Unlike most protein crystals, form IX of bovine pancreatic ribonuclease A
diffracts well when severely dehydrated. Crystal structures have been solved
after 2.5 and 4 days of desiccation with CaSO4, at 1.9 and 2.0 A resolution,
respectively. The two desiccated structures are very similar. An RMS
displacement of 1.6 A is observed for main-chain atoms in each structure when
compared to the hydrated crystal structure with some large rearrangements
observed in loop regions. The structural changes are the result of
intermolecular contacts formed by strong electrostatic interactions in the
absence of a high dielectric medium. The electron density is very diffuse for
some surface loops, consistent with a very disordered structure. This disorder
is related to the conformational changes. These results help explain
conformational changes during the lyophilization of protein and the associated
phenomena of denaturation and molecular memory.
