FIGURE 1.

Schematic representation of the two-step nucleation mechanism of HbS polymers. (a) The nucleation pathway in the space of order parameters. Horizontal axis is HbS concentration, along which dilute solution and dense liquid can be distinguished. Axes of evolution of ordered structures are orthogonal to concentration axis, only two leading to polymers and crystals, respectively, are shown. Other possible structure axes include other crystal polymorphs, disordered aggregates, and gels. Thin dashed arrow along the diagonal indicates pathway of the one-step nucleation mechanism. Thick short-dashed lines indicate pathways of the two-step mechanism leading to polymers or crystals, respectively. (b) Free energy G landscape for different phases possible in HbS solutions. The abscissa is a one-dimensional projection of the full set of order parameters characterizing the phases in the hemoglobin plus solvent system. This coordinate can be approximately thought of as HbS density plus degree of ordering of the HbS molecules. Lower G corresponds to higher stability; thus, the dense liquid is metastable with respect to the solution in HbS solutions and becomes stable upon the addition of PEG (60) or concentrated phosphate (84).