Prospective energy densities in the forisome, a new smart material

The forisome is a protein structure of plants which, in low Ca ²⁺ solutions, assumes a crystalline condensed conformation and, at high Ca²⁺, swells to a dispersed conformation; this transition has been attributed to electrostatic deformation of protein "modules". Forisomes could become an important smart material if the energy density of transformation approached 1 MJ m^{- 3}. Quantitation of the forisome as a charged porous continuum permeated by electrolyte fails by orders of magnitude to achieve this energy density electrostatically. However, condensed → dispersed transitions can be visualized alternatively: (i) an ionic bond near the surface of a forisome crystal dissolves to produce two bound surface charges; (ii) the anionic site bonds to Ca²⁺ becoming less negative; (iii) the two sites repel each other and move apart drawing in water; (iv) electrolyte anions are attracted, bringing with them bound water; (v) this transition propagates throughout the crystal, with incompressible imbibed electrolyte stabilizing the initial separation. With the above transition sequence, it seems possible to achieve an energy density of 0.5 MJ m^{- 3} if the "modules" in the crystal are roughly 10 nm on a side.