In experiments designed to determine the thermal stability and bonding strength of a natural nucleoprotein structure, the loss of birefringence as a function of time and temperature was investigated for both mammalian and nonmammalian sperm nuclei. At a constant temperature, this reaction was found to be first order for both types over a range of temperatures. The methods of chemical kinetics applied to results of these reactions, called birefringence melting reactions, produced values for the enthalpy and entropy of activation in the reactions, which gave some indication of the strength of binding in the nucleoprotein structure; and these results, plus those on the influence of chemicals on the structure, were consistent with the molecular structures which have been proposed by others for the nucleoprotein complex of sperm nuclei. For both bull and human sperm in ethylene glycol, the rate-limiting step in the melting reactions appeared to be the breakage of disulfide bonds. For squid sperm in ethylene glycol, and bull or squid sperm in ethylene glycol plus ß-mercaptoethanol, the identity of this step was more ambiguous, but a possibility consistent with these and other results would be a cooperative breakage of ionic bonds.

In thermal denaturation experiments on sperm cells, described in the accompanying paper, it was found that squid sperm, when melted, lose both birefringence and morphological shape. Bull sperm, on the other hand, show no change of morphology, but their initial negative birefringence becomes positive. Since this suggested the existence of form birefringence, the influence of solvents of different refractive indices on the observed birefringence was investigated, using a new derivation of the Wiener form birefringence equations which allows direct comparison of Wiener's theory with experimental results. Bull sperm showed form birefringence both before and after melting, while squid sperm showed none. Quantitative application of the general form of the Wiener equations to these results gave values for the refractive index and intrinsic birefringence of bull sperm cells. Application of the specific forms of the Wiener equations showed that neither of these descriptions of idealized systems was adequate to describe completely the form birefringence of bull sperm, but that the equation for platelike submicroscopic structures was more nearly an accurate fit to the data than that for rodlike structures.