Polarity of the 14-strand fibers of sickle cell hemoglobin determined by cross-correlation methods.

Images of negatively stained fibers of sickle cell hemoglobin have been analyzed by cross-correlation methods. These methods are used to compensate for the curvature and variable repeat distance characteristic of negatively stained fibers. Averaged images obtained by the correlation procedure display considerably more detail than the filtered images obtained earlier by Fourier methods. The averaged images are sufficiently detailed that the back-projection method for obtaining cross-sections can now be applied directly to the correlation-averaged images. This method was used previously to deduce the 14-strand structure on Fourier-filtered images that incorporated only the near-equatorial maxima. In this way the 14-strand structure has been reconfirmed without utilizing the partial Fourier reconstructions employed earlier that might conceivably have introduced spurious features. In addition, application of the correlation procedure with and without inversion of the reference reveals a consistent polarity in all of the fibers examined. The confirmation of the 14-strand structure by a new procedure and the determination of fiber polarity would appear to eliminate the alternative model of the fibers with a 16-strand structure with equal numbers of strands (eight) of each polarity.