Sedimentation Studies of Gelatin Gels

Structure and properties of rat thymus deoxyribonucleoprotein. I. Electric birefringence and centrifugation studies

Deoxyribonucleoprotein ( DNP ) has been prepared from rat thymus under strictly controlled conditions. It is obtained as an elastic, opaque gel and has been studied in gel form and after dilution, chiefly by the method of electric birefringence. The electric birefringence depends on optical and electrical properties of the particles oriented in an electric field and the decay of birefringence (on removal of the field) depends on the particle length. Birefringence measurements have been made over a range of concentrations and include the effects of ageing, vigorous shaking, temperature change and centrifugation. Preparations of satisfactory stability and with very reproducible properties have been obtained. At all concentrations, particles of apparently the same range of lengths (of up to about 1·6 μ m) are measured in the electric field. On centrifugation of dilutions of the gels, separation into a gel phase and a solution phase occurs; the birefringence properties of the latter are similar to those of unfractionated solutions at the same concentration. It is proposed, therefore, that the gels consist of networks of highly interlinked particles, the networks being broken up partly on dilution into clusters of (reversibly associated) particles. The gel phase is composed of large clusters and the solution phase of much smaller clusters or of single particles. In the electric field all clusters are at least partially broken so that orientation of more or less single particles occurs. Ageing is suggested to be a process of denaturation. It occurs more rapidly in shaken solutions than in controls. Undiluted gels are more resistant than dilutions to ageing, shaking and freezing.