiochemical” reaction, particularly in open bodies of water) and have started examining the role of various structures that might have aided the necessary, complex chemistry. Vesicles, water filled membranes made up of amphiphilic compound such as lipids, are one type of structures that have been particularly promising. The membrane of the vesicle, which is somewhat similar to modern cellular membranes, encloses a sample of aqueous solution and could have contained water-soluble species, concentrated species, and even catalyzed reactions. It has been shown in a number of earlier studies that these structures could have been formed and spontaneously assembly on the prebiotic Earth, providing compartmentalization for the origin of life.

The origin of the use of photochemical energy in metabolism (i.e. photosynthesis or “energy transduction”) is one of the central issues in both the origin and evolution of life. It raises such questions as how energy transduction may have occurred before the development of complex enzymatic systems, such as required by contemporary photosynthesis, and how simple a photochemical system is possible. Vesicle structures may have developed the ability to capture and use light energy. 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.

Encapsulation of colloidal semiconducting mineral particles in vesicles has the potential to provide a simple model for energy transduction that drives protocellular chemistry inside vesicles. In my laboratory, we have demonstrated through testing the system with the redox indicator methyviologen, MV+2 that TiO2 particles in the ~20 nm size range can be incorporated into vesicles and retain their photoactivity even when being concentrated inside vesicles by dehydrate/rehydration. Our current work is aimed at extending this chemistry to biochemical species as the reduction of NAD+ to NADH.

This system, which can drive redox changes inside membrane-bound compartments, represents an experimental model and proof of concept of an energy transduction system in the protocellular stage of the origin of life that can drive chemical changes inside vesicles. This kind of encapsulated photochemistry represents a plausible prebiotic source of activated molecules and could have been used to drive a “protobiochemical” metabolism. More generally, redox reactions at semiconducting surfaces can be considered as potential sources of energy and organic/biochemical species contributing to the origin and early evolution of life. For more details see:
Summers D. P., Noveron J., and Basa R. C. B. (2008) Energy Transduction Inside of Amphiphilic Vesicles: Encapsulation of Photochemically Active Semiconducting Particles., Orig. Life Evol. Biosphere 39, 127–40.

Figure 1. Photographs showing the reduction of MV+2 to the blue colored radical cation by TiO2 encapsulated in vesicles.

Figure 2. Micrograph of the fluorescence of NADH photochemically generated and encapsulated inside of vesicles.