Relationship between β4 hydrogen bond and β6 hydrophobic interactions during aggregate, fiber or crystal formation in oversaturated solutions of hemoglobin A and S

Oversaturated deoxy-alpha(2)beta(T4V)(2) aggregated instantly without a delay time, which is in contrast to the delay time before the generation of fibers of deoxy-HbS and deoxy-alpha(2)beta(E6V,D73H)(2). Solubility of deoxy-alpha(2)beta(T4V)(2) was similar to 10-fold lower than that of deoxy-HbS and was similar to oxy- and deoxy-alpha(2)beta(E6V,T4V)(2). These results indicate that beta 4Val in HbA in the oxy and deoxy forms with or without beta 6Val facilitates hydrophobic interaction of the A-helix with the EF helix of adjacent molecules without forming a beta 4/beta 73 hydrogen bond. Deoxy-HbA generated crystals following aggregation as does HbC-Harlem(alpha(2)beta(E6V,D73N)(2)), while alpha(2)beta(T4V)(2) and alpha(2)beta(D73H)(2) as well as HbS, alpha(2)beta(E6V,D73H)(2) and alpha(2)beta(E6V,T4V)(2) in the oxy and deoxy forms did not form crystals, indicating in addition to the strength of beta 6 amino acid hydrophobicity that the synergism between the 1 beta 4Thr hydrogen bond and beta 6 hydrophobic interaction free energies oil the A-helix play a critical role in formation of fibers versus crystalline nuclei during phase transition. (C) 2008 Elsevier Inc. All rights reserved.