Are protein crystallization mechanisms relevant to understanding and control of polymerization of deoxyhemoglobin S?

We investigated the homogeneous nucleation and subsequent evolution of poly mers of sickle cell hemoglobin (HbS) using differential interference contra st (DIC) microscopy. The same technique was employed to determine the liqui d liquid separation boundaries for a variety of conditions in solution of s ickle cell and normal human hemoglobin. The HbS polymers were also imaged u sing atomic force microscopy. We found that the location of Liquid-Liquid p hase boundary under conditions that mimic those in the erythrocytes is cons istent with previous determinations of the spinodal for this phase transiti on. Increasing the ionic strength shifts this phase boundary to significant ly lower temperatures and fib concentrations. We also found that the nuclea tion of individual HbS fibers indicates that the process is random and foll ows statistics similar to those established for nucleation of crystals or l iquid droplets from vapors. (C) 2001 Elsevier Science B.V. All rights reser ved.