The form birefringence of macromolecules

Perutz has shown that the birefringence changes when the water in a haemoglobin crystal is replaced by salt solution. The effect of this substitution in the birefringenee is calculated and compared with Perutz's observations. There is agreement as to the order of magnitude of the effect, but the optical data indicated a more elongated molecule (a/b ,-~ 1.45) than that deduced from the X-ray data (a/b ,.~ 1.3). This discrepancy is not to be stressed, since the form of the molecule is not yet certain and the measurements of change in birefringence are very approximate. A further exploration of the effect may yield a useful method of estimating the form of other protein molecules.