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

International Tables for Crystallography (2012). Vol. F, ch. 4.2, pp. 125-126   | 1 | 2 |

Section 4.2.4. The `small amphiphile concept'

H. Michela*

aMax-Planck-Institut für Biophysik, Heinrich-Hoffmann-Strasse 7, D-60528 Frankfurt/Main, Germany
Correspondence e-mail:

4.2.4. The `small amphiphile concept'

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From the arguments and observations presented above, it is evident that the size and shape of the detergent micelle are very important in membrane-protein crystallization. Detergent micelles can be made smaller (and their curvatures changed) when small amphiphilic molecules like heptane-1,2,3-triol are added (Timmins et al., 1991[link]; Gast et al., 1994[link]). These compounds form mixed micelles with detergents. When 10% (w/v) heptane-1,2,3-triol is added to 1% solutions of N,N-dimethyldodecyl­amine-N-oxide in water, the number of detergent molecules per micelle decreases from 69 to 34, ∼23 heptane-1,2,3-triol molecules are incorporated and the radius of the micelle is reduced from 20.9 to 16.9 Å (Timmins et al., 1994[link]). This so-called `small amphiphile concept' has been used successfully to crystallize bacterial photosynthetic reaction centres (Michel, 1982[link], 1983[link]; Buchanan et al., 1993[link]), bacterial light-harvesting complexes (Michel, 1991[link]; Koepke et al., 1996[link]) and other membrane proteins (see Table[link]). The light-harvesting complexes from the purple bacterium Rhodospirillum molischianum yield an astonishing number of different crystal forms, but only one diffracts to high resolution. A large amount of heptane-1,2,3-triol had to be added to obtain this crystal form. As a result, heptane-1,2,3-triol itself reached supersaturation during crystallization of the protein. Normally, the protein crystallizes first and heptane-1,2,3-triol later. When heptane-1,2,3-triol crystals form, the protein crystals crack and eventually redissolve. A likely explanation is that the concentration of solubilized heptane-1,2,3-triol is reduced when it forms crystals and the mixed detergent–heptane-1,2,3-triol micelles lose heptane-1,2,3-triol and take up detergent molecules from solution. The micelles expand and the crystals crack. This behaviour caused severe problems when searching for heavy-atom derivatives. Small amphiphiles work well with the N,N-dimethylalkylamine-N-oxides and alkylglucopyranosides as detergents, but not with alkylmaltosides. The most successful small amphiphiles are heptane-1,2,3-triol and benzamidine.


Buchanan, S. K., Fritzsch, G., Ermler, U. & Michel, H. (1993). New crystal form of the photosynthetic reaction centre from Rhodobacter sphaeroides of improved diffraction quality. J. Mol. Biol. 230, 1311–1314.
Gast, P., Hemelrijk, P. & Hoff, A. J. (1994). Determination of the number of detergent molecules associated with the reaction center protein isolated from the photosynthetic bacterium Rhodopseudomonas viridis. Effects of the amphiphilic molecule, 1,2,3-heptanetriol. FEBS Lett. 337, 39–42.
Koepke, J., Hu, X., Muenke, C., Schulten, K. & Michel, H. (1996). The crystal structure of the light-harvesting complex II (B800–850) from Rhodospirilum molischianum. Structure, 4, 581–597.
Michel, H. (1982). Three-dimensional crystals of a membrane protein complex. The photosynthetic reaction centre from Rhodopseudomonas viridis. J. Mol. Biol. 158, 567–572.
Michel, H. (1983). Crystallization of membrane proteins. Trends Biochem. Sci. 8, 56–59.
Michel, H. (1991). Editor. Crystallization of Membrane Proteins. Boca Raton, Florida: CRC Press.
Timmins, P. A., Hauk, J., Wacker, T. & Welte, W. (1991). The influence of heptane-1,2,3-triol on the size and shape of LDAO micelles. Implications for the crystallization of membrane proteins. FEBS Lett. 280, 115–120.
Timmins, P. A., Pebay-Peyroula, E. & Welte, W. (1994). Detergent organisation in solutions and in crystals of membrane proteins. Biophys. Chem. 53, 27–36.

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