THE STABILITY OF THE LATTICE STRUCTURE IN LOW-SALT T-STATE HEMOGLOBINCRYSTALS

The reconstruction of the electron density of molecules in crystals fr om X-ray diffraction measurements depends on the exactness of the pack ing of the molecules in the unit cell and the crystal lattice. Crystal s of T-state haemoglobin, the low affinity form of the molecule, grow from high salt solutions or from low salt solutions in the presence of polyethylene glycol. The low salt lattice has the special property th at it allows the haemoglobin molecule to bind oxygen and other ligands without the crystal breaking up. The stability of the low salt T-stat e crystals appears to arise from a small number of well-defined salt b ridges and hydrogen bonds that are concentrated in specific lattice di rections. These together form a framework within which the molecule ca n make adjustments which are sufficient to accommodate ligand binding but in which the larger quaternary movements normally associated with oxygenation are prevented. In these crystals therefore interactions wi th ligands can be studied directly by X-ray diffraction and the struct ural basis of the T-state's low affinity for oxygen can be analysed.