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Biophys J, August 2000, p. 1016-1022, Vol. 79, No. 2

and
*Department of Physics, Drexel University, Philadelphia,
Pennsylvania 19104, and
Department of Physiology and
Biophysics and Department of Medicine, Albert Einstein College of
Medicine, Bronx, New York 10461 USA
The homogeneous and heterogeneous nucleation kinetics of
sickle hemoglobin (HbS) have been studied for various degrees of solution crowding by substitution of cross-linked hemoglobin A, amounting to 50% of the total hemoglobin. By cross-linking hemoglobin A, hybrid formation between hemoglobin A and hemoglobin S was prevented, thus simplifying the analysis of the results. Polymerization was induced by laser photolysis, and homogeneous nucleation kinetics were determined by observation of the stochastic behavior of the onset
of light scattering. Heterogeneous nucleation was determined by
observing the exponential growth of the progress curves, monitored by
light scattering. At concentrations between 4 and 5 mM tetramer (i.e.,
~30 g/dl), the substitution of 50% HbA for HbS slows the reaction by
a factor of 103 to 104. Using scaled particle
theory to account for the crowding of HbA, the observed decrease in the
homogeneous nucleation rate was accurately predicted, with no variation
of parameters required. Heterogeneous nucleation, on the other hand, is
not well described in the present formulation, and the theory for this
process appears to require modification of the way in which nonideality
is introduced. Nonetheless, the accuracy of the homogeneous nucleation
description suggests that such an approach may be useful for other
assembly processes that occur in a crowded intracellular milieu.
Biophys J, August 2000, p. 1016-1022, Vol. 79, No. 2
© 2000 by the Biophysical Society 0006-3495/00/08/1016/07 $2.00
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