The capacity of zones in density-gradient centrifugation

Zonal rotor centrifugation revisited: new horizons in sorting nanoparticles

Density gradient centrifugation is an effective method for the isolation and purification of small particles. Hollow rotors capable of hosting density gradients replace the need for centrifuge tubes and therefore allow separations at large scales. So far, zonal rotors have been used for biological separations ranging from the purification of whole cells down to serum proteins. We demonstrate that the high-resolution separation method opens up exciting perspectives apart from biology, namely in sorting mixtures of synthetic nanoparticles. Loading and unloading, while the rotor is spinning, avoids perturbations during acceleration and deceleration periods, and thus makes a vital contribution to sorting accuracy. Nowadays one can synthesize nanoscale particles in a wide variety of compositions and shapes. A prominent example for this are “colloidal molecules” or, generally speaking, defined assemblies of nanoparticles that can appear in varying aggregation numbers. Fractionation of such multimodal colloids plays an essential role with regard to their organization into hierarchical organized superstructures such as films, mesocrystals and metamaterials. Zonal rotor centrifugation was found to be a scalable method of getting “colloidal molecules” properly sorted. It allows access to pure fractions of particle monomers, dimers, and trimers, just as well as to fractions that are essentially rich in particle tetramers. Separations were evaluated by differential centrifugal sedimentation, which provides high-resolution size distributions of the collected nanoparticle fractions. The performance achieved in relation to resolution, zone widths, sorting efficiencies and recovery rates clearly demonstrate that zonal rotor centrifugation provides an excellent solution to the fractionation of nanoparticle mixtures.

Hollow bowl-shaped rotors allow for efficient fractionation of nanoparticle mixtures at large scale.

Preparative particle separation in density gradients

The purpose of this review is to summarize the present status of preparative zonal centrifugation and to indicate where the remaining problems, in the reviewer's opinion, may lie. The field is sufficiently new for interest in practical separartions to be uppermost. However, a number of theoretical problems relating to gradient capacity, zonal rotor configuration and particle—particle interactions remain to be fully explored.

Macromolecular distribution near the limits of density-gradient columns. Some applications to the separation and fractionation of glycoproteins

1. Expressions are derived for the distribution at density-gradient equilibrium of macromolecules whose densities are (a) close to the values characterizing the solution limits or (b) outside the span of the gradient. 2. Density-distribution predicted by the expressions agree with those obtained by rigorous methods. 3. The distribution equations are applied to hypothetical mixtures of proteins and glycoproteins in commonly used density-gradient media to simulate separation and fractionation conditions. 4. It is shown that CsBr, although less efficient than CsCl for fractionation, is nevertheless adequate for most purposes; in analytical experiments it may often have advantages over CsCl. Limitations on the use of LiBr are explored. 5. An expression is derived which allows the variance of the partial specific volume of the macromolecular component to be determined from the variance of the buoyant density. It is shown that the relative resolving powers of different salts is expressed by their values of the quantity (formula: see text). 6. The equations are applied to a well-characterized glycoprotein preparation at equilibrium in CsCl and in Cs2SO4:it is shown that the much wider distribution in CsCl than in Cs2SO4 is explicable in terms of the variance in buoyant density and the solvation properties of the salts. 7. Limitations of the expressions arise when dispersity in density is represented by a low apparent molecular weight; realistic simulations can then only be obtained when the component is fully banded.

Simultation of gradient and band propagation in the centrifuge

A technique is developed for simulating the behavior of both the gradient-forming solute and macromolecular bands in a centrifuge. The change with time of the density gradient due to diffusion and sedimentation of the gradient-forming solute is calculated by a finite difference method, making use of the results of earlier work on the theory of the equilibrium density gradient. Using a perturbation technique, the concentration profiles of dilute bands of macromolecules are then calculated as they sediment and diffuse through the varying supporting gradient. Results of the stimulaion techniques are compared with experiment.

The effects of overloading in density-gradient centrifugation

The effects of overloading of the sample zone in density gradient centrifugation have been studied by use of a three-component shelf-lavered sample in which the total protein concentration was increased by addition of different amounts of albumin. It is found that overloading of the gradient gives rise to particle movements which are not predictable from the Svedberg equation. The two typical effects of overloading are dislocation of the zone mass centres and changes in the zone shapes. It is found that the magnitude of the calculated sedimentation coefficients increases nearly linearly with increasing sample load. The changes in zone shapes are found to depend on the specific load and two different patterns may be distinguished. The zone of the sample component which causes the overloading is defined as primarily overloaded and the others as secondarily overloaded. In primarily overloaded zones the original Gaussian shape is lost, while in secondarily overloaded zones the Gaussian zone shape is maintained, although a zone broadening is seen. Extreme high loads are found to be able to divide single zones. As a whole these experiments show that evidence for a non-overloaded set of experimental conditions must be provided, when density gradient centrifugation is used for determination of sedimentation coefficients. For preparative gradient centrifugations the power of resolution will decrease with increasing sample load. A simple method to detect overloading in density gradient centrifugations is described.