Orthorhombic crystal form of bacteriorhodopsin nucleated on benzamidine diffracting to 3.6 A resolution.

Freshly formed benzamidine crystals were found to provide a suitable nucleation surface for crystallisation of bacteriorhodopsin. At 20 degrees C and 1% octylglucoside pseudohexagonal needles of bacteriorhodopsin nucleated on the benzamidine surface. At 4 degrees C and reduced detergent concentration (0.5%) a new, better-ordered orthorhombic crystal form of bacteriorhodopsin was formed by heterogeneous nucleation on benzamidine. Polarised absorption spectroscopy and flash photolysis experiments were used to show that the crystalline bacteriorhodopsin is photoactive in both forms. The M-intermediate absorbs maximally at 405nm and formation of M does not disturb the crystal lattice. The plate-shaped crystals diffract to a resolution of 3.6 A along the a and b directions and to 4.2 A in the c direction. The most likely space group of the crystals is C222 (a = 107.5 A, b = 117.0 A, c = 69.5 A). The crystals are built from layers of bacteriorhodopsin molecules that are tilted from the c axis by about 45 degrees. In addition, as a background to the discovery of the new crystal form, the influence of different detergents, additives and precipitants on the formation of pseudohexagonal needles is presented.