Flotation

Westbrook, E. M.

doi:10.1107/97809553602060000664

International
Tables for
Crystallography
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

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International Tables for Crystallography (2006). Vol. F. ch. 5.2, p. 119 | 1 | 2 |

Section 5.2.6.5. Flotation

E. M. Westbrook^a ^*

^a Molecular Biology Consortium, Argonne, Illinois 60439, USA
Correspondence e-mail: westbrook@anl.gov

5.2.6.5. Flotation

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The crystal must first be wiped completely free of external liquid and then immersed in a mixture of organic solvents, the density of which is adjusted (by addition of denser or lighter solvents) until the crystal neither rises nor sinks. Note that if the liquid used were aqueous, the crystal density would change as the surrounding liquid density is changed (e.g. by adding salt), since the crystal's free-solvent compartment would exchange with the external liquid. In this case, the equilibrium density, $[\rho_{e}]$ , is a function only of the hydration number, w, and the macromolecule's partial specific volume, $[\overline{\upsilon}_{m}]$ : $[\rho_{e} = (w + 1)/(w + \overline{\upsilon}_{m}). \eqno(5.2.6.2)]$ $[\rho_{e}]$ is about $[1.25 \hbox{ g ml}^{-1}]$ for all protein crystals, regardless of packing arrangements or molecular weights, since $[w \simeq 0.25]$ and $[\overline{\upsilon}_{m} \simeq 0.74\hbox{ cm}^{3}\hbox{ g}^{-1}]$ .

When the crystal just floats, the liquid's density (which now equals the crystal density) can be measured by standard techniques with high accuracy. Flotation measurements can be made with small samples (Bernal & Crowfoot, 1934) and with slurries of microcrystals. Centrifugation should be used to accelerate the crystal settling rate each time the liquid density is altered. The method can be tedious, so its practitioners rarely achieve an accuracy better than 0.2–1.0% (Low & Richards, 1952a).

References

Bernal, J. D. & Crowfoot, D. (1934). Use of the centrifuge in determining the density of small crystals. Nature (London), 134, 809–810.Google Scholar

Low, B. W. & Richards, F. M. (1952a). The use of the gradient tube for the determination of crystal densities. J. Am. Chem. Soc. 74, 1660–1666.Google Scholar

International Tables for Crystallography (2006). Vol. F. ch. 5.2, p. 119