Density-gradient centrifugation establishes media with different densities, allowing cells, organelles, membranous particles, macromolecular complexes, and nucleic acids to be fractionated according to their sedimentation behavior or buoyant density. Colloidal silica media, polysucrose media, iodixanol, sucrose, and CsCl have different physicochemical properties and application ranges.

Keywords: density-gradient centrifugation; colloidal silica; polysucrose; iodixanol; sucrose; CsCl

1 Physical Basis of Density-Gradient Separation

1.1 Particle Sedimentation and Buoyant Density

(1) Density Difference Between Particles and the Medium

Particle migration in a centrifugal field is jointly determined by centrifugal force, buoyancy, and viscous drag. When the particle density is higher than that of the surrounding medium, its net migration is directed toward the bottom of the centrifuge tube. As the particle enters gradient regions of progressively increasing density, the density difference between the particle and the medium decreases, and its sedimentation velocity declines accordingly. When the medium density approaches the particle’s buoyant density, the net driving force for further migration approaches zero.

(2) Particle Size, Shape, and Medium Viscosity

Buoyant density is not the only factor determining particle migration. For particles with similar densities, differences in size and shape can still produce different sedimentation velocities, while increased medium viscosity increases resistance to particle movement. Therefore, rate-zonal separation primarily exploits differences in particle sedimentation coefficients, whereas isopycnic separation depends more strongly on the buoyant densities of the particles themselves.

(3) Stabilization of Separation Zones by the Gradient

A medium density that gradually increases along the axis of the centrifuge tube suppresses liquid convection and component mixing, allowing different particles to remain in relatively concentrated separation zones. The density gradient therefore regulates particle migration while also stabilizing the separation interfaces.

1.2 Rate-Zonal, Isopycnic, and Density-Barrier Separation

(1) Rate-Zonal Centrifugation

Rate-zonal centrifugation separates particles primarily according to their sedimentation velocities. The sample is usually loaded onto the top of the gradient, and particles with larger sedimentation coefficients migrate farther under the same centrifugation conditions. Because the maximum density of the gradient generally remains below the buoyant density of the target particles, their separation positions are strongly time-dependent; excessive centrifugation can cause different components to continue migrating toward the bottom of the tube. Sucrose-gradient analysis of ribosomal subunits, monosomes, and polysomes is a typical application.

(2) Isopycnic Centrifugation

Isopycnic centrifugation separates particles primarily according to their buoyant densities. After migrating along the gradient to a region where the medium density approaches their own buoyant density, the net sedimentation force on the particles reaches equilibrium, and a stable zone is formed. Organelles, membranous particles, viruses, and nucleic acids can all be fractionated according to this principle in appropriate media.

(3) Density-Barrier Separation

Density-barrier separation places a medium layer of a defined density beneath the sample. Components capable of entering and passing through the medium layer continue migrating downward, whereas components that cannot effectively pass through it become enriched at the interface. Preformulated polysucrose density-separation media used to isolate peripheral blood mononuclear cells (PBMCs) are a typical application of this mode.

1.3 Step Gradients, Continuous Gradients, and Self-Generated Gradients

Step gradients consist of a series of medium layers with different densities, and target particles usually become enriched at the interfaces between adjacent density layers. They are suitable for fractionating components with relatively well-defined density ranges. In continuous gradients, medium density changes smoothly along the axis of the centrifuge tube, improving the resolution of components with similar densities. Some media can also redistribute in a centrifugal field to form self-generated gradients. Colloidal silica media, iodixanol, and CsCl can all establish continuous density changes under appropriate conditions.

2 Colloidal Silica Media

2.1 Density Characteristics of Colloidal Silica

(1) Colloidal Particles Provide Medium Density

Colloidal silica density-gradient media use colloidal silica particles as their primary source of density and may employ polyvinylpyrrolidone (PVP)-coated or silane-modified silica particles. The density of these media is primarily provided by high-density colloidal particles dispersed in the liquid phase, giving them a different physical basis from systems that rely on large quantities of low-molecular-weight solutes to increase density.

(2) Low Viscosity and Mild Osmotic Conditions

Because colloidal particles provide the density, colloidal silica media maintain relatively low viscosity even at higher medium densities and reduce the osmotic burden caused by high concentrations of small-molecule media. Following appropriate adjustment of the buffer and ionic composition, they can provide a separation environment suitable for intact cells and subcellular structures.

2.2 Preformed and Self-Generated Gradients

(1) Preformed Gradients

Colloidal silica working solutions of different densities can be prepared in advance as step or continuous gradients. During centrifugation, target cells or organelles enter the corresponding density regions and form bands according to their sedimentation behavior and buoyant density.

