Buoyant Density
 
The buoyant density of a macromolecule is its effective density in solution. It is defined relative to the fluid medium in which it resides and/or through which it is sedimented. The dimensions of buoyant density are in cm3/g. If a macromolecule is sedimented through a fluid column in which the density of the column is adjusted so that it spans a gradient of density both less than and greater than the macromolecule, the macromolecule moves to a fixed position in the fluid column. At this position the density of the macromolecule is equal to that of the solvent. This is an equilibrium position that is independent of time. This process or technique is termed density gradient sedimentation-equilibrium, equilibrium banding or isopycnic gradient centrifugation. These techniques have uses that range from the analysis of macromolecule solvation to the separation of both living cells and subcellular organelle systems (1). Recently the evaluation of macromolecular values of buoyant density have been been employed in a variety of research situations including (1) the analysis of viral infectivity during therapy (2, 3); (2) the nature of circulating immune complexes of viruses (4); (3) as a preselection method for PCR methods (5).
References
1. S. Daenen, W. Huiges, E. Modderman, and M. R. Halie (1993) Leukemia Res. 17, 37–41. 
2. A. Nagasaka, S. Hige, T. Matsushima, J. Yoshida, Y. Sasaki, I. Tsunematsu, and M. Asaka (1997) J. Med. Virol. 52, 190–194. 
3. T. Kanto, N. Hayashi, T. Takehara, H. Hagiwara, E. Mita, M. Naito, A. Kasahara, H. Fusamoto, and T. Kamada (1995) J. Hepatol. 22, 440–448. 
4. M. Hijikata, Y. K. Shimizu, H. Kato, A. Iwamoto, J. W. Shih, H. J. Alter, R. H. Purcell, and H. Yoshikura (1993) J. Virol. 67, 1953–1958. 
5. R. J. Carrick, G. G. Schlauder, D. A. Peterson, and I. K. Mushahwar (1992) J. Virol. Meth. 39, 279-290.