Structure of hemoglobin S fibers: optical determination of the molecular orientation in sickled erythrocytes.

Possible orientations of deoxyhemoglobin S molecules within sickle-cell fibers are delimited by polarized absorption measurements on single sickled cells and single crystals of deoxyhemoglobin A. The polarization ratio of cells provides a lower limit for that of an individual fiber and, coupled with the absorption properties of the deoxyhemoglobin molecule, restricts the orientation of the long molecular (x) axis to within 22 degrees of the fiber axis. Adopting the stacked ring model of Finch et al. for the molecular positions and the additional constraint that at least one mutated (beta6) site is part of an intermolecular contact, our optical result requires that the true molecular dyad (y) axis pass through some part of an adjacent molecule in the same ring. This range of orientations for the y axis is approximately perpendicular to those described in existing models and places at least one beta6 residue in position to be part of a contact between molecules in the same ring.