(2) Self-Generated Gradients

Colloidal silica particles can sediment and redistribute under relatively high centrifugal forces. Under appropriate rotor, centrifugal-force, and centrifugation-time conditions, a relatively homogeneous colloidal silica suspension can gradually form a continuous density gradient.

2.3 Separation of Leukocytes and Subcellular Particles

(1) Leukocyte Subsets

Colloidal silica media can use multiple density layers to further distinguish neutrophils, eosinophils, monocytes, and other leukocyte subsets with different densities. Compared with a fixed-density polysucrose barrier, their density range can be adjusted more flexibly, making them suitable for further fractionation of leukocyte subsets.

(2) Organelles

Crude fractions containing mitochondria, lysosomes, peroxisomes, secretory granules, and other organelles can first be obtained by differential centrifugation and then further purified with colloidal silica media to remove membrane fragments and other organelles with different densities.

(3) Tissue Debris and Low-Density Components

Colloidal silica media can also be used to remove myelin, dead cells, and low-density debris during the preparation of single-cell suspensions from certain tissues, reducing interference from noncellular components in flow cytometry and single-cell analysis.

2.4 Application Boundaries of Colloidal Silica Media

Colloidal silica media are colloidal-particle systems, and recovered target fractions generally require dilution, washing, or repeated centrifugation to reduce the amount of residual medium. Residual colloidal particles may interfere with certain optical measurements and protein quantification methods; therefore, the residual colloidal medium usually needs to be removed thoroughly before proteomic analysis, enzyme-activity assays, and similar studies. Iodixanol generally offers advantages for smaller membranous particles that require higher buoyant-density resolution.

3 Polysucrose Media

3.1 Highly Branched Polysucrose Structure

Polysucrose media can consist of highly branched hydrophilic polymers formed by crosslinking sucrose with epichlorohydrin. Their structures are compact, strongly hydrophilic, and essentially free of ionic groups, allowing them to serve as polymeric media in density-separation systems for biological samples. Polysucrose materials with different average molecular weights can be used for cell separation and studies of polysucrose-medium properties.

3.2 Differences Between Polysucrose Raw Materials and Preformulated Polysucrose Separation Media

(1) Polysucrose as a Polymeric Medium Component

Polysucrose materials with different average molecular weights and specifications can be used as density-medium components when constructing separation systems, with their applications determined mainly by molecular weight, concentration, and system design.

(2) Preformulated Polysucrose Density-Separation Media

Preformulated polysucrose density-separation media are not composed of polysucrose polymer alone but contain polysucrose together with high-density components in ready-to-use cell-separation systems. Their density and osmotic properties can be adjusted for specific blood-cell separation requirements, giving them a different application positioning from polysucrose raw materials.

3.3 Density-Barrier Separation of Blood Cells

(1) Red Blood Cells and Most Granulocytes Migrate Downward

After whole blood is layered over a polysucrose-based density-separation medium and centrifuged, red blood cells and most granulocytes, which have relatively high effective densities, can enter the separation medium and continue migrating downward. They are mainly distributed beneath the medium or form a pellet.

(2) PBMCs Become Enriched at the Interface

Lymphocytes and monocytes cannot pass through the density barrier as effectively as red blood cells and most granulocytes and therefore become enriched primarily at the interface between the plasma and the separation medium. This process represents cell stratification mediated by a fixed-density barrier rather than the migration of each cell type to a precise isopycnic position.

3.4 Preformulated Polysucrose Separation Systems with Different Densities

Different preformulated polysucrose density-separation media can establish different cell-separation windows by using different densities. Classical systems with a density of approximately 1.077 g/mL are primarily used to isolate PBMCs, whereas higher-density media such as 1.084 g/mL can be used for mononuclear cells or other blood-cell systems with different density ranges.

3.5 Co-Separation of Low-Density Granulocytes

Low-density neutrophils can occur under pathological conditions such as infection, chronic inflammation, and cancer. After their effective density decreases, these cells can become enriched at the interface together with PBMCs. Therefore, the polysucrose-medium interface obtained from disease samples cannot be regarded as a completely granulocyte-free PBMC population, and its cellular composition should still be confirmed using surface markers or additional separation methods.

4 Nonionic Iodinated Medium: Iodixanol

4.1 High Density and Near-Isotonic Properties

(1) High Molecular Weight and High Iodine Content

Iodixanol is a nonionic dimeric iodinated compound. Its relatively high molecular weight and high iodine content allow it to achieve high medium densities at relatively low molar concentrations.

