from the American Chemical Society in 1970 and the Helen Hay Whitney Foundation Duckett Jones Award in 1972.

Active with a variety of problems, Professor Vinograd was best known for two major areas of scientific accomplishment: the theory and application of density gradient ultracentrifugation and the study of the properties of closed circular DNA rings.

DENSITY GRADIENT ULTRACENTRIFUGATION

Vinograd’s initial major contribution was the development of density gradient ultracentrifugation. This was stimulated by Matthew Meselson and Frank Stahl, who were seeking a means to implement their bold experiment to verify the hypothesis that DNA replication involved the separation of the two parental strands, one going into each of the two daughter DNA molecules.

Meselson and Stahl initially wanted to make the parental strands heavier than normal by incorporation of 5-bromouracil and sought Vinograd’s advice as to whether 5-bromouracil-containing DNA could be separated from normal DNA by velocity ultracentrifugation. Vinograd indicated this seemed unlikely unless the velocity difference could be magnified by approximately matching the density of the DNA of the strands with a salt solution. From this germinated the concept of equilibrium sedimentation of macromolecules in density gradients and subsequently the famous experiment of Meselson and Stahl using the isotope of N15 instead of bromouracil.

In the equilibrium sedimentation method the macromolecule (DNA) is dissolved in a salt solution (CsCl) of the appropriate density and centrifuged to equilibrium (approximately 24 hours). At equilibrium, driven by sedimentation and diffusion, the CsCl will form a stable gradient of concentration, increasing in density with the radius. The larger



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