Solubilities of deoxygenated sickle cell hemoglobin (deoxy-Hb S), at varying pH and temperature over a range of concentrations encompassing those found in erythrocytes, were measured. The technique involved ultracentrifugation, which gave values of the supernatant concentration and the mass of the sedimented material. The data establish that the solubility of deoxy-Hb S is the saturation concentration and is independent of initial concentration. The mass of the pellet phase increases linearly with initial concentration. Moreover, the saturation concentration represents the critical concentration above which monomers are in equilibrium with polymers. These polymers are the putative cause of erythrocyte deformation associated with sickle cell anemia. The solubility-pH profiles of deoxy-Hb S at various temperatures, unlike those of other proteins, show no minima at the isoelectric pH, but instead snow a marked decrease in solubility below pH 7.0, indicating the predominance of polymerization over the expected increase in solubility. Deoxy-Hb S, within specified ranges of temperature and pH, possesses a negative temperature coefficient of solubility, a property characteristic of hydrophobic interactions. The saturation concentration is, however, temperature independent at conditions close to physiological. The enthalpy of polymerization (3.5 kcal/mol) is temperature independent from 60 to 220 for all pH values between 6.45 and 7.40. In the range of 220 to 38°, this parameter becomes less endothermic, having a value of 2.5 kcal/mol at pH 6.45 and a value of zero at pH 7.20. Such behavior of the system suggests a phase transition near 220. Within the range of.conditions examined the polymerization is entropically driven.Sickle cell hemoglobin (Hb S) is a variant of normal adult hemoglobin (Hb A) in which a substitution of valine for glutamic acid occurs at position 6 in both ,3 chains. Deoxygenated solutions of Hb S, at concentrations comparable to those within erythrocytes, form liquid crystals or tactoids. These nematic crystals of deoxy-Hb S, the putative cause of erythrocyte sickling, have been characterized optically by their birefringence and polarization dichroism (1, 2). Structural information derived from studies of both x-ray diffraction and optical diffraction of electron micrographs has provided evidence that these paracrystalline arrays are composed of helical microtubules packed into square lattices (3, 4).The term lowest gelling point has been used (5) to denote the concentration at which a solution of deoxy-Hb S loses its fluidity. Binary mixtures of Hb S with Hb A, and with mutant hemoglobins, have been characterized by their "lowest gelling point" as well. More recent experiments, using somewhat different techniques, have evaluated the effect of a diverse population of hemoglobins, both liganded and unliganded, on what has been termed minimum gelling concentration (6). Gelation of deoxy-Hb S has also been studied by sedimentation equilibrium experiments using either interference (7) or schlie...

line)LW(CO)s with triphenylphosphine in toluene at 35.4°was studied for several ratios of [A] to [L] ? The resulting plots of (log x)(At-Ablank) vs. t, exhibited in Figure 1, indicate reaction 2, = W(CO)5, to be markedly inhibited by added amine.8 In the presence of 10-20-fold excesses of amine the reactions fail to attain equilibrium over the time allowed for equilibration by A-I (2 days).1 The activation parameters are readily explicable on this basis: the reactions proceed further toward equilibrium at higher temperature, resulting in the previously discussed temperature sensitivity of "Aeq."Acknowledgments. The support of this research by the Robert A. Welch Foundation under Grant B-434 and the North Texas State University Faculty Research Fund is greatly appreciated. The experimental contribution of Mr. P. T. Chang is also acknowledged.Registry No. (Aniline)W(CO)5, 16969-72-5; triphenylphosphine, 603-35-0. Supplementary Material Available. A table of all rate observations will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy or $2.00 for microfiche, referring to code number INORG-74-1790.(7) Specific ratios of [A] to [I.] were not given in ref t. However, from the concentration ranges of amine and L employed in that study ([amine], (100-1600) X 10'4 M\ [L], (2-100) X 1(T4 M) and from the magnitudes of the reported values of K^, it can be inferred that 10-20-fold exesses of amine relative to triphenylphosphine conservatively represent conditions employed in those studies.(8) For triphenylstibine at an [A] to [L]ratio of 9.90, was observed to be 22.2 (2) hr.

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