(2) High Density and Near-Isotonic Conditions

Iodixanol can establish a near-isotonic environment at relatively high medium densities, making high-density conditions compatible with a comparatively mild osmotic environment. This property provides an important basis for its use in separating intact membranous particles and organelles.

4.2 Preservation of Membranous-Particle Structure

Organelles, extracellular vesicles (EVs), and enveloped viruses all possess lipid membranes, and their particle volumes and buoyant densities are readily affected by external osmotic pressure. Hyperosmotic media can cause particles to lose water and shrink, thereby altering their actual buoyant densities. Iodixanol maintains a relatively mild osmotic environment at high medium densities, reducing changes in the physical state of particles caused by the medium itself.

4.3 Separation of Organelles, EVs, and Viruses

(1) Organelles

Mitochondria, peroxisomes, lysosomes, nuclei, endoplasmic reticulum, Golgi apparatus, and plasma membranes can all be fractionated by density using iodixanol. Near-isotonic conditions can reduce artificial shifts in buoyant density caused by organelle shrinkage or swelling.

(2) Extracellular Vesicles

EVs overlap in particle size with lipoproteins, protein aggregates, and other non-vesicular particles, making them difficult to distinguish adequately by differential centrifugation alone. Iodixanol gradients introduce buoyant density as an additional separation dimension and can reduce contamination by some co-sedimenting components.

(3) Virus Particles

Virus particles at different stages of maturation or assembly can have different buoyant densities and form corresponding bands in iodixanol gradients. For enveloped viruses, near-isotonic conditions can also reduce the effects of hyperosmotic environments on the viral envelope and particle volume.

4.4 Application Boundaries of Iodixanol

Iodixanol separation still depends on the presence of effective buoyant-density differences among particles. Particles with highly overlapping buoyant densities require further purification using orthogonal separation methods such as size exclusion or affinity capture. Residual iodixanol in recovered samples should also be removed or exchanged for another buffer according to the requirements of downstream analyses.

5 Sucrose: A Classical Medium for Rate-Zonal Centrifugation and Subcellular Fractionation

5.1 Concentration, Density, Viscosity, and Osmotic Pressure

(1) Concentration and Medium Density

Sucrose has high water solubility, and its concentration can be adjusted to continuously regulate medium density and construct step or continuous gradients. Its readily controllable concentration and well-established protocols make it a classical biological density-gradient medium.

(2) Concentration and Medium Viscosity

Increasing the sucrose concentration simultaneously increases solution viscosity, thereby reducing the migration velocity of particles in a centrifugal field. Consequently, particle positions in sucrose rate-zonal separation are jointly influenced by gradient composition, centrifugal force, and centrifugation time.

(3) Concentration and Osmotic Pressure

High concentrations of sucrose also generate high osmotic pressure. For particles with intact membrane structures, transmembrane water movement can cause volume contraction and further alter their buoyant density and banding behavior.

5.2 Ribosomes and RNP Complexes

(1) Ribosomes and Polysomes

Linear sucrose gradients are widely used for ribosome sedimentation analysis. Small ribosomal subunits, large ribosomal subunits, monosomes, and polysomes of different lengths have different sedimentation coefficients and can be distributed across different gradient regions under defined centrifugation conditions, allowing the evaluation of ribosome assembly states and translation-complex composition.

(2) RNPs and Protein Complexes

Ribonucleoprotein complexes (RNPs) and other large protein complexes can be fractionated in sucrose gradients according to differences in their sedimentation coefficients. Such experiments primarily represent rate-zonal centrifugation and reflect the relative sedimentation behavior of different complexes under defined centrifugation conditions.

5.3 Fractionation of Organelles and Viruses

(1) Purification of Subcellular Particles

Crude mitochondrial, endoplasmic-reticulum, or other membranous fractions obtained by differential centrifugation can be further fractionated through sucrose gradients according to density and sedimentation characteristics, thereby reducing contamination by other subcellular components.

(2) Fractionation of Virus Particles

Sucrose cushions, step gradients, and continuous gradients can all be used to concentrate and fractionate certain virus particles. For enveloped viruses that are sensitive to osmotic conditions, the effects of high sucrose concentrations on particle volume and membrane structure must also be considered.

5.4 Research Compatibility of Different Sucrose Grades

(1) Molecular-Biology and Nuclease-Free Grades

Studies of ribosomes, polysomes, and RNA-associated RNPs require preservation of RNA integrity. Molecular-biology-grade or RNase- and DNase-free sucrose is therefore suitable for reducing the effects of exogenous nucleases on samples.

