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I41 S T R Y
Ultracentrifuge Studies with Rayleigh Interference Optics. 11. Low-Speed Sedimentation Equilibrium of Homogeneous Systems" E. G. Richards, D. C . Teller,? and H. K. Schachman
ABSTRACT : Procedures
are presented for the determination of molecular weights by the low-speed sedimentation equilibrium method coupled with a synthetic boundary experiment which is used for the labeling of the Rayleigh interference fringes. Emphasis is placed on experimental details concerning the planning and performance of the experiments and also on the reading of interference patterns. A method is described for determination of the optimum rotor velocity leading to resolvable fringes a t the bottom of the cell for solutions of varying concentrations and column heights. General equations are used which permit the calculation of both weight and z-average molecular weights for polydisperse, ideal systems, as well as the molecular weight and second virial coefficient for two-component, nonideal systems. Labeling of fringes is accomplished by both conservation of mass in the cell and by location of the hinge point with the aid of the white-light fringe. Agreement of results by the two methods generates confidence that (1) all material in the cell was accounted for. (2) the fringe labeling was correct, and (3) no calculation errors were made, To eliminate tedious calculations necessary for thorough evaluation of the available data on the interference patterns, a computer program
D
espite the long-standing and sound theoretical foundation of the sedimentation equilibrium method (Svedberg, 1925; Svedberg and Pedersen, 1940), until recently it was used only rarely for the determination of molecular weights of large and small molecules. The resurgence of the method in the past 6 years stems principally from the large reduction in the time required for an experiment and the greatly increased accuracy in the optical registration of concentration
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* From the Department of Biochemistry and Virus Laboratory (H. I
