Molecular packing in a second monoclinic crystal of deoxygenated sickle hemoglobin

A close correspondence has been demonstrated between double filaments of deoxygenated hemoglobin S molecules as found in monoclinic crystals, forms I and II, and in sickle fibers. We have carried out a low resolution study of monoclinic form II by X-ray diffraction analysis. Its structure differs from that of form I solely by a shift along the a-axis of the molecular centers of the asymmetric unit, which forms the double filament. The magnitude of the translation was determined from a minimum residual calculation. The x co-ordinates of the symmetry related molecular centers of antipolar double filaments are approximately the same. This means that the double filaments are nearly in register. A minor component associated with form II crystals proved to be form I. The possible existence of additional forms is discussed.The significance of the molecular arrangement in form II is related to its presence in sickle fibers. We have determined the contacts between antipolar double filaments in this form as well as a number in form I not tabulated previously. These new contacts represent additional stabilizing interactions that might provide targets for the design of stereospecific antisickling agents.