Dynamic Equilibrium at the Bacteriorhodopsin Crystal Edge

The purple membrane (PM) exists in the plasma membrane of Halobacterium halobium, and its constituent protein, bacteriorhodopsin (bR), functions as a light-driven proton pump. In the PM, bR monomers are associated to form a trimeric oligomer and the trimers are arranged in a hexagonal lattice (see also chapters 17 and 18). However, several aspects in the crystal formation remain open for example, (1) trimer-trimer interaction sites and association energy and (2) existence of preformed trimers in the fluidic noncrystal region. In the 2D crystal of bR and any crystals in general, they are in dynamic equilibrium with the constituents at the interface between the crystal and the liquid phase. Here, we visualized dynamic events at the interface in the PM to provide information of the crystal formation and intermolecular interactions.  

Because the type 11 interaction contains two elementary bonds, whereas the type 111 interaction contains three, the energy difference fiii-fii corresponds to the association energy of the single elementary bond. From the ratio T2/T3 = 0.22 and Eq. 19.2, this elementary association energy is estimated to be about -1.5 k T, which corresponds to -0.9 kcal/mol at 300 This value is approximately consistent with that estimated by differential scanning calorimetiy.  

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