The standard molecular model of the fiber of the
sickle hemoglobin (HbS: β6 Glu → Val) has been
revised to allow both β6 mutation sites to participate
in intermolecular contacts, rather than only one β6
site as previously thought, for four molecules per 14-molecule
fiber cross section. This structure accurately predicts
the copolymerization of hybridized mixtures of HbS with
HbA or HbC (β6 Glu → Lys), which could not be
reconciled with prior models in which only half the β6
sites were required for assembly. This model suggests new
contacts within the fiber and raises the question of whether
these cross-linked double strands could possess added stability
important in such processes as nucleation.