. Elementary biophysics: selected topics . Fig. 40. The situation in a CsCl density gradient in which the solution den- sity varies from at the inner edge of the tube to at the outer edge of the tube. The point of density is indicated, and particles of that density will be found in a band at that point. If particles were to the left of the band, they would be more dense than the solution and thus would be moved to the right by the centrifuging force. If to the right of the band, they would be less dense than the solution and thus would be floated inward to the left. Thus, after man
. Elementary biophysics: selected topics . Fig. 40. The situation in a CsCl density gradient in which the solution den- sity varies from at the inner edge of the tube to at the outer edge of the tube. The point of density is indicated, and particles of that density will be found in a band at that point. If particles were to the left of the band, they would be more dense than the solution and thus would be moved to the right by the centrifuging force. If to the right of the band, they would be less dense than the solution and thus would be floated inward to the left. Thus, after many hours of spinning the mixture of particles and CsCl solution, the situation sketched will be obtained. be presented below. This is called the method of the sedimentation velocity. (c) If the centrifuge is run for a sufficiently long time (days), all the particles will tend to be at the bottom of the tube. But, due to the Brownian (heat) motion, some of the particles will actually go backward toward the axis of rotation. When these two processes (centrifugal sedi- mentation and backward diffusion) have come to equilibrium, there will be a gradient of particle concentration from essentially zero at the inner edge of the tube to a maximum at the outer edge. This gradient can be made visible by the optical methods referred to above. The bigger the particles, the more they will tend to be concentrated near the outer edge of the tube. From a measurement of the actual concentration gradient, then, the molecular weight of the particles may be directly deduced. This is called the method of sedimentation equilibrium. (d) If a low molecular weight salt were in solution, the sedimentation equilibrium method would result in a very gradual gradient. Indeed, it should be obvious that a salt could be chosen writh so low a molecular weight that it is hardly sedimented at all. Table salt (NaCl), for exam- ple, would hardly be expected to be recovered from a solution by this method. But it will
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