A light scattering study of the interaction of sodium desoxyribonucleate with horse serum albumin

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Abstract
The properties of soluble complexes formed in acetate buffer solutions from sodium desoxyribonucleate and serum albumin have been studied by a light scattering method. It has been established that the molecular weight, size, and shape of the complexes formed are influenced by pH, ionic strength, and the proportions and total concentration of the components. At ionic strength 0.04 and about ‐a 1:1 weight ratio of the components, no binding of albumin by nucleate has been observed at pH higher than 6.0. With continuously decreasing pH, an increasing number of albumin molecules is bound by individual nucleate molecules; this causes stretching of the nucleate molecules and an increase in the excluded volume. At pH 5.5 crosslinking begins. This causes the nucleate molecules to shrink. The concentration of non‐bound albumin becomes negligible, and forces of attraction act between the complex particles in solution. With further lowering of the pH, the molecular weight of the complex increases until, at pH 5.1, precipitation of the complex occurs. Qualitatively, the same picture is obtained if we follow the influence of ionic strength at pH 5.3 and the weight ratio of the components. The formation of the complex begins at an ionic strength below 0.20, and crosslinking and complex precipitation occur at ionic strengths of 0.06 and 0.02, respectively. The lowering of the nucleate to albumin ratio has an effect similar to that of lowering of pH or ionic strength. The molecular weight, size, and shape of the complex are stable only in a limited concentration range. At too high concentrations, complex precipitation occurs, and at too low concentrations, there is dissociation of the complex. Ultraviolet absorption measurements indicate that purine and pyrimidine bases of nucleate do not participate in the binding of albumin. It is assumed that albumin is attached to the nucleate by electrostatic forces between positively charged groups of protein and negatively charged phosphate groups of nucleate only.