Studies of monoclinic hen egg-white lysozyme. IV. X-ray refinement at 1.8 Å resolution and a comparison of the variable regions in the polymorphic forms

Monoclinic crystals of hen egg-white lysozyme (E.C. 3.2.1.17, HEL) grown at low pH in the presence of NaNO₃ belong to space group P2₁ with unit-cell dimensions, a = 28.0, b = 62.5, c = 60.9 Å and β= 90.8° with two molecules in the asymmetric unit. 1.8 Å resolution intensity data, collected on a CAD-4 diffractometer, contained 17 524 reflections with F > 3σ (93% complete). Our earlier preliminary 1.8 Å model was refitted and refined using X-PLOR to an R value of 0.189. The deviations in the model from ideal geometry are 0.013 Å in bond lengths and 2.8° in bond angles. The r.m.s. deviation in the backbone atoms between the two molecules is 0.42 Å. A comparison of HEL in different polymorphic crystal forms reveals that the prominent structural variability among them resides in two exposed regions 45–50 and 65–73 which are also regions of lattice contacts.