Crystallization and preliminary X-ray diffraction analysis of double-helical RNA octamers

Single crystals of a chemically synthesized self-complementary RNA octamer with sequence r(CCCCGGGG) have been obtained by screening 50 different conditions at room temperature using a standard sparse-matrix sampling method. Two crystal forms with different morphologies grew under diverse crystallization conditions within days by hanging-drop vapor diffusion. Hexagonal crystals with space group P6₁22 (one strand per asymmetric unit) and unit-cell dimensions a = b = 39.73 and c = 58.55 Å, diffracted to 2.6  Å. Rhombohedral crystals with space group R32 (one duplex per asymmetric unit) and unit-cell dimensions a = b = 42.38 and c = 131.70 Å, (hexagonal setting) diffracted beyond 1.5 Å. Data sets for both crystal forms were collected on image-plate/rotating-anode generator equipment and structure determinations and refinements are under way.