(2) Cell-Culture Grades

Systems involving intact cells and organelles place greater emphasis on the biocompatibility of the medium. Sucrose products intended for cell culture, insect cell culture, or plant cell culture can be selected for biological samples of the corresponding origins.

(3) High-Purity, Pharmacopoeial, and Low-Endotoxin Grades

ACS, AR, PharmPure™, and GMP-related grades can be selected according to raw-material purity, endotoxin limits, and quality-control requirements. Differences among these grades primarily affect the background quality of the medium and its compatibility with downstream applications without changing the fundamental separation mechanism of sucrose gradients.

6 CsCl: High-Density Isopycnic Gradients and Nucleic Acid Separation

6.1 Self-Generated CsCl Isopycnic Gradients

Cesium chloride (CsCl) has high solubility and can form high-density solutions. In an ultracentrifugal field, CsCl establishes a stable continuous density gradient through sedimentation and diffusion, after which sample macromolecules migrate to regions corresponding to their own buoyant densities. CsCl systems therefore represent typical self-generated isopycnic gradients.

6.2 Buoyant-Density Separation of DNA and RNA

(1) Nucleic Acid Composition and Buoyant Density

The equilibrium positions of DNA and RNA in CsCl gradients are influenced by nucleic acid composition, conformation, and hydration state. After different nucleic acid components reach their isopycnic positions, they form relatively concentrated bands, making the separation results primarily reflective of buoyant-density differences.

(2) Density Resolution of Differences in DNA Composition

DNA base composition can affect its buoyant density. CsCl isopycnic gradients can therefore be used to fractionate different DNA components, genomic DNA, and associated nucleic acid particles, providing a separation dimension distinct from separation according to molecular size.

6.3 Purification of Nucleic Acids and Separation of Associated Particles

CsCl gradients can be used for the high-purity separation of DNA, RNA, and certain nucleic acid-associated particles. Their broad high-density range is suitable for fine fractionation based on differences in the intrinsic buoyant densities of nucleic acids.

6.4 Application Boundaries of CsCl Systems

CsCl isopycnic gradients generally require high centrifugal forces and long centrifugation times, and the high salt concentration must be removed after separation. Column-based, magnetic-bead, or precipitation methods are more convenient for routine rapid nucleic acid purification, whereas CsCl is primarily suitable for studies requiring fine fractionation according to buoyant density.

7 Comparison and Selection of Density-Gradient Media

7.1 Performance Comparison of Major Density-Gradient Media

7.2 Medium Selection for Different Research Targets

7.3 Blood-Cell Separation: Polysucrose and Colloidal Silica Media

Polysucrose and preformulated polysucrose density-separation media primarily use a fixed-density barrier to achieve rapid stratification of PBMCs from red blood cells and granulocytes, making them suitable for mononuclear-cell enrichment. When further separation of neutrophils, eosinophils, or other leukocyte subsets with different density characteristics is required, multilayer or continuous colloidal silica gradients provide greater flexibility in density settings.

7.4 Organelle Separation: Colloidal Silica, Iodixanol, and Sucrose

Crude organelle fractions are usually first obtained by differential centrifugation, after which a gradient medium is selected according to purity and structural-preservation requirements. Colloidal silica media are suitable for further purification under relatively mild conditions; iodixanol is suitable for osmotically sensitive membranous organelles and fine buoyant-density fractionation; and sucrose is suitable for classical subcellular fractionation systems.

7.5 EVs and Viruses: Iodixanol and Sucrose

EV separation requires the introduction of buoyant-density information in addition to particle size while preserving membrane structure and particle volume as much as possible, making iodixanol a commonly used medium for fine density separation. For virus separation, sucrose cushions or gradients can be selected for concentration and fractionation according to the research objective, whereas iodixanol can be used to improve buoyant-density resolution and reduce the effects of hyperosmotic environments on enveloped particles.

7.6 Ribosomes and RNPs: Sucrose

Ribosomes, RNPs, and large protein complexes are primarily fractionated according to differences in sedimentation coefficients, and linear sucrose gradients remain a classical rate-zonal system. When RNA integrity must be controlled, molecular-biology-grade or RNase- and DNase-free sucrose can be selected.

7.7 Nucleic Acids: CsCl

CsCl is suitable for isopycnic fractionation of DNA, RNA, and associated nucleic acid components according to differences in buoyant density. Compared with routine column-based or magnetic-bead purification, its principal value lies in providing buoyant density as an independent separation dimension rather than serving as a routine rapid nucleic acid extraction method.

8 Products

References

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[3] Böyum A. Isolation of Mononuclear Cells and Granulocytes from Human Blood. Scand J Clin Lab Invest Suppl. 1968;97:77-89.

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