. Elementary biophysics: selected topics. Biophysics. 84 METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE. Axis of rotation Fig. 38. Tubes spinning about an axis of rotation with an angular velocity a) and with the particle of interest at a distance r from the axis. The particle is at P and is suspended in a liquid L. Inner edge > Boundary I Aijfw*:** Axis of rotation Particles in solution moving to the right Particles which have reached the outer end of the tube and have formed a pellet Fig. 39. A representation of the situation some time after a tube containing particles denser than the s
. Elementary biophysics: selected topics. Biophysics. 84 METHODS FOR DETERMINING MOLECULAR SIZE AND SHAPE. Axis of rotation Fig. 38. Tubes spinning about an axis of rotation with an angular velocity a) and with the particle of interest at a distance r from the axis. The particle is at P and is suspended in a liquid L. Inner edge > Boundary I Aijfw*:** Axis of rotation Particles in solution moving to the right Particles which have reached the outer end of the tube and have formed a pellet Fig. 39. A representation of the situation some time after a tube containing particles denser than the suspending liquid has been started spinning in a cen- trifuge. The particles which were at the inner edge at time zero move together to the right and thus form a boundary whose movement can be photographed for determination of the velocity of the movement. (b) The rate at which a particle will move when being spun at a given frequency w at a given radius r will depend on its density, the density and viscosity of the liquid, and on the size of the particle, as wall be demonstrated below. The individual particle cannot be studied, but the particles at the inner edge of the tube form a boundary which moves radially outward as a result of the centrifligation, as indicated in Fig. 39. This boundary can be made visible by optical methods, so that the rate of displacement of the particles can be deduced by photographing the transparent tube at various times after starting the centrifuge. Since the density and vis- cosity of the liquid can be measured, it is possible to compute the ratio of the mass of the particle (its molecular weight) to its size. If the particle is spherical, the molecular weight can be deduced directly. If it is not spherical, a somewhat more complicated procedure is required which gives us the size of the particle by measuring the diffusion of particles through a liquid. Then, by a combination of these measurements, the molecular weight can be deduced. The mathema
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